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       perf_event_open - set up performance monitoring


       #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.


       Given   a   list   of  parameters,  perf_event_open()  returns  a  file
       descriptor, 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
       measuring 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
       occur.  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).

       The pid and cpu arguments allow specifying which  process  and  CPU  to

       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/kernel/perf_event_paranoid value of less than 1.

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

       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

       The flags argument is formed by ORing together  zero  or  more  of  the
       following 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
              creation 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).

              This flag allows creating an event as part of an event group but
              having no group leader.  It is unclear why this is useful.

              This flag reroutes the output from an event to the group leader.

       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
              monitor  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
       information 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 */
                     __reserved_1   : 41;

               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 config1;          /* extension of config */

               union {
                   __u64 bp_len;           /* breakpoint length */
                   __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 */
               __u32 __reserved_2;         /* Align to u64 */


       The fields of the  perf_event_attr  structure  are  described  in  more
       detail below:

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

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

                     This  indicates  one  of  the   software-defined   events
                     provided  by  the  kernel (even if no hardware support is

                     This  indicates  a  tracepoint  provided  by  the  kernel
                     tracepoint infrastructure.

                     This  indicates  a  hardware  cache  event.   This  has a
                     special  encoding,  described   in   the   config   field

                     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
                     address as well as execution of an instruction address.

              dynamic PMU
                     Since  Linux  2.6.39,   perf_event_open()   can   support
                     multiple  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
                     filesystem: 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.

       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

              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_ATR_SIZE_VER3  is  96
              corresponding   to   the   addition   of   sample_regs_user  and
              sample_stack_user in Linux 3.7.

       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.

              The  most  significant  bit  (bit  63)  of config signifies CPU-
              specific  (raw)  counter  configuration  data;   if   the   most
              significant  bit is unset, the next 7 bits are an event type and
              the rest of the bits are the event identifier.

              There are  various  ways  to  set  the  config  field  that  are
              dependent  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
              generalized hardware CPU events.  Not all of these are available
              on all platforms.  Set config to one of the following:

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

                          Retired instructions.   Be  careful,  these  can  be
                          affected  by  various  issues, most notably hardware
                          interrupt counts.

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

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

                          Retired branch instructions.  Prior to Linux 2.6.34,
                          this used the wrong event on AMD processors.

                          Mispredicted branch instructions.

                          Bus  cycles,  which  can  be  different  from  total

                   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:

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

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

                          This reports the number of page faults.

                          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.

                          This reports the number of  times  the  process  has
                          migrated to a new CPU.

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

                          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

                   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
                          negatively 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

              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:

                             for measuring Level 1 Data Cache

                             for measuring Level 1 Instruction Cache

                             for measuring Last-Level Cache

                             for measuring the Data TLB

                             for measuring the Instruction TLB

                             for measuring the branch prediction unit

                      PERF_COUNT_HW_CACHE_NODE (since Linux 3.0)
                             for measuring local memory accesses

                  and perf_hw_cache_op_id is one of

                             for read accesses

                             for write accesses

                             for prefetch accesses

                  and perf_hw_cache_op_result_id is one of

                             to measure accesses

                             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
              library  can  be  used  to  translate  from  the  name  in   the
              architectural  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.

       sample_period, sample_freq
              A  "sampling" counter is one that generates an interrupt every N
              events, where N is given by sample_period.  A  sampling  counter
              has  sample_period  >  0.   When  an  overflow interrupt occurs,
              requested data is recorded in the mmap buffer.  The  sample_type
              field controls what data is recorded on each interrupt.

              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.

              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

                     Records instruction pointer.

                     Records the process and thread IDs.

                     Records a timestamp.

                     Records an address, if applicable.

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

                     Records the callchain (stack backtrace).

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

                     Records CPU number.

                     Records the current sampling period.

                     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

                     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
                     hardware 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 only available 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
                     different sample_type settings.  Parsing the event stream
                     properly 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).

                     This  new  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

                     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

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

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

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

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

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

              The disabled  bit  specifies  whether  the  counter  starts  out
              disabled  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
              initialized  with  disabled  set  to  1 and any child events are
              initialized with disabled set to 0.  Despite disabled  being  0,
              the  child  events  will  not  start  until  the group leader is

              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

              Inherit does not work for  some  combinations  of  read_formats,
              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
              counters  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
              subsequently enabled or disabled.

              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).

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

              If  this  bit  is  set, the count excludes events that happen in

              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.

              If set, don't count when the CPU is idle.

       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
              modified by the exec(2)  and  prctl(PR_SET_NAME)  system  calls.
              Unfortunately  for  tools,  there  is  no way to distinguish one
              system call versus the other.

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

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

              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.

              If  set,  have  a  sampling  interrupt  happen when we cross the
              wakeup_watermark boundary.  Otherwise  interrupts  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 values of this 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

       mmap_data (since Linux 2.6.36)
              The  counterpart  of the mmap field.  This enables generation 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
              sample_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)
              Do not measure time spent in VM host.

       exclude_guest (since Linux 3.2)
              Do not measure time spent in VM guest.

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

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

       wakeup_events, wakeup_watermark
              This  union  sets  how  many  samples  (wakeup_events)  or bytes
              (wakeup_watermark) happen before  an  overflow  signal  happens.
              Which one is used is selected by the watermark bitflag.

              wakeup_events  only  counts PERF_RECORD_SAMPLE record types.  To
              receive  a  signal  for  every  incoming  PERF_RECORD  type  set
              wakeup_watermark to 1.

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

                     No breakpoint.

                     Count when we read the memory location.

                     Count when we write the memory location.

                     Count when we read or write the memory location.

                     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)
              bp_addr  address  of  the breakpoint.  For execution breakpoints
              this is the memory address of the instruction of  interest;  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
              OFFCORE_EVENTS on Nehalem/Westmere/SandyBridge use this field on
              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,
              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 following values.  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.

                     Branch target is in user space.

                     Branch target is in kernel space.

                     Branch target is in hypervisor.

                     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.

                     Any branch type.

                     Any call branch.

                     Any return branch.

                     Indirect calls.

              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.

       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-
              specific     and     described     in    the    kernel    header

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

   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 ENOSPC is returned

       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 */
                     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.  Only available if
              PERF_FORMAT_GROUP was specified.

       time_enabled, time_running
              Total  time  the  event was enabled and running.  Normally these
              are the same.   If  more  events  are  started,  then  available
              counter  slots  on the PMU, then multiplexing happens and events
              run only part of the time.  In that case, the  time_enabled  and
              time  running values can be used to scale an estimated value for
              the count.

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

       id     A globally unique value for this particular event, only there 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
       information such as where the ring-buffer head is.

       Before  kernel  2.6.39,  there  is a bug that means you must allocate a
       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 1k */
               __u64 data_head;         /* head in the data section */
               __u64 data_tail;         /* user-space written tail */

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

              Version number of this structure.

              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 the event was active.

              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 3.12 these are renamed to cap_bit0 and you should
              use the new 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;
                      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);

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

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

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

              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

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

              This  points  to  the  head  of  the  data  section.   The value
              continuously increases, it does not wrap.  The value needs to be
              manually 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().

              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.

       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.   That  way  a  newer  file  will  be supported by older perf tools, with these new
       optional 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

       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
              following (note these are not bit masks, only one can be set  at
              a time):

                     Unknown CPU mode.

                     Sample happened in the kernel.

                     Sample happened in user code.

                     Sample happened in the hypervisor.

                     Sample happened in the guest kernel.

                     Sample happened in guest user code.

              In addition, one of the following bits can be set:

                     This is set when the mapping is not executable; otherwise
                     the mapping is executable.

                     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.

                     This  indicates  there   is   extended   data   available
                     (currently not used).

       type   The  type  value  is  one  of  the  below.   The  values  in the
              corresponding record (that follows the  header)  depend  on  the
              type selected as shown.

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

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

                  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   is the number of events that were lost.

                  This record indicates a change in the process name.

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

                  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;

                  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;

                  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;

                  This record indicates a read event.

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

                  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 */
                          u64   weight;     /* if PERF_SAMPLE_WEIGHT */
                          u64   data_src;   /* if PERF_SAMPLE_DATA_SRC */
                          u64   transaction;/* if PERF_SAMPLE_TRANSACTION */

                      If PERF_SAMPLE_IDENTIFIER is enabled, a 64-bit unique ID
                      is   included.    This   is   a   duplication   of   the
                      PERF_SAMPLE_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.

                      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.

                      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
                      included.   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.

                      If  PERF_SAMPLE_STREAM_ID is enabled, a 64-bit unique ID
                      is included.  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.

                  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.

                      If  PERF_SAMPLE_PERIOD  is  enabled,  a   64-bit   value
                      indicating 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
                      number  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
                      indicating 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,
                      followed  by bnr perf_branch_entry structures which each
                      include the fields:

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

                      to     The branch target.

                             The branch target was mispredicted.

                             The branch target was predicted.

                      in_tx (since Linux 3.11)
                             The   branch   was   in  a  transactional  memory

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

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

                      Support for mispred and predicted is  optional;  if  not
                      supported, both 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
                      sample_regs_user bit mask.

                  size, data[size], dyn_size
                      If  PERF_SAMPLE_STACK_USER  is  enabled, then record the
                      user stack to enable  backtracing.   size  is  the  size
                      requested  by  the  user  in stack_user_size or else the
                      maximum record size.  data is the stack data.   dyn_size
                      is  the amount of data actually dumped (can be less than

                      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.

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

                          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

                          Memory  hierarchy  level  hit  or  miss,  a  bitwise
                          combination of:

                          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

                          Snoop mode, a bitwise combination of:

                          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

                          Lock instruction, a bitwise combination of:

                          PERF_MEM_LOCK_NA        Not available
                          PERF_MEM_LOCK_LOCKED    Locked transaction

                          TLB access hit or miss, a bitwise combination of:

                          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

                      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

                             Abort from an elision  type  transaction  (Intel-

                             Abort from a generic transaction.

                             Synchronous   abort   (related  to  the  reported

                             Asynchronous abort (not related to  the  reported

                             Retryable  abort  (retrying  the  transaction may
                             have succeeded).

                             Abort due to memory conflicts with other threads.

                             Abort due to write capacity overflow.

                             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

   Signal overflow
       Events can be set to deliver a signal when a threshold is crossed.  The
       signal handler is set up using the  poll(2),  select(2),  epoll(2)  and
       fcntl(2), system calls.

       To  generate signals, sampling must be enabled (sample_period must have
       a nonzero value).

       There are two ways to generate signals.

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

       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, a POLL_IN signal is sent on overflow, but once the value
       reaches 0, a signal is sent of type POLL_HUP and the  underlying  event
       is disabled.

       Note:  on  newer  kernels  (definitely  noticed  with  3.2) a signal is
       provided for every overflow, even if wakeup_events is not set.

   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.

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

              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
              specified is not the group leader (but see BUGS).

              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
              disabling 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
              specified is not the group leader (but see BUGS).

              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.   A signal with POLL_IN set will
              happen on each overflow until the count  reaches  0;  when  that
              happens  a  signal  with  POLL_HUP  set is sent and the event is
              disabled.  Using  an  argument  of  0  is  considered  undefined

              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
              specified is not the group leader (but see BUGS).

              This updates the overflow period for the event.

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

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

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

              This tells the kernel  to  report  event  notifications  to  the
              specified file descriptor rather than the default one.  The file
              descriptors 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

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

   Using prctl
       A  process can enable or disable all the event groups that are attached
       to   it   using    the    prctl(2)    PR_TASK_PERF_EVENTS_ENABLE    and
       PR_TASK_PERF_EVENTS_DISABLE  operations.   This applies to all counters
       on the calling process, whether created by this process or by  another,
       and does not affect any counters that this process has created on other
       processes.  It enables or disables only  the  group  leaders,  not  any
       other members in the groups.

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


                  The  perf_event_paranoid  file can be set to restrict access
                  to the performance counters.

                  2   only allow user-space measurements.

                  1   allow both kernel and user measurements (default).

                  0   allow access to CPU-specific data but not raw tracepoint

                  -1  no restrictions.

                  The   existence  of  the  perf_event_paranoid  file  is  the
                  official  method  for  determining  if  a  kernel   supports


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


                  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
           available 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

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

           /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

           /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
                  expressed  in  terms  of  the fields found in the previously
                  mentioned ./format/ directory.  These  are  not  necessarily
                  complete 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
                  separated  by  commas.   Each  entry  has  an optional value
                  (either hex or decimal).  If no value is specified, then  it
                  is  assumed  to be a single-bit field with a value of 1.  An
                  example entry may look like this: event=0x2,inv,ldlat=3.

                  This file  is  the  standard  kernel  device  interface  for
                  injecting 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.


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


       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

       EACCES Returned  when  the  requested  event   requires   CAP_SYS_ADMIN
              permissions  (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;
              monitoring all processes on a given CPU  (i.e.,  specifying  the
              pid  argument  as  -1);  and not setting exclude_kernel when the
              paranoid 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.

       EFAULT Returned if  the  attr  pointer  points  at  an  invalid  memory

       EINVAL Returned  if  the  specified  event  is invalid.  There are many
              possible reasons for this.  A not-exhaustive  list:  sample_freq
              is  higher than the maximum setting; the cpu to monitor does not
              exist; 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

       EMFILE Each  opened  event uses one file descriptor.  If a large number
              of events are opened the per-user file descriptor  limit  (often
              1024) will be hit 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.

              Returned  if  an  event requiring a specific hardware feature is
              requested but there  is  no  hardware  support.   This  includes
              requesting  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.

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

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

       ESRCH  Returned  if  attempting  to  attach  to a process that does not


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


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


       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


       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
       kernel.   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

       In older Linux 2.6 versions, refreshing an event group leader refreshed
       all siblings, and refreshing with a parameter  of  0  enabled  infinite
       refresh.  This behavior is unsupported and should not be relied on.

       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.


       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;

       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
", pe.config);

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

           printf("Measuring instruction count for this printf

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

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



       fcntl(2), mmap(2), open(2), prctl(2), read(2)


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