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PERF_EVENT_OPEN(2)         Linux Programmer's Manual        PERF_EVENT_OPEN(2)

NAME
       perf_event_open - set up performance monitoring

SYNOPSIS
       #include <linux/perf_event.h>
       #include <linux/hw_breakpoint.h>

       int perf_event_open(struct perf_event_attr *attr,
                           pid_t pid, int cpu, int group_fd,
                           unsigned long flags);

       Note: There is no glibc wrapper for this system call; see NOTES.

DESCRIPTION
       Given  a  list of parameters, perf_event_open() returns a file descrip-
       tor, for use in subsequent system calls  (read(2),  mmap(2),  prctl(2),
       fcntl(2), etc.).

       A  call to perf_event_open() creates a file descriptor that allows mea-
       suring performance information.  Each file  descriptor  corresponds  to
       one  event  that  is measured; these can be grouped together to measure
       multiple events simultaneously.

       Events can be enabled and disabled in two ways: via  ioctl(2)  and  via
       prctl(2).   When  an  event  is  disabled it does not count or generate
       overflows but does continue to exist and maintain its count value.

       Events come in two flavors: counting and sampled.  A counting event  is
       one  that  is used for counting the aggregate number of events that oc-
       cur.  In general, counting event results are gathered  with  a  read(2)
       call.   A  sampling  event periodically writes measurements to a buffer
       that can then be accessed via mmap(2).

   Arguments
       The pid and cpu arguments allow specifying which  process  and  CPU  to
       monitor:

       pid == 0 and cpu == -1
              This measures the calling process/thread on any CPU.

       pid == 0 and cpu >= 0
              This  measures  the  calling process/thread only when running on
              the specified CPU.

       pid > 0 and cpu == -1
              This measures the specified process/thread on any CPU.

       pid > 0 and cpu >= 0
              This measures the specified process/thread only when running  on
              the specified CPU.

       pid == -1 and cpu >= 0
              This  measures all processes/threads on the specified CPU.  This
              requires   CAP_SYS_ADMIN   capability   or   a    /proc/sys/ker-
              nel/perf_event_paranoid value of less than 1.

       pid == -1 and cpu == -1
              This setting is invalid and will return an error.

       When  pid  is greater than zero, permission to perform this system call
       is governed by a ptrace access mode  PTRACE_MODE_READ_REALCREDS  check;
       see ptrace(2).

       The  group_fd  argument  allows  event  groups to be created.  An event
       group has one event which is the group leader.  The leader  is  created
       first,  with  group_fd = -1.  The rest of the group members are created
       with subsequent perf_event_open() calls with group_fd being set to  the
       file  descriptor  of  the  group leader.  (A single event on its own is
       created with group_fd = -1 and is considered to be a group with only  1
       member.)   An  event group is scheduled onto the CPU as a unit: it will
       be put onto the CPU only if all of the events in the group can  be  put
       onto  the  CPU.  This means that the values of the member events can be
       meaningfully compared--added, divided (to get ratios), and so  on--with
       each other, since they have counted events for the same set of executed
       instructions.

       The flags argument is formed by ORing together zero or more of the fol-
       lowing values:

       PERF_FLAG_FD_CLOEXEC (since Linux 3.14)
              This  flag  enables the close-on-exec flag for the created event
              file descriptor, so that the file  descriptor  is  automatically
              closed  on  execve(2).   Setting the close-on-exec flags at cre-
              ation time, rather than later with  fcntl(2),  avoids  potential
              race    conditions    where    the    calling   thread   invokes
              perf_event_open() and fcntl(2)  at  the  same  time  as  another
              thread calls fork(2) then execve(2).

       PERF_FLAG_FD_NO_GROUP
              This  flag  tells the event to ignore the group_fd parameter ex-
              cept for the purpose of setting up output redirection using  the
              PERF_FLAG_FD_OUTPUT flag.

       PERF_FLAG_FD_OUTPUT (broken since Linux 2.6.35)
              This flag re-routes the event's sampled output to instead be in-
              cluded in the mmap buffer of the event specified by group_fd.

       PERF_FLAG_PID_CGROUP (since Linux 2.6.39)
              This flag activates  per-container  system-wide  monitoring.   A
              container is an abstraction that isolates a set of resources for
              finer-grained control (CPUs, memory, etc.).  In this  mode,  the
              event  is  measured  only if the thread running on the monitored
              CPU belongs to the designated container (cgroup).  The cgroup is
              identified  by passing a file descriptor opened on its directory
              in the cgroupfs filesystem.  For instance, if the cgroup to mon-
              itor   is   called  test,  then  a  file  descriptor  opened  on
              /dev/cgroup/test (assuming cgroupfs is mounted  on  /dev/cgroup)
              must  be  passed  as  the  pid  parameter.  cgroup monitoring is
              available only for system-wide events and may therefore  require
              extra permissions.

       The  perf_event_attr structure provides detailed configuration informa-
       tion for the event being created.

           struct perf_event_attr {
               __u32 type;                 /* Type of event */
               __u32 size;                 /* Size of attribute structure */
               __u64 config;               /* Type-specific configuration */

               union {
                   __u64 sample_period;    /* Period of sampling */
                   __u64 sample_freq;      /* Frequency of sampling */
               };

               __u64 sample_type;  /* Specifies values included in sample */
               __u64 read_format;  /* Specifies values returned in read */

               __u64 disabled       : 1,   /* off by default */
                     inherit        : 1,   /* children inherit it */
                     pinned         : 1,   /* must always be on PMU */
                     exclusive      : 1,   /* only group on PMU */
                     exclude_user   : 1,   /* don't count user */
                     exclude_kernel : 1,   /* don't count kernel */
                     exclude_hv     : 1,   /* don't count hypervisor */
                     exclude_idle   : 1,   /* don't count when idle */
                     mmap           : 1,   /* include mmap data */
                     comm           : 1,   /* include comm data */
                     freq           : 1,   /* use freq, not period */
                     inherit_stat   : 1,   /* per task counts */
                     enable_on_exec : 1,   /* next exec enables */
                     task           : 1,   /* trace fork/exit */
                     watermark      : 1,   /* wakeup_watermark */
                     precise_ip     : 2,   /* skid constraint */
                     mmap_data      : 1,   /* non-exec mmap data */
                     sample_id_all  : 1,   /* sample_type all events */
                     exclude_host   : 1,   /* don't count in host */
                     exclude_guest  : 1,   /* don't count in guest */
                     exclude_callchain_kernel : 1,
                                           /* exclude kernel callchains */
                     exclude_callchain_user   : 1,
                                           /* exclude user callchains */
                     mmap2          :  1,  /* include mmap with inode data */
                     comm_exec      :  1,  /* flag comm events that are
                                              due to exec */
                     use_clockid    :  1,  /* use clockid for time fields */
                     context_switch :  1,  /* context switch data */

                     __reserved_1   : 37;

               union {
                   __u32 wakeup_events;    /* wakeup every n events */
                   __u32 wakeup_watermark; /* bytes before wakeup */
               };

               __u32     bp_type;          /* breakpoint type */

               union {
                   __u64 bp_addr;          /* breakpoint address */
                   __u64 kprobe_func;      /* for perf_kprobe */
                   __u64 uprobe_path;      /* for perf_uprobe */
                   __u64 config1;          /* extension of config */
               };

               union {
                   __u64 bp_len;           /* breakpoint length */
                   __u64 kprobe_addr;      /* with kprobe_func == NULL */
                   __u64 probe_offset;     /* for perf_[k,u]probe */
                   __u64 config2;          /* extension of config1 */
               };
               __u64 branch_sample_type;   /* enum perf_branch_sample_type */
               __u64 sample_regs_user;     /* user regs to dump on samples */
               __u32 sample_stack_user;    /* size of stack to dump on
                                              samples */
               __s32 clockid;              /* clock to use for time fields */
               __u64 sample_regs_intr;     /* regs to dump on samples */
               __u32 aux_watermark;        /* aux bytes before wakeup */
               __u16 sample_max_stack;     /* max frames in callchain */
               __u16 __reserved_2;         /* align to u64 */

           };

       The fields of the perf_event_attr structure are described in  more  de-
       tail below:

       type   This  field specifies the overall event type.  It has one of the
              following values:

              PERF_TYPE_HARDWARE
                     This indicates one of the "generalized"  hardware  events
                     provided  by the kernel.  See the config field definition
                     for more details.

              PERF_TYPE_SOFTWARE
                     This indicates one of the  software-defined  events  pro-
                     vided  by  the  kernel  (even  if  no hardware support is
                     available).

              PERF_TYPE_TRACEPOINT
                     This indicates a tracepoint provided by the kernel trace-
                     point infrastructure.

              PERF_TYPE_HW_CACHE
                     This  indicates  a hardware cache event.  This has a spe-
                     cial encoding, described in the config field definition.

              PERF_TYPE_RAW
                     This indicates a "raw" implementation-specific  event  in
                     the config field.

              PERF_TYPE_BREAKPOINT (since Linux 2.6.33)
                     This  indicates  a hardware breakpoint as provided by the
                     CPU.  Breakpoints can be read/write accesses  to  an  ad-
                     dress as well as execution of an instruction address.

              dynamic PMU
                     Since  Linux 2.6.38, perf_event_open() can support multi-
                     ple PMUs.  To enable this, a value exported by the kernel
                     can  be  used  in the type field to indicate which PMU to
                     use.  The value to use can be found in the sysfs filesys-
                     tem:  there  is  a  subdirectory  per  PMU instance under
                     /sys/bus/event_source/devices.   In   each   subdirectory
                     there is a type file whose content is an integer that can
                     be   used   in   the   type   field.     For    instance,
                     /sys/bus/event_source/devices/cpu/type contains the value
                     for the core CPU PMU, which is usually 4.

              kprobe and uprobe (since Linux 4.17)
                     These two dynamic PMUs create a kprobe/uprobe and  attach
                     it  to  the file descriptor generated by perf_event_open.
                     The kprobe/uprobe will be destroyed on the destruction of
                     the   file   descriptor.   See  fields  kprobe_func,  up-
                     robe_path, kprobe_addr, and  probe_offset  for  more  de-
                     tails.

       size   The  size  of the perf_event_attr structure for forward/backward
              compatibility.  Set this using sizeof(struct perf_event_attr) to
              allow  the kernel to see the struct size at the time of compila-
              tion.

              The related define PERF_ATTR_SIZE_VER0 is set to  64;  this  was
              the  size of the first published struct.  PERF_ATTR_SIZE_VER1 is
              72, corresponding  to  the  addition  of  breakpoints  in  Linux
              2.6.33.  PERF_ATTR_SIZE_VER2 is 80 corresponding to the addition
              of branch sampling in Linux 3.4.  PERF_ATTR_SIZE_VER3 is 96 cor-
              responding   to   the  addition  of  sample_regs_user  and  sam-
              ple_stack_user in Linux 3.7.  PERF_ATTR_SIZE_VER4 is 104  corre-
              sponding  to  the  addition  of  sample_regs_intr in Linux 3.19.
              PERF_ATTR_SIZE_VER5 is 112  corresponding  to  the  addition  of
              aux_watermark in Linux 4.1.

       config This  specifies  which  event  you want, in conjunction with the
              type field.  The config1 and config2 fields are also taken  into
              account  in  cases  where 64 bits is not enough to fully specify
              the event.  The encoding of these fields are event dependent.

              There are various ways to set the config field that  are  depen-
              dent  on the value of the previously described type field.  What
              follows are various possible settings for config  separated  out
              by type.

              If  type is PERF_TYPE_HARDWARE, we are measuring one of the gen-
              eralized hardware CPU events.  Not all of these are available on
              all platforms.  Set config to one of the following:

                   PERF_COUNT_HW_CPU_CYCLES
                          Total  cycles.   Be  wary of what happens during CPU
                          frequency scaling.

                   PERF_COUNT_HW_INSTRUCTIONS
                          Retired instructions.  Be careful, these can be  af-
                          fected  by various issues, most notably hardware in-
                          terrupt counts.

                   PERF_COUNT_HW_CACHE_REFERENCES
                          Cache accesses.  Usually this indicates  Last  Level
                          Cache  accesses  but this may vary depending on your
                          CPU.  This may include prefetches and coherency mes-
                          sages; again this depends on the design of your CPU.

                   PERF_COUNT_HW_CACHE_MISSES
                          Cache  misses.   Usually  this  indicates Last Level
                          Cache misses; this is intended to be  used  in  con-
                          junction   with  the  PERF_COUNT_HW_CACHE_REFERENCES
                          event to calculate cache miss rates.

                   PERF_COUNT_HW_BRANCH_INSTRUCTIONS
                          Retired branch instructions.  Prior to Linux 2.6.35,
                          this used the wrong event on AMD processors.

                   PERF_COUNT_HW_BRANCH_MISSES
                          Mispredicted branch instructions.

                   PERF_COUNT_HW_BUS_CYCLES
                          Bus  cycles,  which  can be different from total cy-
                          cles.

                   PERF_COUNT_HW_STALLED_CYCLES_FRONTEND (since Linux 3.0)
                          Stalled cycles during issue.

                   PERF_COUNT_HW_STALLED_CYCLES_BACKEND (since Linux 3.0)
                          Stalled cycles during retirement.

                   PERF_COUNT_HW_REF_CPU_CYCLES (since Linux 3.3)
                          Total cycles; not affected by CPU frequency scaling.

              If type is PERF_TYPE_SOFTWARE, we are measuring software  events
              provided by the kernel.  Set config to one of the following:

                   PERF_COUNT_SW_CPU_CLOCK
                          This  reports  the CPU clock, a high-resolution per-
                          CPU timer.

                   PERF_COUNT_SW_TASK_CLOCK
                          This reports a clock count specific to the task that
                          is running.

                   PERF_COUNT_SW_PAGE_FAULTS
                          This reports the number of page faults.

                   PERF_COUNT_SW_CONTEXT_SWITCHES
                          This  counts  context switches.  Until Linux 2.6.34,
                          these were all reported as user-space events,  after
                          that they are reported as happening in the kernel.

                   PERF_COUNT_SW_CPU_MIGRATIONS
                          This reports the number of times the process has mi-
                          grated to a new CPU.

                   PERF_COUNT_SW_PAGE_FAULTS_MIN
                          This counts the number of minor page faults.   These
                          did not require disk I/O to handle.

                   PERF_COUNT_SW_PAGE_FAULTS_MAJ
                          This  counts the number of major page faults.  These
                          required disk I/O to handle.

                   PERF_COUNT_SW_ALIGNMENT_FAULTS (since Linux 2.6.33)
                          This counts the number of alignment  faults.   These
                          happen  when  unaligned  memory accesses happen; the
                          kernel can handle these but it reduces  performance.
                          This  happens  only  on some architectures (never on
                          x86).

                   PERF_COUNT_SW_EMULATION_FAULTS (since Linux 2.6.33)
                          This counts the number  of  emulation  faults.   The
                          kernel sometimes traps on unimplemented instructions
                          and emulates them for user space.   This  can  nega-
                          tively impact performance.

                   PERF_COUNT_SW_DUMMY (since Linux 3.12)
                          This  is  a  placeholder  event that counts nothing.
                          Informational sample record types such  as  mmap  or
                          comm  must be associated with an active event.  This
                          dummy event allows gathering  such  records  without
                          requiring a counting event.

              If  type  is  PERF_TYPE_TRACEPOINT, then we are measuring kernel
              tracepoints.  The value to use in config can  be  obtained  from
              under  debugfs tracing/events/*/*/id if ftrace is enabled in the
              kernel.

              If type is PERF_TYPE_HW_CACHE, then we are measuring a  hardware
              CPU  cache event.  To calculate the appropriate config value use
              the following equation:

                      (perf_hw_cache_id) | (perf_hw_cache_op_id << 8) |
                      (perf_hw_cache_op_result_id << 16)

                  where perf_hw_cache_id is one of:

                      PERF_COUNT_HW_CACHE_L1D
                             for measuring Level 1 Data Cache

                      PERF_COUNT_HW_CACHE_L1I
                             for measuring Level 1 Instruction Cache

                      PERF_COUNT_HW_CACHE_LL
                             for measuring Last-Level Cache

                      PERF_COUNT_HW_CACHE_DTLB
                             for measuring the Data TLB

                      PERF_COUNT_HW_CACHE_ITLB
                             for measuring the Instruction TLB

                      PERF_COUNT_HW_CACHE_BPU
                             for measuring the branch prediction unit

                      PERF_COUNT_HW_CACHE_NODE (since Linux 3.1)
                             for measuring local memory accesses

                  and perf_hw_cache_op_id is one of:

                      PERF_COUNT_HW_CACHE_OP_READ
                             for read accesses

                      PERF_COUNT_HW_CACHE_OP_WRITE
                             for write accesses

                      PERF_COUNT_HW_CACHE_OP_PREFETCH
                             for prefetch accesses

                  and perf_hw_cache_op_result_id is one of:

                      PERF_COUNT_HW_CACHE_RESULT_ACCESS
                             to measure accesses

                      PERF_COUNT_HW_CACHE_RESULT_MISS
                             to measure misses

              If type is PERF_TYPE_RAW, then a custom "raw"  config  value  is
              needed.   Most  CPUs  support events that are not covered by the
              "generalized" events.  These  are  implementation  defined;  see
              your  CPU  manual (for example the Intel Volume 3B documentation
              or the AMD BIOS and Kernel Developer Guide).   The  libpfm4  li-
              brary  can  be  used to translate from the name in the architec-
              tural manuals to the raw hex value perf_event_open() expects  in
              this field.

              If  type is PERF_TYPE_BREAKPOINT, then leave config set to zero.
              Its parameters are set in other places.

              If type is kprobe or uprobe, set retprobe (bit 0 of config,  see
              /sys/bus/event_source/devices/[k,u]probe/format/retprobe)    for
              kretprobe/uretprobe.   See  fields   kprobe_func,   uprobe_path,
              kprobe_addr, and probe_offset for more details.

       kprobe_func, uprobe_path, kprobe_addr, and probe_offset
              These  fields describe the kprobe/uprobe for dynamic PMUs kprobe
              and uprobe.  For kprobe: use kprobe_func  and  probe_offset,  or
              use  kprobe_addr and leave kprobe_func as NULL.  For uprobe: use
              uprobe_path and probe_offset.

       sample_period, sample_freq
              A "sampling" event is one that generates an  overflow  notifica-
              tion  every N events, where N is given by sample_period.  A sam-
              pling event has sample_period > 0.  When an overflow occurs, re-
              quested  data  is  recorded in the mmap buffer.  The sample_type
              field controls what data is recorded on each overflow.

              sample_freq can be used if you wish to use frequency rather than
              period.   In  this case, you set the freq flag.  The kernel will
              adjust the sampling period to try and achieve the desired  rate.
              The rate of adjustment is a timer tick.

       sample_type
              The  various  bits in this field specify which values to include
              in the sample.  They will be recorded in a ring-buffer, which is
              available  to  user space using mmap(2).  The order in which the
              values are saved in the sample are documented in the MMAP Layout
              subsection  below;  it  is not the enum perf_event_sample_format
              order.

              PERF_SAMPLE_IP
                     Records instruction pointer.

              PERF_SAMPLE_TID
                     Records the process and thread IDs.

              PERF_SAMPLE_TIME
                     Records a timestamp.

              PERF_SAMPLE_ADDR
                     Records an address, if applicable.

              PERF_SAMPLE_READ
                     Record counter values for all events in a group, not just
                     the group leader.

              PERF_SAMPLE_CALLCHAIN
                     Records the callchain (stack backtrace).

              PERF_SAMPLE_ID
                     Records a unique ID for the opened event's group leader.

              PERF_SAMPLE_CPU
                     Records CPU number.

              PERF_SAMPLE_PERIOD
                     Records the current sampling period.

              PERF_SAMPLE_STREAM_ID
                     Records  a  unique  ID  for  the  opened  event.   Unlike
                     PERF_SAMPLE_ID the actual ID is returned, not  the  group
                     leader.   This  ID  is  the  same  as the one returned by
                     PERF_FORMAT_ID.

              PERF_SAMPLE_RAW
                     Records additional data, if applicable.  Usually returned
                     by tracepoint events.

              PERF_SAMPLE_BRANCH_STACK (since Linux 3.4)
                     This provides a record of recent branches, as provided by
                     CPU branch sampling hardware (such as Intel  Last  Branch
                     Record).  Not all hardware supports this feature.

                     See  the branch_sample_type field for how to filter which
                     branches are reported.

              PERF_SAMPLE_REGS_USER (since Linux 3.7)
                     Records the current user-level CPU  register  state  (the
                     values in the process before the kernel was called).

              PERF_SAMPLE_STACK_USER (since Linux 3.7)
                     Records the user level stack, allowing stack unwinding.

              PERF_SAMPLE_WEIGHT (since Linux 3.10)
                     Records  a  hardware provided weight value that expresses
                     how costly the sampled event was.  This allows the  hard-
                     ware to highlight expensive events in a profile.

              PERF_SAMPLE_DATA_SRC (since Linux 3.10)
                     Records  the  data  source: where in the memory hierarchy
                     the data associated with  the  sampled  instruction  came
                     from.   This is available only if the underlying hardware
                     supports this feature.

              PERF_SAMPLE_IDENTIFIER (since Linux 3.12)
                     Places the SAMPLE_ID value in a  fixed  position  in  the
                     record, either at the beginning (for sample events) or at
                     the end (if a non-sample event).

                     This was necessary  because  a  sample  stream  may  have
                     records from various different event sources with differ-
                     ent sample_type settings.  Parsing the event stream prop-
                     erly  was  not  possible because the format of the record
                     was needed to find SAMPLE_ID, but the format could not be
                     found  without  knowing what event the sample belonged to
                     (causing a circular dependency).

                     The PERF_SAMPLE_IDENTIFIER setting makes the event stream
                     always parsable by putting SAMPLE_ID in a fixed location,
                     even though it means having duplicate SAMPLE_ID values in
                     records.

              PERF_SAMPLE_TRANSACTION (since Linux 3.13)
                     Records  reasons  for  transactional  memory abort events
                     (for example, from Intel TSX  transactional  memory  sup-
                     port).

                     The  precise_ip  setting  must  be  greater  than 0 and a
                     transactional memory abort event must be measured  or  no
                     values  will be recorded.  Also note that some perf_event
                     measurements, such as sampled cycle counting,  may  cause
                     extraneous  aborts  (by  causing  an  interrupt  during a
                     transaction).

              PERF_SAMPLE_REGS_INTR (since Linux 3.19)
                     Records a subset of the current  CPU  register  state  as
                     specified    by   sample_regs_intr.    Unlike   PERF_SAM-
                     PLE_REGS_USER the register values will return kernel reg-
                     ister state if the overflow happened while kernel code is
                     running.  If the CPU supports hardware sampling of regis-
                     ter state (i.e., PEBS on Intel x86) and precise_ip is set
                     higher than zero then the register  values  returned  are
                     those captured by hardware at the time of the sampled in-
                     struction's retirement.

       read_format
              This field specifies the format of the data returned by  read(2)
              on a perf_event_open() file descriptor.

              PERF_FORMAT_TOTAL_TIME_ENABLED
                     Adds  the 64-bit time_enabled field.  This can be used to
                     calculate estimated totals if the  PMU  is  overcommitted
                     and multiplexing is happening.

              PERF_FORMAT_TOTAL_TIME_RUNNING
                     Adds  the 64-bit time_running field.  This can be used to
                     calculate estimated totals if the  PMU  is  overcommitted
                     and multiplexing is happening.

              PERF_FORMAT_ID
                     Adds  a 64-bit unique value that corresponds to the event
                     group.

              PERF_FORMAT_GROUP
                     Allows all counter values in an event group  to  be  read
                     with one read.

       disabled
              The  disabled  bit specifies whether the counter starts out dis-
              abled or enabled.  If disabled, the event can later  be  enabled
              by ioctl(2), prctl(2), or enable_on_exec.

              When creating an event group, typically the group leader is ini-
              tialized with disabled set to 1 and any child  events  are  ini-
              tialized  with disabled set to 0.  Despite disabled being 0, the
              child events will not start until the group leader is enabled.

       inherit
              The inherit bit specifies that this counter should count  events
              of child tasks as well as the task specified.  This applies only
              to new children, not to any existing children at  the  time  the
              counter  is  created  (nor to any new children of existing chil-
              dren).

              Inherit does not work for some combinations of read_format  val-
              ues, such as PERF_FORMAT_GROUP.

       pinned The  pinned  bit  specifies that the counter should always be on
              the CPU if at all possible.  It applies only to  hardware  coun-
              ters  and  only to group leaders.  If a pinned counter cannot be
              put onto the CPU (e.g., because there are  not  enough  hardware
              counters  or  because of a conflict with some other event), then
              the counter goes into an 'error' state, where reads return  end-
              of-file  (i.e.,  read(2)  returns 0) until the counter is subse-
              quently enabled or disabled.

       exclusive
              The exclusive bit specifies that when this counter's group is on
              the  CPU,  it should be the only group using the CPU's counters.
              In the future this may allow monitoring programs to support  PMU
              features  that  need  to  run  alone so that they do not disrupt
              other hardware counters.

              Note that many unexpected situations may prevent events with the
              exclusive  bit  set  from ever running.  This includes any users
              running a system-wide measurement as well as any kernel  use  of
              the  performance  counters  (including  the commonly enabled NMI
              Watchdog Timer interface).

       exclude_user
              If this bit is set, the count excludes  events  that  happen  in
              user space.

       exclude_kernel
              If  this  bit  is  set, the count excludes events that happen in
              kernel space.

       exclude_hv
              If this bit is set, the count excludes events that happen in the
              hypervisor.   This is mainly for PMUs that have built-in support
              for handling this (such as POWER).  Extra support is needed  for
              handling hypervisor measurements on most machines.

       exclude_idle
              If  set,  don't  count  when  the  CPU is running the idle task.
              While you can currently enable this for any event  type,  it  is
              ignored for all but software events.

       mmap   The  mmap bit enables generation of PERF_RECORD_MMAP samples for
              every mmap(2) call that has PROT_EXEC set.  This allows tools to
              notice  new executable code being mapped into a program (dynamic
              shared libraries for example) so that addresses  can  be  mapped
              back to the original code.

       comm   The  comm  bit enables tracking of process command name as modi-
              fied by the exec(2) and prctl(PR_SET_NAME) system calls as  well
              as  writing  to  /proc/self/comm.  If the comm_exec flag is also
              successfully set (possible since Linux 3.16), then the misc flag
              PERF_RECORD_MISC_COMM_EXEC  can  be  used  to  differentiate the
              exec(2) case from the others.

       freq   If this bit is set, then sample_frequency not  sample_period  is
              used when setting up the sampling interval.

       inherit_stat
              This  bit  enables  saving of event counts on context switch for
              inherited tasks.  This is meaningful only if the  inherit  field
              is set.

       enable_on_exec
              If  this  bit is set, a counter is automatically enabled after a
              call to exec(2).

       task   If this bit is set, then fork/exit notifications are included in
              the ring buffer.

       watermark
              If  set,  have an overflow notification happen when we cross the
              wakeup_watermark boundary.   Otherwise,  overflow  notifications
              happen after wakeup_events samples.

       precise_ip (since Linux 2.6.35)
              This controls the amount of skid.  Skid is how many instructions
              execute between an event of interest happening  and  the  kernel
              being able to stop and record the event.  Smaller skid is better
              and allows more accurate reporting of which events correspond to
              which instructions, but hardware is often limited with how small
              this can be.

              The possible values of this field are the following:

              0  SAMPLE_IP can have arbitrary skid.

              1  SAMPLE_IP must have constant skid.

              2  SAMPLE_IP requested to have 0 skid.

              3  SAMPLE_IP must have 0 skid.   See  also  the  description  of
                 PERF_RECORD_MISC_EXACT_IP.

       mmap_data (since Linux 2.6.36)
              This is the counterpart of the mmap field.  This enables genera-
              tion of PERF_RECORD_MMAP samples for mmap(2) calls that  do  not
              have PROT_EXEC set (for example data and SysV shared memory).

       sample_id_all (since Linux 2.6.38)
              If  set, then TID, TIME, ID, STREAM_ID, and CPU can additionally
              be included in non-PERF_RECORD_SAMPLEs if the corresponding sam-
              ple_type is selected.

              If  PERF_SAMPLE_IDENTIFIER  is  specified, then an additional ID
              value is included as the last value to ease parsing  the  record
              stream.  This may lead to the id value appearing twice.

              The layout is described by this pseudo-structure:

                  struct sample_id {
                      { u32 pid, tid; }   /* if PERF_SAMPLE_TID set */
                      { u64 time;     }   /* if PERF_SAMPLE_TIME set */
                      { u64 id;       }   /* if PERF_SAMPLE_ID set */
                      { u64 stream_id;}   /* if PERF_SAMPLE_STREAM_ID set  */
                      { u32 cpu, res; }   /* if PERF_SAMPLE_CPU set */
                      { u64 id;       }   /* if PERF_SAMPLE_IDENTIFIER set */
                  };

       exclude_host (since Linux 3.2)
              When  conducting  measurements that include processes running VM
              instances (i.e., have executed a KVM_RUN ioctl(2)), only measure
              events happening inside a guest instance.  This is only meaning-
              ful outside the guests; this  setting  does  not  change  counts
              gathered  inside  of  a guest.  Currently, this functionality is
              x86 only.

       exclude_guest (since Linux 3.2)
              When conducting measurements that include processes  running  VM
              instances  (i.e., have executed a KVM_RUN ioctl(2)), do not mea-
              sure events happening inside  guest  instances.   This  is  only
              meaningful  outside  the  guests;  this  setting does not change
              counts gathered inside of a guest.  Currently, this  functional-
              ity is x86 only.

       exclude_callchain_kernel (since Linux 3.7)
              Do not include kernel callchains.

       exclude_callchain_user (since Linux 3.7)
              Do not include user callchains.

       mmap2 (since Linux 3.16)
              Generate an extended executable mmap record that contains enough
              additional information to  uniquely  identify  shared  mappings.
              The mmap flag must also be set for this to work.

       comm_exec (since Linux 3.16)
              This is purely a feature-detection flag, it does not change ker-
              nel behavior.  If this flag can successfully be set, then,  when
              comm is enabled, the PERF_RECORD_MISC_COMM_EXEC flag will be set
              in the misc field of a comm record header if  the  rename  event
              being  reported  was  caused  by a call to exec(2).  This allows
              tools to distinguish between the various types of process renam-
              ing.

       use_clockid (since Linux 4.1)
              This  allows  selecting  which  internal Linux clock to use when
              generating timestamps via the clockid field.  This can  make  it
              easier  to correlate perf sample times with timestamps generated
              by other tools.

       context_switch (since Linux 4.3)
              This enables the generation of PERF_RECORD_SWITCH records when a
              context  switch  occurs.   It  also  enables  the  generation of
              PERF_RECORD_SWITCH_CPU_WIDE records when  sampling  in  CPU-wide
              mode.   This functionality is in addition to existing tracepoint
              and software events for measuring context switches.  The  advan-
              tage  of  this method is that it will give full information even
              with strict perf_event_paranoid settings.

       wakeup_events, wakeup_watermark
              This union  sets  how  many  samples  (wakeup_events)  or  bytes
              (wakeup_watermark)  happen  before an overflow notification hap-
              pens.  Which one is used is selected by the watermark bit flag.

              wakeup_events counts only PERF_RECORD_SAMPLE record  types.   To
              receive  overflow  notification for all PERF_RECORD types choose
              watermark and set wakeup_watermark to 1.

              Prior to Linux 3.0, setting wakeup_events to 0  resulted  in  no
              overflow  notifications; more recent kernels treat 0 the same as
              1.

       bp_type (since Linux 2.6.33)
              This chooses the breakpoint type.  It is one of:

              HW_BREAKPOINT_EMPTY
                     No breakpoint.

              HW_BREAKPOINT_R
                     Count when we read the memory location.

              HW_BREAKPOINT_W
                     Count when we write the memory location.

              HW_BREAKPOINT_RW
                     Count when we read or write the memory location.

              HW_BREAKPOINT_X
                     Count when we execute code at the memory location.

              The values can be combined via a bitwise or, but the combination
              of  HW_BREAKPOINT_R  or  HW_BREAKPOINT_W with HW_BREAKPOINT_X is
              not allowed.

       bp_addr (since Linux 2.6.33)
              This is the address of the  breakpoint.   For  execution  break-
              points,  this is the memory address of the instruction of inter-
              est; for read and write breakpoints, it is the memory address of
              the memory location of interest.

       config1 (since Linux 2.6.39)
              config1  is  used for setting events that need an extra register
              or otherwise do not fit in the regular config field.   Raw  OFF-
              CORE_EVENTS  on  Nehalem/Westmere/SandyBridge  use this field on
              Linux 3.3 and later kernels.

       bp_len (since Linux 2.6.33)
              bp_len is the length of the breakpoint being measured if type is
              PERF_TYPE_BREAKPOINT.     Options    are    HW_BREAKPOINT_LEN_1,
              HW_BREAKPOINT_LEN_2,    HW_BREAKPOINT_LEN_4,    and    HW_BREAK-
              POINT_LEN_8.    For   an   execution  breakpoint,  set  this  to
              sizeof(long).

       config2 (since Linux 2.6.39)
              config2 is a further extension of the config1 field.

       branch_sample_type (since Linux 3.4)
              If PERF_SAMPLE_BRANCH_STACK is enabled, then this specifies what
              branches to include in the branch record.

              The  first  part of the value is the privilege level, which is a
              combination of one of the values listed below.  If the user does
              not  set  privilege  level  explicitly,  the kernel will use the
              event's privilege level.  Event and branch privilege  levels  do
              not have to match.

              PERF_SAMPLE_BRANCH_USER
                     Branch target is in user space.

              PERF_SAMPLE_BRANCH_KERNEL
                     Branch target is in kernel space.

              PERF_SAMPLE_BRANCH_HV
                     Branch target is in hypervisor.

              PERF_SAMPLE_BRANCH_PLM_ALL
                     A  convenience  value  that is the three preceding values
                     ORed together.

              In addition to the privilege value, at least one or more of  the
              following bits must be set.

              PERF_SAMPLE_BRANCH_ANY
                     Any branch type.

              PERF_SAMPLE_BRANCH_ANY_CALL
                     Any  call  branch (includes direct calls, indirect calls,
                     and far jumps).

              PERF_SAMPLE_BRANCH_IND_CALL
                     Indirect calls.

              PERF_SAMPLE_BRANCH_CALL (since Linux 4.4)
                     Direct calls.

              PERF_SAMPLE_BRANCH_ANY_RETURN
                     Any return branch.

              PERF_SAMPLE_BRANCH_IND_JUMP (since Linux 4.2)
                     Indirect jumps.

              PERF_SAMPLE_BRANCH_COND (since Linux 3.16)
                     Conditional branches.

              PERF_SAMPLE_BRANCH_ABORT_TX (since Linux 3.11)
                     Transactional memory aborts.

              PERF_SAMPLE_BRANCH_IN_TX (since Linux 3.11)
                     Branch in transactional memory transaction.

              PERF_SAMPLE_BRANCH_NO_TX (since Linux 3.11)
                     Branch   not   in   transactional   memory   transaction.
                     PERF_SAMPLE_BRANCH_CALL_STACK (since Linux 4.1) Branch is
                     part of a hardware-generated call stack.   This  requires
                     hardware  support,  currently  only  found  on  Intel x86
                     Haswell or newer.

       sample_regs_user (since Linux 3.7)
              This bit mask defines the set of user CPU registers to  dump  on
              samples.   The  layout of the register mask is architecture-spe-
              cific and is described in the kernel header  file  arch/ARCH/in-
              clude/uapi/asm/perf_regs.h.

       sample_stack_user (since Linux 3.7)
              This  defines  the  size  of the user stack to dump if PERF_SAM-
              PLE_STACK_USER is specified.

       clockid (since Linux 4.1)
              If use_clockid is set, then this field  selects  which  internal
              Linux timer to use for timestamps.  The available timers are de-
              fined  in  linux/time.h,   with   CLOCK_MONOTONIC,   CLOCK_MONO-
              TONIC_RAW,  CLOCK_REALTIME,  CLOCK_BOOTTIME,  and CLOCK_TAI cur-
              rently supported.

       aux_watermark (since Linux 4.1)
              This  specifies  how  much  data  is  required  to   trigger   a
              PERF_RECORD_AUX sample.

       sample_max_stack (since Linux 4.8)
              When  sample_type  includes  PERF_SAMPLE_CALLCHAIN,  this  field
              specifies how many stack frames to report  when  generating  the
              callchain.

   Reading results
       Once a perf_event_open() file descriptor has been opened, the values of
       the events can be read from the file descriptor.  The values  that  are
       there  are  specified by the read_format field in the attr structure at
       open time.

       If you attempt to read into a buffer that is not big enough to hold the
       data, the error ENOSPC results.

       Here is the layout of the data returned by a read:

       * If  PERF_FORMAT_GROUP  was specified to allow reading all events in a
         group at once:

             struct read_format {
                 u64 nr;            /* The number of events */
                 u64 time_enabled;  /* if PERF_FORMAT_TOTAL_TIME_ENABLED */
                 u64 time_running;  /* if PERF_FORMAT_TOTAL_TIME_RUNNING */
                 struct {
                     u64 value;     /* The value of the event */
                     u64 id;        /* if PERF_FORMAT_ID */
                 } values[nr];
             };

       * If PERF_FORMAT_GROUP was not specified:

             struct read_format {
                 u64 value;         /* The value of the event */
                 u64 time_enabled;  /* if PERF_FORMAT_TOTAL_TIME_ENABLED */
                 u64 time_running;  /* if PERF_FORMAT_TOTAL_TIME_RUNNING */
                 u64 id;            /* if PERF_FORMAT_ID */
             };

       The values read are as follows:

       nr     The number of events in this file descriptor.  Available only if
              PERF_FORMAT_GROUP was specified.

       time_enabled, time_running
              Total  time  the  event was enabled and running.  Normally these
              values are the same.  Multiplexing  happens  if  the  number  of
              events  is  more than the number of available PMU counter slots.
              In that case the events run  only  part  of  the  time  and  the
              time_enabled and time running values can be used to scale an es-
              timated value for the count.

       value  An unsigned 64-bit value containing the counter result.

       id     A globally unique value for this particular event; only  present
              if PERF_FORMAT_ID was specified in read_format.

   MMAP layout
       When using perf_event_open() in sampled mode, asynchronous events (like
       counter overflow or PROT_EXEC mmap tracking) are logged  into  a  ring-
       buffer.  This ring-buffer is created and accessed through mmap(2).

       The mmap size should be 1+2^n pages, where the first page is a metadata
       page (struct perf_event_mmap_page) that contains various bits of infor-
       mation such as where the ring-buffer head is.

       Before  kernel  2.6.39,  there is a bug that means you must allocate an
       mmap ring buffer when sampling even if you do not plan to access it.

       The structure of the first metadata mmap page is as follows:

           struct perf_event_mmap_page {
               __u32 version;        /* version number of this structure */
               __u32 compat_version; /* lowest version this is compat with */
               __u32 lock;           /* seqlock for synchronization */
               __u32 index;          /* hardware counter identifier */
               __s64 offset;         /* add to hardware counter value */
               __u64 time_enabled;   /* time event active */
               __u64 time_running;   /* time event on CPU */
               union {
                   __u64   capabilities;
                   struct {
                       __u64 cap_usr_time / cap_usr_rdpmc / cap_bit0 : 1,
                             cap_bit0_is_deprecated : 1,
                             cap_user_rdpmc         : 1,
                             cap_user_time          : 1,
                             cap_user_time_zero     : 1,
                   };
               };
               __u16 pmc_width;
               __u16 time_shift;
               __u32 time_mult;
               __u64 time_offset;
               __u64 __reserved[120];   /* Pad to 1 k */
               __u64 data_head;         /* head in the data section */
               __u64 data_tail;         /* user-space written tail */
               __u64 data_offset;       /* where the buffer starts */
               __u64 data_size;         /* data buffer size */
               __u64 aux_head;
               __u64 aux_tail;
               __u64 aux_offset;
               __u64 aux_size;

           }

       The following list describes the  fields  in  the  perf_event_mmap_page
       structure in more detail:

       version
              Version number of this structure.

       compat_version
              The lowest version this is compatible with.

       lock   A seqlock for synchronization.

       index  A unique hardware counter identifier.

       offset When  using  rdpmc  for reads this offset value must be added to
              the one returned by rdpmc to get the current total event count.

       time_enabled
              Time the event was active.

       time_running
              Time the event was running.

       cap_usr_time / cap_usr_rdpmc / cap_bit0 (since Linux 3.4)
              There  was  a  bug  in  the  definition  of   cap_usr_time   and
              cap_usr_rdpmc  from  Linux 3.4 until Linux 3.11.  Both bits were
              defined to point to the same location, so it was  impossible  to
              know if cap_usr_time or cap_usr_rdpmc were actually set.

              Starting  with Linux 3.12, these are renamed to cap_bit0 and you
              should use the cap_user_time and cap_user_rdpmc fields instead.

       cap_bit0_is_deprecated (since Linux 3.12)
              If set, this bit indicates that the kernel supports the properly
              separated cap_user_time and cap_user_rdpmc bits.

              If  not-set, it indicates an older kernel where cap_usr_time and
              cap_usr_rdpmc map to the same bit and thus both features  should
              be used with caution.

       cap_user_rdpmc (since Linux 3.12)
              If the hardware supports user-space read of performance counters
              without syscall (this is the "rdpmc" instruction on  x86),  then
              the following code can be used to do a read:

                  u32 seq, time_mult, time_shift, idx, width;
                  u64 count, enabled, running;
                  u64 cyc, time_offset;

                  do {
                      seq = pc->lock;
                      barrier();
                      enabled = pc->time_enabled;
                      running = pc->time_running;

                      if (pc->cap_usr_time && enabled != running) {
                          cyc = rdtsc();
                          time_offset = pc->time_offset;
                          time_mult   = pc->time_mult;
                          time_shift  = pc->time_shift;
                      }

                      idx = pc->index;
                      count = pc->offset;

                      if (pc->cap_usr_rdpmc && idx) {
                          width = pc->pmc_width;
                          count += rdpmc(idx - 1);
                      }

                      barrier();
                  } while (pc->lock != seq);

       cap_user_time (since Linux 3.12)
              This  bit  indicates  the hardware has a constant, nonstop time-
              stamp counter (TSC on x86).

       cap_user_time_zero (since Linux 3.12)
              Indicates the presence of time_zero which allows  mapping  time-
              stamp values to the hardware clock.

       pmc_width
              If cap_usr_rdpmc, this field provides the bit-width of the value
              read using the rdpmc or equivalent  instruction.   This  can  be
              used to sign extend the result like:

                  pmc <<= 64 - pmc_width;
                  pmc >>= 64 - pmc_width; // signed shift right
                  count += pmc;

       time_shift, time_mult, time_offset

              If  cap_usr_time,  these  fields can be used to compute the time
              delta since time_enabled (in nanoseconds) using rdtsc  or  simi-
              lar.

                  u64 quot, rem;
                  u64 delta;
                  quot = (cyc >> time_shift);
                  rem = cyc & (((u64)1 << time_shift) - 1);
                  delta = time_offset + quot * time_mult +
                          ((rem * time_mult) >> time_shift);

              Where  time_offset,  time_mult,  time_shift, and cyc are read in
              the seqcount loop described above.  This delta can then be added
              to enabled and possible running (if idx), improving the scaling:

                  enabled += delta;
                  if (idx)
                      running += delta;
                  quot = count / running;
                  rem  = count % running;
                  count = quot * enabled + (rem * enabled) / running;

       time_zero (since Linux 3.12)

              If  cap_usr_time_zero  is  set, then the hardware clock (the TSC
              timestamp counter on x86) can be calculated from the  time_zero,
              time_mult, and time_shift values:

                  time = timestamp - time_zero;
                  quot = time / time_mult;
                  rem  = time % time_mult;
                  cyc = (quot << time_shift) + (rem << time_shift) / time_mult;

              And vice versa:

                  quot = cyc >> time_shift;
                  rem  = cyc & (((u64)1 << time_shift) - 1);
                  timestamp = time_zero + quot * time_mult +
                      ((rem * time_mult) >> time_shift);

       data_head
              This points to the head of the data section.  The value continu-
              ously increases, it does not wrap.  The value needs to be  manu-
              ally wrapped by the size of the mmap buffer before accessing the
              samples.

              On SMP-capable platforms, after  reading  the  data_head  value,
              user space should issue an rmb().

       data_tail
              When  the  mapping  is PROT_WRITE, the data_tail value should be
              written by user space to reflect the last read  data.   In  this
              case, the kernel will not overwrite unread data.

       data_offset (since Linux 4.1)
              Contains  the  offset  of  the location in the mmap buffer where
              perf sample data begins.

       data_size (since Linux 4.1)
              Contains the size of the perf sample region within the mmap buf-
              fer.

       aux_head, aux_tail, aux_offset, aux_size (since Linux 4.1)
              The AUX region allows mmaping a separate sample buffer for high-
              bandwidth data streams (separate from the main perf sample  buf-
              fer).   An  example  of  a  high-bandwidth stream is instruction
              tracing support, as is found in newer Intel processors.

              To set up an AUX area, first aux_offset needs to be set with  an
              offset  greater than data_offset+data_size and aux_size needs to
              be set to the desired buffer size.  The desired offset and  size
              must  be  page  aligned,  and  the  size must be a power of two.
              These values are then passed to mmap in order  to  map  the  AUX
              buffer.   Pages  in  the  AUX buffer are included as part of the
              RLIMIT_MEMLOCK resource limit (see setrlimit(2)),  and  also  as
              part of the perf_event_mlock_kb allowance.

              By  default, the AUX buffer will be truncated if it will not fit
              in the available space in the ring buffer.  If the AUX buffer is
              mapped  as a read only buffer, then it will operate in ring buf-
              fer mode where old data will be overwritten by  new.   In  over-
              write mode, it might not be possible to infer where the new data
              began, and it is the consumer's job to disable measurement while
              reading to avoid possible data races.

              The aux_head and aux_tail ring buffer pointers have the same be-
              havior and ordering rules as the  previous  described  data_head
              and data_tail.

       The following 2^n ring-buffer pages have the layout described below.

       If perf_event_attr.sample_id_all is set, then all event types will have
       the sample_type selected fields related  to  where/when  (identity)  an
       event   took  place  (TID,  TIME,  ID,  CPU,  STREAM_ID)  described  in
       PERF_RECORD_SAMPLE  below,  it  will  be   stashed   just   after   the
       perf_event_header  and  the  fields  already  present  for the existing
       fields, that is, at the end  of  the  payload.   This  allows  a  newer
       perf.data  file  to  be supported by older perf tools, with the new op-
       tional fields being ignored.

       The mmap values start with a header:

           struct perf_event_header {
               __u32   type;
               __u16   misc;
               __u16   size;
           };

       Below, we describe the perf_event_header fields in  more  detail.   For
       ease  of  reading,  the  fields with shorter descriptions are presented
       first.

       size   This indicates the size of the record.

       misc   The misc field contains additional information about the sample.

              The CPU mode can be determined from this value by  masking  with
              PERF_RECORD_MISC_CPUMODE_MASK and looking for one of the follow-
              ing (note these are not bit masks, only one  can  be  set  at  a
              time):

              PERF_RECORD_MISC_CPUMODE_UNKNOWN
                     Unknown CPU mode.

              PERF_RECORD_MISC_KERNEL
                     Sample happened in the kernel.

              PERF_RECORD_MISC_USER
                     Sample happened in user code.

              PERF_RECORD_MISC_HYPERVISOR
                     Sample happened in the hypervisor.

              PERF_RECORD_MISC_GUEST_KERNEL (since Linux 2.6.35)
                     Sample happened in the guest kernel.

              PERF_RECORD_MISC_GUEST_USER  (since Linux 2.6.35)
                     Sample happened in guest user code.

              Since  the  following  three statuses are generated by different
              record types, they alias to the same bit:

              PERF_RECORD_MISC_MMAP_DATA (since Linux 3.10)
                     This is set when the mapping is not executable; otherwise
                     the mapping is executable.

              PERF_RECORD_MISC_COMM_EXEC (since Linux 3.16)
                     This is set for a PERF_RECORD_COMM record on kernels more
                     recent than Linux 3.16  if  a  process  name  change  was
                     caused by an exec(2) system call.

              PERF_RECORD_MISC_SWITCH_OUT (since Linux 4.3)
                     When  a PERF_RECORD_SWITCH or PERF_RECORD_SWITCH_CPU_WIDE
                     record is generated, this bit indicates that the  context
                     switch  is away from the current process (instead of into
                     the current process).

              In addition, the following bits can be set:

              PERF_RECORD_MISC_EXACT_IP
                     This indicates that the content of PERF_SAMPLE_IP  points
                     to  the actual instruction that triggered the event.  See
                     also perf_event_attr.precise_ip.

              PERF_RECORD_MISC_EXT_RESERVED (since Linux 2.6.35)
                     This indicates there is  extended  data  available  (cur-
                     rently not used).

              PERF_RECORD_MISC_PROC_MAP_PARSE_TIMEOUT
                     This  bit  is  not set by the kernel.  It is reserved for
                     the   user-space   perf   utility   to   indicate    that
                     /proc/i[pid]/maps  parsing  was  taking  too long and was
                     stopped, and thus the mmap records may be truncated.

       type   The type value is one of the below.  The values  in  the  corre-
              sponding record (that follows the header) depend on the type se-
              lected as shown.

              PERF_RECORD_MMAP
                  The MMAP events record the PROT_EXEC mappings so that we can
                  correlate  user-space  IPs to code.  They have the following
                  structure:

                      struct {
                          struct perf_event_header header;
                          u32    pid, tid;
                          u64    addr;
                          u64    len;
                          u64    pgoff;
                          char   filename[];
                      };

                  pid    is the process ID.

                  tid    is the thread ID.

                  addr   is the address of the allocated memory.  len  is  the
                         length  of  the  allocated memory.  pgoff is the page
                         offset of the allocated memory.  filename is a string
                         describing the backing of the allocated memory.

              PERF_RECORD_LOST
                  This record indicates when events are lost.

                      struct {
                          struct perf_event_header header;
                          u64    id;
                          u64    lost;
                          struct sample_id sample_id;
                      };

                  id     is  the  unique  event  ID  for the samples that were
                         lost.

                  lost   is the number of events that were lost.

              PERF_RECORD_COMM
                  This record indicates a change in the process name.

                      struct {
                          struct perf_event_header header;
                          u32    pid;
                          u32    tid;
                          char   comm[];
                          struct sample_id sample_id;
                      };

                  pid    is the process ID.

                  tid    is the thread ID.

                  comm   is a string containing the new name of the process.

              PERF_RECORD_EXIT
                  This record indicates a process exit event.

                      struct {
                          struct perf_event_header header;
                          u32    pid, ppid;
                          u32    tid, ptid;
                          u64    time;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_THROTTLE, PERF_RECORD_UNTHROTTLE
                  This record indicates a throttle/unthrottle event.

                      struct {
                          struct perf_event_header header;
                          u64    time;
                          u64    id;
                          u64    stream_id;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_FORK
                  This record indicates a fork event.

                      struct {
                          struct perf_event_header header;
                          u32    pid, ppid;
                          u32    tid, ptid;
                          u64    time;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_READ
                  This record indicates a read event.

                      struct {
                          struct perf_event_header header;
                          u32    pid, tid;
                          struct read_format values;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_SAMPLE
                  This record indicates a sample.

                      struct {
                          struct perf_event_header header;
                          u64    sample_id;   /* if PERF_SAMPLE_IDENTIFIER */
                          u64    ip;          /* if PERF_SAMPLE_IP */
                          u32    pid, tid;    /* if PERF_SAMPLE_TID */
                          u64    time;        /* if PERF_SAMPLE_TIME */
                          u64    addr;        /* if PERF_SAMPLE_ADDR */
                          u64    id;          /* if PERF_SAMPLE_ID */
                          u64    stream_id;   /* if PERF_SAMPLE_STREAM_ID */
                          u32    cpu, res;    /* if PERF_SAMPLE_CPU */
                          u64    period;      /* if PERF_SAMPLE_PERIOD */
                          struct read_format v;
                                              /* if PERF_SAMPLE_READ */
                          u64    nr;          /* if PERF_SAMPLE_CALLCHAIN */
                          u64    ips[nr];     /* if PERF_SAMPLE_CALLCHAIN */
                          u32    size;        /* if PERF_SAMPLE_RAW */
                          char  data[size];   /* if PERF_SAMPLE_RAW */
                          u64    bnr;         /* if PERF_SAMPLE_BRANCH_STACK */
                          struct perf_branch_entry lbr[bnr];
                                              /* if PERF_SAMPLE_BRANCH_STACK */
                          u64    abi;         /* if PERF_SAMPLE_REGS_USER */
                          u64    regs[weight(mask)];
                                              /* if PERF_SAMPLE_REGS_USER */
                          u64    size;        /* if PERF_SAMPLE_STACK_USER */
                          char   data[size];  /* if PERF_SAMPLE_STACK_USER */
                          u64    dyn_size;    /* if PERF_SAMPLE_STACK_USER &&
                                                 size != 0 */
                          u64    weight;      /* if PERF_SAMPLE_WEIGHT */
                          u64    data_src;    /* if PERF_SAMPLE_DATA_SRC */
                          u64    transaction; /* if PERF_SAMPLE_TRANSACTION */
                          u64    abi;         /* if PERF_SAMPLE_REGS_INTR */
                          u64    regs[weight(mask)];
                                              /* if PERF_SAMPLE_REGS_INTR */
                      };

                  sample_id
                      If PERF_SAMPLE_IDENTIFIER is enabled, a 64-bit unique ID
                      is  included.   This  is  a duplication of the PERF_SAM-
                      PLE_ID id value, but included at the  beginning  of  the
                      sample so parsers can easily obtain the value.

                  ip  If  PERF_SAMPLE_IP is enabled, then a 64-bit instruction
                      pointer value is included.

                  pid, tid
                      If PERF_SAMPLE_TID is enabled, then a 32-bit process  ID
                      and 32-bit thread ID are included.

                  time
                      If  PERF_SAMPLE_TIME is enabled, then a 64-bit timestamp
                      is included.  This is obtained via  local_clock()  which
                      is  a  hardware  timestamp  if available and the jiffies
                      value if not.

                  addr
                      If PERF_SAMPLE_ADDR is enabled, then a 64-bit address is
                      included.   This is usually the address of a tracepoint,
                      breakpoint, or software event; otherwise the value is 0.

                  id  If PERF_SAMPLE_ID is enabled, a 64-bit unique ID is  in-
                      cluded.  If the event is a member of an event group, the
                      group leader ID is returned.  This ID is the same as the
                      one returned by PERF_FORMAT_ID.

                  stream_id
                      If  PERF_SAMPLE_STREAM_ID is enabled, a 64-bit unique ID
                      is included.  Unlike PERF_SAMPLE_ID the actual ID is re-
                      turned,  not  the  group leader.  This ID is the same as
                      the one returned by PERF_FORMAT_ID.

                  cpu, res
                      If PERF_SAMPLE_CPU is enabled, this is  a  32-bit  value
                      indicating  which  CPU  was being used, in addition to a
                      reserved (unused) 32-bit value.

                  period
                      If PERF_SAMPLE_PERIOD is enabled, a 64-bit  value  indi-
                      cating the current sampling period is written.

                  v   If  PERF_SAMPLE_READ  is  enabled,  a  structure of type
                      read_format is included which has values for all  events
                      in  the  event group.  The values included depend on the
                      read_format value used at perf_event_open() time.

                  nr, ips[nr]
                      If PERF_SAMPLE_CALLCHAIN is enabled, then a 64-bit  num-
                      ber  is  included  which  indicates  how  many following
                      64-bit instruction pointers will follow.   This  is  the
                      current callchain.

                  size, data[size]
                      If PERF_SAMPLE_RAW is enabled, then a 32-bit value indi-
                      cating size is included followed by an  array  of  8-bit
                      values  of length size.  The values are padded with 0 to
                      have 64-bit alignment.

                      This RAW record data is opaque with respect to the  ABI.
                      The  ABI  doesn't  make any promises with respect to the
                      stability of its  content,  it  may  vary  depending  on
                      event, hardware, and kernel version.

                  bnr, lbr[bnr]
                      If  PERF_SAMPLE_BRANCH_STACK  is  enabled, then a 64-bit
                      value indicating the number of records is included, fol-
                      lowed by bnr perf_branch_entry structures which each in-
                      clude the fields:

                      from   This indicates the source instruction (may not be
                             a branch).

                      to     The branch target.

                      mispred
                             The branch target was mispredicted.

                      predicted
                             The branch target was predicted.

                      in_tx (since Linux 3.11)
                             The branch was in a transactional memory transac-
                             tion.

                      abort (since Linux 3.11)
                             The branch was in an aborted transactional memory
                             transaction.

                      cycles (since Linux 4.3)
                             This  reports  the number of cycles elapsed since
                             the previous branch stack update.

                      The entries are from most to least recent, so the  first
                      entry has the most recent branch.

                      Support  for mispred, predicted, and cycles is optional;
                      if not supported, those values will be 0.

                      The type  of  branches  recorded  is  specified  by  the
                      branch_sample_type field.

                  abi, regs[weight(mask)]
                      If  PERF_SAMPLE_REGS_USER  is enabled, then the user CPU
                      registers are recorded.

                      The  abi  field  is  one  of  PERF_SAMPLE_REGS_ABI_NONE,
                      PERF_SAMPLE_REGS_ABI_32 or PERF_SAMPLE_REGS_ABI_64.

                      The  regs  field  is  an array of the CPU registers that
                      were specified by the sample_regs_user attr field.   The
                      number  of  values is the number of bits set in the sam-
                      ple_regs_user bit mask.

                  size, data[size], dyn_size
                      If PERF_SAMPLE_STACK_USER  is  enabled,  then  the  user
                      stack  is  recorded.  This can be used to generate stack
                      backtraces.  size is the size requested by the  user  in
                      sample_stack_user or else the maximum record size.  data
                      is the stack data (a raw dump of the memory  pointed  to
                      by the stack pointer at the time of sampling).  dyn_size
                      is the amount of data actually dumped (can be less  than
                      size).  Note that dyn_size is omitted if size is 0.

                  weight
                      If  PERF_SAMPLE_WEIGHT  is  enabled, then a 64-bit value
                      provided by the hardware is recorded that indicates  how
                      costly  the  event was.  This allows expensive events to
                      stand out more clearly in profiles.

                  data_src
                      If PERF_SAMPLE_DATA_SRC is enabled, then a 64-bit  value
                      is recorded that is made up of the following fields:

                      mem_op
                          Type of opcode, a bitwise combination of:

                          PERF_MEM_OP_NA          Not available
                          PERF_MEM_OP_LOAD        Load instruction
                          PERF_MEM_OP_STORE       Store instruction
                          PERF_MEM_OP_PFETCH      Prefetch
                          PERF_MEM_OP_EXEC        Executable code

                      mem_lvl
                          Memory hierarchy level hit or miss, a bitwise combi-
                          nation   of   the   following,   shifted   left   by
                          PERF_MEM_LVL_SHIFT:

                          PERF_MEM_LVL_NA         Not available
                          PERF_MEM_LVL_HIT        Hit
                          PERF_MEM_LVL_MISS       Miss
                          PERF_MEM_LVL_L1         Level 1 cache
                          PERF_MEM_LVL_LFB        Line fill buffer
                          PERF_MEM_LVL_L2         Level 2 cache
                          PERF_MEM_LVL_L3         Level 3 cache
                          PERF_MEM_LVL_LOC_RAM    Local DRAM
                          PERF_MEM_LVL_REM_RAM1   Remote DRAM 1 hop
                          PERF_MEM_LVL_REM_RAM2   Remote DRAM 2 hops
                          PERF_MEM_LVL_REM_CCE1   Remote cache 1 hop
                          PERF_MEM_LVL_REM_CCE2   Remote cache 2 hops
                          PERF_MEM_LVL_IO         I/O memory
                          PERF_MEM_LVL_UNC        Uncached memory

                      mem_snoop
                          Snoop  mode, a bitwise combination of the following,
                          shifted left by PERF_MEM_SNOOP_SHIFT:

                          PERF_MEM_SNOOP_NA       Not available
                          PERF_MEM_SNOOP_NONE     No snoop
                          PERF_MEM_SNOOP_HIT      Snoop hit
                          PERF_MEM_SNOOP_MISS     Snoop miss
                          PERF_MEM_SNOOP_HITM     Snoop hit modified

                      mem_lock
                          Lock instruction, a bitwise combination of the  fol-
                          lowing, shifted left by PERF_MEM_LOCK_SHIFT:

                          PERF_MEM_LOCK_NA        Not available
                          PERF_MEM_LOCK_LOCKED    Locked transaction

                      mem_dtlb
                          TLB access hit or miss, a bitwise combination of the
                          following, shifted left by PERF_MEM_TLB_SHIFT:

                          PERF_MEM_TLB_NA         Not available
                          PERF_MEM_TLB_HIT        Hit
                          PERF_MEM_TLB_MISS       Miss
                          PERF_MEM_TLB_L1         Level 1 TLB
                          PERF_MEM_TLB_L2         Level 2 TLB
                          PERF_MEM_TLB_WK         Hardware walker
                          PERF_MEM_TLB_OS         OS fault handler

                  transaction
                      If the  PERF_SAMPLE_TRANSACTION  flag  is  set,  then  a
                      64-bit  field  is recorded describing the sources of any
                      transactional memory aborts.

                      The field is a bitwise combination of the following val-
                      ues:

                      PERF_TXN_ELISION
                             Abort  from  an  elision type transaction (Intel-
                             CPU-specific).

                      PERF_TXN_TRANSACTION
                             Abort from a generic transaction.

                      PERF_TXN_SYNC
                             Synchronous abort (related to  the  reported  in-
                             struction).

                      PERF_TXN_ASYNC
                             Asynchronous  abort  (not related to the reported
                             instruction).

                      PERF_TXN_RETRY
                             Retryable abort  (retrying  the  transaction  may
                             have succeeded).

                      PERF_TXN_CONFLICT
                             Abort due to memory conflicts with other threads.

                      PERF_TXN_CAPACITY_WRITE
                             Abort due to write capacity overflow.

                      PERF_TXN_CAPACITY_READ
                             Abort due to read capacity overflow.

                      In addition, a user-specified abort code can be obtained
                      from the high 32 bits of the field by shifting right  by
                      PERF_TXN_ABORT_SHIFT   and   masking   with   the  value
                      PERF_TXN_ABORT_MASK.

                  abi, regs[weight(mask)]
                      If PERF_SAMPLE_REGS_INTR is enabled, then the  user  CPU
                      registers are recorded.

                      The  abi  field  is  one  of  PERF_SAMPLE_REGS_ABI_NONE,
                      PERF_SAMPLE_REGS_ABI_32, or PERF_SAMPLE_REGS_ABI_64.

                      The regs field is an array of  the  CPU  registers  that
                      were  specified by the sample_regs_intr attr field.  The
                      number of values is the number of bits set in  the  sam-
                      ple_regs_intr bit mask.

              PERF_RECORD_MMAP2
                  This  record  includes extended information on mmap(2) calls
                  returning executable mappings.  The  format  is  similar  to
                  that of the PERF_RECORD_MMAP record, but includes extra val-
                  ues that allow uniquely identifying shared mappings.

                      struct {
                          struct perf_event_header header;
                          u32    pid;
                          u32    tid;
                          u64    addr;
                          u64    len;
                          u64    pgoff;
                          u32    maj;
                          u32    min;
                          u64    ino;
                          u64    ino_generation;
                          u32    prot;
                          u32    flags;
                          char   filename[];
                          struct sample_id sample_id;
                      };

                  pid    is the process ID.

                  tid    is the thread ID.

                  addr   is the address of the allocated memory.

                  len    is the length of the allocated memory.

                  pgoff  is the page offset of the allocated memory.

                  maj    is the major ID of the underlying device.

                  min    is the minor ID of the underlying device.

                  ino    is the inode number.

                  ino_generation
                         is the inode generation.

                  prot   is the protection information.

                  flags  is the flags information.

                  filename
                         is a string describing the backing of  the  allocated
                         memory.

              PERF_RECORD_AUX (since Linux 4.1)

                  This  record reports that new data is available in the sepa-
                  rate AUX buffer region.

                      struct {
                          struct perf_event_header header;
                          u64    aux_offset;
                          u64    aux_size;
                          u64    flags;
                          struct sample_id sample_id;
                      };

                  aux_offset
                         offset in the AUX mmap region where the new data  be-
                         gins.

                  aux_size
                         size of the data made available.

                  flags  describes the AUX update.

                         PERF_AUX_FLAG_TRUNCATED
                                if  set,  then the data returned was truncated
                                to fit the available buffer size.

                         PERF_AUX_FLAG_OVERWRITE
                                if set, then the data returned has overwritten
                                previous data.

              PERF_RECORD_ITRACE_START (since Linux 4.1)

                  This  record  indicates  which  process has initiated an in-
                  struction trace event, allowing tools to properly  correlate
                  the  instruction addresses in the AUX buffer with the proper
                  executable.

                      struct {
                          struct perf_event_header header;
                          u32    pid;
                          u32    tid;
                      };

                  pid    process ID of  the  thread  starting  an  instruction
                         trace.

                  tid    thread  ID  of  the  thread  starting  an instruction
                         trace.

              PERF_RECORD_LOST_SAMPLES (since Linux 4.2)

                  When using hardware  sampling  (such  as  Intel  PEBS)  this
                  record  indicates  some number of samples that may have been
                  lost.

                      struct {
                          struct perf_event_header header;
                          u64    lost;
                          struct sample_id sample_id;
                      };

                  lost   the number of potentially lost samples.

              PERF_RECORD_SWITCH (since Linux 4.3)

                  This record indicates a context switch  has  happened.   The
                  PERF_RECORD_MISC_SWITCH_OUT  bit in the misc field indicates
                  whether it was a context switch into or away from  the  cur-
                  rent process.

                      struct {
                          struct perf_event_header header;
                          struct sample_id sample_id;
                      };

              PERF_RECORD_SWITCH_CPU_WIDE (since Linux 4.3)

                  As  with  PERF_RECORD_SWITCH this record indicates a context
                  switch has happened, but it only  occurs  when  sampling  in
                  CPU-wide  mode  and  provides  additional information on the
                  process       being       switched       to/from.        The
                  PERF_RECORD_MISC_SWITCH_OUT  bit in the misc field indicates
                  whether it was a context switch into or away from  the  cur-
                  rent process.

                      struct {
                          struct perf_event_header header;
                          u32 next_prev_pid;
                          u32 next_prev_tid;
                          struct sample_id sample_id;
                      };

                  next_prev_pid
                         The  process  ID of the previous (if switching in) or
                         next (if switching out) process on the CPU.

                  next_prev_tid
                         The thread ID of the previous (if  switching  in)  or
                         next (if switching out) thread on the CPU.

   Overflow handling
       Events  can be set to notify when a threshold is crossed, indicating an
       overflow.  Overflow conditions can be captured by monitoring the  event
       file  descriptor  with poll(2), select(2), or epoll(7).  Alternatively,
       the overflow events can be captured via sa signal handler, by  enabling
       I/O  signaling  on the file descriptor; see the discussion of the F_SE-
       TOWN and F_SETSIG operations in fcntl(2).

       Overflows are generated only by  sampling  events  (sample_period  must
       have a nonzero value).

       There are two ways to generate overflow notifications.

       The first is to set a wakeup_events or wakeup_watermark value that will
       trigger if a certain number of samples or bytes have  been  written  to
       the mmap ring buffer.  In this case, POLL_IN is indicated.

       The  other  way  is  by  use of the PERF_EVENT_IOC_REFRESH ioctl.  This
       ioctl adds to a counter that decrements each time the event  overflows.
       When  nonzero,  POLL_IN  is  indicated,  but once the counter reaches 0
       POLL_HUP is indicated and the underlying event is disabled.

       Refreshing an event group leader refreshes all siblings and  refreshing
       with  a  parameter of 0 currently enables infinite refreshes; these be-
       haviors are unsupported and should not be relied on.

       Starting with Linux 3.18, POLL_HUP is indicated if the event being mon-
       itored is attached to a different process and that process exits.

   rdpmc instruction
       Starting  with  Linux  3.4 on x86, you can use the rdpmc instruction to
       get low-latency reads without having to enter the  kernel.   Note  that
       using  rdpmc  is  not necessarily faster than other methods for reading
       event values.

       Support for this can be detected with the cap_usr_rdpmc  field  in  the
       mmap  page; documentation on how to calculate event values can be found
       in that section.

       Originally, when rdpmc support was enabled, any process (not just  ones
       with  an  active  perf event) could use the rdpmc instruction to access
       the counters.  Starting with Linux 4.0, rdpmc support is  only  allowed
       if  an  event  is currently enabled in a process's context.  To restore
       the old behavior, write the value 2 to /sys/devices/cpu/rdpmc.

   perf_event ioctl calls
       Various ioctls act on perf_event_open() file descriptors:

       PERF_EVENT_IOC_ENABLE
              This enables the individual event or event  group  specified  by
              the file descriptor argument.

              If  the  PERF_IOC_FLAG_GROUP  bit  is set in the ioctl argument,
              then all events in a group are enabled, even if the event speci-
              fied is not the group leader (but see BUGS).

       PERF_EVENT_IOC_DISABLE
              This disables the individual counter or event group specified by
              the file descriptor argument.

              Enabling or disabling the leader of a group enables or  disables
              the  entire  group; that is, while the group leader is disabled,
              none of the counters in the group will count.  Enabling or  dis-
              abling  a  member  of a group other than the leader affects only
              that counter; disabling a non-leader  stops  that  counter  from
              counting but doesn't affect any other counter.

              If  the  PERF_IOC_FLAG_GROUP  bit  is set in the ioctl argument,
              then all events in a group are disabled, even if the event spec-
              ified is not the group leader (but see BUGS).

       PERF_EVENT_IOC_REFRESH
              Non-inherited overflow counters can use this to enable a counter
              for a number of overflows specified by the argument, after which
              it is disabled.  Subsequent calls of this ioctl add the argument
              value to the  current  count.   An  overflow  notification  with
              POLL_IN set will happen on each overflow until the count reaches
              0; when that happens a notification with POLL_HUP  set  is  sent
              and the event is disabled.  Using an argument of 0 is considered
              undefined behavior.

       PERF_EVENT_IOC_RESET
              Reset the event count specified by the file descriptor  argument
              to  zero.  This resets only the counts; there is no way to reset
              the multiplexing time_enabled or time_running values.

              If the PERF_IOC_FLAG_GROUP bit is set  in  the  ioctl  argument,
              then  all  events in a group are reset, even if the event speci-
              fied is not the group leader (but see BUGS).

       PERF_EVENT_IOC_PERIOD
              This updates the overflow period for the event.

              Since Linux 3.7 (on ARM) and Linux  3.14  (all  other  architec-
              tures),  the new period takes effect immediately.  On older ker-
              nels, the new period did not take effect until  after  the  next
              overflow.

              The  argument  is a pointer to a 64-bit value containing the de-
              sired new period.

              Prior to Linux 2.6.36, this ioctl always failed due to a bug  in
              the kernel.

       PERF_EVENT_IOC_SET_OUTPUT
              This tells the kernel to report event notifications to the spec-
              ified file descriptor rather than the default one.  The file de-
              scriptors must all be on the same CPU.

              The  argument  specifies  the  desired file descriptor, or -1 if
              output should be ignored.

       PERF_EVENT_IOC_SET_FILTER (since Linux 2.6.33)
              This adds an ftrace filter to this event.

              The argument is a pointer to the desired ftrace filter.

       PERF_EVENT_IOC_ID (since Linux 3.12)
              This returns the event ID value for the  given  event  file  de-
              scriptor.

              The  argument  is a pointer to a 64-bit unsigned integer to hold
              the result.

       PERF_EVENT_IOC_SET_BPF (since Linux 4.1)
              This allows attaching a Berkeley Packet Filter (BPF) program  to
              an  existing  kprobe  tracepoint  event.  You need CAP_SYS_ADMIN
              privileges to use this ioctl.

              The argument is a BPF program file descriptor that  was  created
              by a previous bpf(2) system call.

       PERF_EVENT_IOC_PAUSE_OUTPUT (since Linux 4.7)
              This  allows  pausing  and  resuming the event's ring-buffer.  A
              paused ring-buffer does not prevent generation of  samples,  but
              simply  discards  them.   The  discarded  samples are considered
              lost, and cause a PERF_RECORD_LOST sample to be  generated  when
              possible.  An overflow signal may still be triggered by the dis-
              carded sample even though the ring-buffer remains empty.

              The argument is an unsigned 32-bit  integer.   A  nonzero  value
              pauses the ring-buffer, while a zero value resumes the ring-buf-
              fer.

       PERF_EVENT_MODIFY_ATTRIBUTES (since Linux 4.17)
              This allows modifying an existing event without the overhead  of
              closing  and reopening a new event.  Currently this is supported
              only for breakpoint events.

              The argument is a pointer to a  perf_event_attr  structure  con-
              taining the updated event settings.

       PERF_EVENT_IOC_QUERY_BPF (since Linux 4.16)
              This allows querying which Berkeley Packet Filter (BPF) programs
              are attached to an existing kprobe tracepoint.  You can only at-
              tach one BPF program per event, but you can have multiple events
              attached to a tracepoint.  Querying this value on one tracepoint
              event  returns the id of all BPF programs in all events attached
              to the tracepoint.  You need  CAP_SYS_ADMIN  privileges  to  use
              this ioctl.

              The argument is a pointer to a structure
                  struct perf_event_query_bpf {
                      __u32    ids_len;
                      __u32    prog_cnt;
                      __u32    ids[0];
                  };

              The  ids_len  field  indicates the number of ids that can fit in
              the provided ids array.  The prog_cnt value is filled in by  the
              kernel  with the number of attached BPF programs.  The ids array
              is filled with the id of each attached BPF  program.   If  there
              are  more  programs  than will fit in the array, then the kernel
              will return ENOSPC and ids_len will indicate the number of  pro-
              gram IDs that were successfully copied.

   Using prctl(2)
       A  process  can enable or disable all currently open event groups using
       the prctl(2) PR_TASK_PERF_EVENTS_ENABLE and PR_TASK_PERF_EVENTS_DISABLE
       operations.  This applies only to events created locally by the calling
       process.  This does not apply to events created by other processes  at-
       tached  to  the  calling  process  or  inherited  events  from a parent
       process.  Only group leaders are enabled and disabled,  not  any  other
       members of the groups.

   perf_event related configuration files
       Files in /proc/sys/kernel/

           /proc/sys/kernel/perf_event_paranoid
                  The  perf_event_paranoid  file can be set to restrict access
                  to the performance counters.

                  2   allow only user-space measurements (default since  Linux
                      4.6).
                  1   allow  both kernel and user measurements (default before
                      Linux 4.6).
                  0   allow access to CPU-specific data but not raw tracepoint
                      samples.
                  -1  no restrictions.

                  The  existence  of the perf_event_paranoid file is the offi-
                  cial  method  for   determining   if   a   kernel   supports
                  perf_event_open().

           /proc/sys/kernel/perf_event_max_sample_rate
                  This  sets  the  maximum sample rate.  Setting this too high
                  can allow users to sample at a rate that impacts overall ma-
                  chine  performance and potentially lock up the machine.  The
                  default value is 100000 (samples per second).

           /proc/sys/kernel/perf_event_max_stack
                  This file sets the maximum depth of stack frame entries  re-
                  ported when generating a call trace.

           /proc/sys/kernel/perf_event_mlock_kb
                  Maximum  number  of pages an unprivileged user can mlock(2).
                  The default is 516 (kB).

       Files in /sys/bus/event_source/devices/

           Since Linux 2.6.34, the kernel supports having multiple PMUs avail-
           able  for monitoring.  Information on how to program these PMUs can
           be found under /sys/bus/event_source/devices/.   Each  subdirectory
           corresponds to a different PMU.

           /sys/bus/event_source/devices/*/type (since Linux 2.6.38)
                  This  contains an integer that can be used in the type field
                  of perf_event_attr to indicate that you  wish  to  use  this
                  PMU.

           /sys/bus/event_source/devices/cpu/rdpmc (since Linux 3.4)
                  If this file is 1, then direct user-space access to the per-
                  formance counter registers is allowed via the rdpmc instruc-
                  tion.  This can be disabled by echoing 0 to the file.

                  As  of  Linux  4.0  the  behavior has changed, so that 1 now
                  means only  allow  access  to  processes  with  active  perf
                  events, with 2 indicating the old allow-anyone-access behav-
                  ior.

           /sys/bus/event_source/devices/*/format/ (since Linux 3.4)
                  This subdirectory contains information on the  architecture-
                  specific  subfields  available  for  programming the various
                  config fields in the perf_event_attr struct.

                  The content of each file is the name of  the  config  field,
                  followed  by  a  colon,  followed by a series of integer bit
                  ranges separated by commas.  For example, the file event may
                  contain  the  value  config1:1,6-10,44  which indicates that
                  event is an attribute that occupies bits 1,6-10, and  44  of
                  perf_event_attr::config1.

           /sys/bus/event_source/devices/*/events/ (since Linux 3.4)
                  This  subdirectory  contains  files  with predefined events.
                  The contents are strings describing the event  settings  ex-
                  pressed  in terms of the fields found in the previously men-
                  tioned ./format/ directory.  These are not necessarily  com-
                  plete  lists of all events supported by a PMU, but usually a
                  subset of events deemed useful or interesting.

                  The content of each file is a list of attribute names  sepa-
                  rated  by  commas.  Each entry has an optional value (either
                  hex or decimal).  If no value is specified, then it  is  as-
                  sumed  to be a single-bit field with a value of 1.  An exam-
                  ple entry may look like this: event=0x2,inv,ldlat=3.

           /sys/bus/event_source/devices/*/uevent
                  This file is the standard kernel device  interface  for  in-
                  jecting hotplug events.

           /sys/bus/event_source/devices/*/cpumask (since Linux 3.7)
                  The cpumask file contains a comma-separated list of integers
                  that indicate a representative CPU number  for  each  socket
                  (package)  on  the motherboard.  This is needed when setting
                  up uncore or  northbridge  events,  as  those  PMUs  present
                  socket-wide events.

RETURN VALUE
       perf_event_open()  returns  the  new file descriptor, or -1 if an error
       occurred (in which case, errno is set appropriately).

ERRORS
       The errors returned by perf_event_open() can be inconsistent,  and  may
       vary across processor architectures and performance monitoring units.

       E2BIG  Returned if the perf_event_attr size value is too small (smaller
              than PERF_ATTR_SIZE_VER0), too big (larger than the page  size),
              or  larger  than the kernel supports and the extra bytes are not
              zero.  When E2BIG is returned, the perf_event_attr size field is
              overwritten by the kernel to be the size of the structure it was
              expecting.

       EACCES Returned when the requested event requires CAP_SYS_ADMIN permis-
              sions  (or a more permissive perf_event paranoid setting).  Some
              common cases where an unprivileged process  may  encounter  this
              error:  attaching  to a process owned by a different user; moni-
              toring all processes on a given CPU (i.e.,  specifying  the  pid
              argument  as  -1); and not setting exclude_kernel when the para-
              noid setting requires it.

       EBADF  Returned if the group_fd file descriptor is not  valid,  or,  if
              PERF_FLAG_PID_CGROUP  is  set, the cgroup file descriptor in pid
              is not valid.

       EBUSY (since Linux 4.1)
              Returned if another event already has exclusive  access  to  the
              PMU.

       EFAULT Returned  if  the  attr  pointer points at an invalid memory ad-
              dress.

       EINVAL Returned if the specified event is invalid.  There are many pos-
              sible  reasons  for this.  A not-exhaustive list: sample_freq is
              higher than the maximum setting; the cpu to monitor does not ex-
              ist;  read_format  is out of range; sample_type is out of range;
              the flags value is out of range; exclusive or pinned set and the
              event  is not a group leader; the event config values are out of
              range or set reserved bits; the generic event  selected  is  not
              supported;  or  there  is  not  enough  room to add the selected
              event.

       EINTR  Returned when trying to mix perf and ftrace handling for  a  up-
              robe.

       EMFILE Each  opened  event uses one file descriptor.  If a large number
              of events are opened, the per-process limit  on  the  number  of
              open file descriptors will be reached, and no more events can be
              created.

       ENODEV Returned when the event involves a feature not supported by  the
              current CPU.

       ENOENT Returned  if  the type setting is not valid.  This error is also
              returned for some unsupported generic events.

       ENOSPC Prior to Linux 3.3, if there was not enough room for the  event,
              ENOSPC  was returned.  In Linux 3.3, this was changed to EINVAL.
              ENOSPC is still returned if  you  try  to  add  more  breakpoint
              events than supported by the hardware.

       ENOSYS Returned  if PERF_SAMPLE_STACK_USER is set in sample_type and it
              is not supported by hardware.

       EOPNOTSUPP
              Returned if an event requiring a specific  hardware  feature  is
              requested  but  there is no hardware support.  This includes re-
              questing low-skid events if not supported, branch tracing if  it
              is not available, sampling if no PMU interrupt is available, and
              branch stacks for software events.

       EOVERFLOW (since Linux 4.8)
              Returned  if  PERF_SAMPLE_CALLCHAIN  is   requested   and   sam-
              ple_max_stack   is   larger   than   the  maximum  specified  in
              /proc/sys/kernel/perf_event_max_stack.

       EPERM  Returned on many (but not all) architectures when an unsupported
              exclude_hv,  exclude_idle,  exclude_user, or exclude_kernel set-
              ting is specified.

              It can also happen, as with EACCES, when the requested event re-
              quires   CAP_SYS_ADMIN   permissions   (or   a  more  permissive
              perf_event paranoid setting).  This includes  setting  a  break-
              point on a kernel address, and (since Linux 3.13) setting a ker-
              nel function-trace tracepoint.

       ESRCH  Returned if attempting to attach to a process that does not  ex-
              ist.

VERSION
       perf_event_open()  was  introduced  in  Linux  2.6.31  but  was  called
       perf_counter_open().  It was renamed in Linux 2.6.32.

CONFORMING TO
       This perf_event_open() system call Linux-specific  and  should  not  be
       used in programs intended to be portable.

NOTES
       Glibc  does  not  provide a wrapper for this system call; call it using
       syscall(2).  See the example below.

       The official way of knowing if perf_event_open() support is enabled  is
       checking    for    the    existence    of   the   file   /proc/sys/ker-
       nel/perf_event_paranoid.

BUGS
       The F_SETOWN_EX option to fcntl(2) is needed to properly  get  overflow
       signals in threads.  This was introduced in Linux 2.6.32.

       Prior  to  Linux 2.6.33 (at least for x86), the kernel did not check if
       events could be scheduled together until read time.  The  same  happens
       on all known kernels if the NMI watchdog is enabled.  This means to see
       if a given set of events works you have  to  perf_event_open(),  start,
       then read before you know for sure you can get valid measurements.

       Prior  to Linux 2.6.34, event constraints were not enforced by the ker-
       nel.  In that case, some events would silently return "0" if the kernel
       scheduled them in an improper counter slot.

       Prior  to  Linux  2.6.34,  there  was a bug when multiplexing where the
       wrong results could be returned.

       Kernels from Linux 2.6.35 to Linux 2.6.39 can quickly crash the  kernel
       if "inherit" is enabled and many threads are started.

       Prior  to  Linux  2.6.35,  PERF_FORMAT_GROUP did not work with attached
       processes.

       There is a bug in the kernel code between Linux 2.6.36  and  Linux  3.0
       that  ignores  the  "watermark" field and acts as if a wakeup_event was
       chosen if the union has a nonzero value in it.

       From Linux 2.6.31 to Linux 3.4, the PERF_IOC_FLAG_GROUP ioctl  argument
       was  broken  and would repeatedly operate on the event specified rather
       than iterating across all sibling events in a group.

       From Linux 3.4 to Linux 3.11, the mmap cap_usr_rdpmc  and  cap_usr_time
       bits  mapped  to  the  same  location.   Code should migrate to the new
       cap_user_rdpmc and cap_user_time fields instead.

       Always double-check your results!  Various generalized events have  had
       wrong  values.   For example, retired branches measured the wrong thing
       on AMD machines until Linux 2.6.35.

EXAMPLE
       The following is a short example that measures  the  total  instruction
       count of a call to printf(3).

       #include <stdlib.h>
       #include <stdio.h>
       #include <unistd.h>
       #include <string.h>
       #include <sys/ioctl.h>
       #include <linux/perf_event.h>
       #include <asm/unistd.h>

       static long
       perf_event_open(struct perf_event_attr *hw_event, pid_t pid,
                       int cpu, int group_fd, unsigned long flags)
       {
           int ret;

           ret = syscall(__NR_perf_event_open, hw_event, pid, cpu,
                          group_fd, flags);
           return ret;
       }

       int
       main(int argc, char **argv)
       {
           struct perf_event_attr pe;
           long long count;
           int fd;

           memset(&pe, 0, sizeof(struct perf_event_attr));
           pe.type = PERF_TYPE_HARDWARE;
           pe.size = sizeof(struct perf_event_attr);
           pe.config = PERF_COUNT_HW_INSTRUCTIONS;
           pe.disabled = 1;
           pe.exclude_kernel = 1;
           pe.exclude_hv = 1;

           fd = perf_event_open(&pe, 0, -1, -1, 0);
           if (fd == -1) {
              fprintf(stderr, "Error opening leader %llx\n", pe.config);
              exit(EXIT_FAILURE);
           }

           ioctl(fd, PERF_EVENT_IOC_RESET, 0);
           ioctl(fd, PERF_EVENT_IOC_ENABLE, 0);

           printf("Measuring instruction count for this printf\n");

           ioctl(fd, PERF_EVENT_IOC_DISABLE, 0);
           read(fd, &count, sizeof(long long));

           printf("Used %lld instructions\n", count);

           close(fd);
       }

SEE ALSO
       perf(1), fcntl(2), mmap(2), open(2), prctl(2), read(2)

       Documentation/admin-guide/perf-security.rst in the kernel source tree

COLOPHON
       This  page  is  part of release 5.05 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

Linux                             2020-02-09                PERF_EVENT_OPEN(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSION | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON