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    Command:

    perf_event_open

    
    
    

    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
           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).
    
       Arguments
           The pid and cpu arguments allow specifying which  process  and  CPU  to
           monitor:
    
           pid == 0 and cpu == -1
                  This measures the current process/thread on any CPU.
    
           pid == 0 and cpu >= 0
                  This  measures  the  current 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.  Mea-
                  surements such as this require the CAP_SYS_ADMIN capability or a
                  /proc/sys/kernel/perf_event_paranoid value of less than 1.
    
           since  they  have  counted events for the same set of executed instruc-
           tions.
    
           The flags argument is formed by ORing together zero or more of the fol-
           lowing values:
    
           PERF_FLAG_FD_NO_GROUP
                  This flag allows creating an event as part of an event group but
                  having no group leader.  It is unclear why this is useful.
    
           PERF_FLAG_FD_OUTPUT
                  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  grain  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 */
                                                /* 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:
    
                  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
                         /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  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_ATR_SIZE_VER3 is 96 cor-
                  responding  to  the  addition  of  sample_regs_user   and   sam-
                  ple_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-spe-
                  cific  (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 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
                              affected by various issues,  most  notably  hardware
                              interrupt 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_BUS_CYCLES
                              Bus  cycles,  which  can  be  different  from  total
                              cycles.
    
                       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
                              migrated 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)
    
                  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 3.0)
                                 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
    
                  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.
    
           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.
    
                         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 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 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).
    
                         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  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).
    
           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
                         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_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  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.
    
    
           comm   The  comm  bit enables tracking of process command name as modi-
                  fied by the exec(2) and prctl(PR_SET_NAME) system calls.  Unfor-
                  tunately  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.
    
           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  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
                         PERF_RECORD_MISC_EXACT_IP.
    
           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
                          { 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:
    
                  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)
                  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 mem-
                  ory location of interest.
    
                  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.
    
                  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.
    
                  PERF_SAMPLE_BRANCH_ANY_RETURN
                         Any return branch.
    
                  PERF_SAMPLE_BRANCH_IND_CALL
                         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.
    
           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 */
                     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.  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 than 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  run-
                  ning  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).
    
                   __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 looks at 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 3.12 these are renamed to cap_bit0 and you  should
                  use the new cap_user_time and cap_user_rdpmc fields instead.
                  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  times-
                  tamp counter (TSC on x86).
    
           cap_user_time_zero (since Linux 3.12)
                  Indicates  the presence of time_zero which allows mapping times-
                  tamp 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.
                          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);
    
           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.
    
           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
           perf.data  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;
               };
                  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
                         Sample happened in the guest kernel.
    
                  PERF_RECORD_MISC_GUEST_USER
                         Sample happened in guest user code.
    
                  In addition, one of the following bits can be set:
    
                  PERF_RECORD_MISC_MMAP_DATA
                         This is set when the mapping is not executable; otherwise
                         the mapping is executable.
    
                  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
                         This  indicates  there  is  extended data available (cur-
                         rently not used).
    
           type   The type value is one of the below.  The values  in  the  corre-
                  sponding  record  (that  follows  the header) depend on the type
                  selected 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[];
                          };
    
    
                  PERF_RECORD_COMM
                      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;
                          };
    
                  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;
                              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 */
                          };
    
                      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
                          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.
    
                      stream_id
                          If  PERF_SAMPLE_STREAM_ID is enabled, a 64-bit unique ID
                          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
                          include 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.
    
                          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.
                          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 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
                          size).
    
                      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:
    
                              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:
    
    
                              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 transac-
                          tional 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
                                 instruction).
    
                          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
                          PERF_TXN_ABORT_MASK.
    
       Signal overflow
           Events can be set to deliver a signal when a threshold is crossed.  The
           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  pro-
           vided 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
    
           PERF_EVENT_IOC_ENABLE
                  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
                  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.  A signal with POLL_IN set will hap-
                  pen  on  each overflow until the count reaches 0; when that hap-
                  pens a signal with POLL_HUP set is sent and the  event  is  dis-
                  abled.  Using an argument of 0 is considered undefined behavior.
                  updated immediately.
    
                  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.
    
           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
                  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)
                  Returns the event ID value for the given event fd.
    
                  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 current 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/
    
               /proc/sys/kernel/perf_event_paranoid
    
                      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
                          samples.
    
    
                      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 you wish to use this PMU.
    
               /sys/bus/event_source/devices/*/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.
    
               /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
                      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 sepa-
                      rated by commas.  Each entry has an optional  value  (either
                      hex  or  decimal).   If  no  value  is  specified than 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.
    
               /sys/bus/event_source/devices/*/uevent
                      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
    
                  expecting.
    
           EINVAL Returned if the specified event is not available.
    
           ENOSPC Prior to Linux 3.3, if there was not enough room for the  event,
                  ENOSPC  was  returned.   Linus  did  not like this, and this was
                  changed to EINVAL.  ENOSPC is still returned if you try to  read
                  results into too small of a buffer.
    
    
    

    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.
    
           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.
    
           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);
    
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