libperf-counting - Man Page

counting interface

Description

The counting interface provides API to measure and get count for specific perf events.

The following test tries to explain count on counting.c example.

It is by no means complete guide to counting, but shows libperf basic API for counting.

The counting.c comes with libperf package and can be compiled and run like:

$ gcc -o counting counting.c -lperf
$ sudo ./counting
count 176792, enabled 176944, run 176944
count 176242, enabled 176242, run 176242

It requires root access, because of the PERF_COUNT_SW_CPU_CLOCK event, which is available only for root.

The counting.c example monitors two events on the current process and displays their count, in a nutshell it:

creates events
adds them to the event list
opens and enables events through the event list
does some workload
disables events
reads and displays event counts
destroys the event list

The first thing you need to do before using libperf is to call init function:

  8 static int libperf_print(enum libperf_print_level level,
  9                          const char *fmt, va_list ap)
 10 {
 11         return vfprintf(stderr, fmt, ap);
 12 }

 14 int main(int argc, char **argv)
 15 {
 ...
 35         libperf_init(libperf_print);

It will setup the library and sets function for debug output from library.

The libperf_print callback will receive any message with its debug level, defined as:

enum libperf_print_level {
        LIBPERF_ERR,
        LIBPERF_WARN,
        LIBPERF_INFO,
        LIBPERF_DEBUG,
        LIBPERF_DEBUG2,
        LIBPERF_DEBUG3,
};

Once the setup is complete we start by defining specific events using the struct perf_event_attr.

We create software events for cpu and task:

 20         struct perf_event_attr attr1 = {
 21                 .type        = PERF_TYPE_SOFTWARE,
 22                 .config      = PERF_COUNT_SW_CPU_CLOCK,
 23                 .read_format = PERF_FORMAT_TOTAL_TIME_ENABLED|PERF_FORMAT_TOTAL_TIME_RUNNING,
 24                 .disabled    = 1,
 25         };
 26         struct perf_event_attr attr2 = {
 27                 .type        = PERF_TYPE_SOFTWARE,
 28                 .config      = PERF_COUNT_SW_TASK_CLOCK,
 29                 .read_format = PERF_FORMAT_TOTAL_TIME_ENABLED|PERF_FORMAT_TOTAL_TIME_RUNNING,
 30                 .disabled    = 1,
 31         };

The read_format setup tells perf to include timing details together with each count.

Next step is to prepare threads map.

In this case we will monitor current process, so we create threads map with single pid (0):

 37         threads = perf_thread_map__new_dummy();
 38         if (!threads) {
 39                 fprintf(stderr, "failed to create threads\n");
 40                 return -1;
 41         }
 42
 43         perf_thread_map__set_pid(threads, 0, 0);

Now we create libperf’s event list, which will serve as holder for the events we want:

 45         evlist = perf_evlist__new();
 46         if (!evlist) {
 47                 fprintf(stderr, "failed to create evlist\n");
 48                 goto out_threads;
 49         }

We create libperf’s events for the attributes we defined earlier and add them to the list:

 51         evsel = perf_evsel__new(&attr1);
 52         if (!evsel) {
 53                 fprintf(stderr, "failed to create evsel1\n");
 54                 goto out_evlist;
 55         }
 56
 57         perf_evlist__add(evlist, evsel);
 58
 59         evsel = perf_evsel__new(&attr2);
 60         if (!evsel) {
 61                 fprintf(stderr, "failed to create evsel2\n");
 62                 goto out_evlist;
 63         }
 64
 65         perf_evlist__add(evlist, evsel);

Configure event list with the thread map and open events:

 67         perf_evlist__set_maps(evlist, NULL, threads);
 68
 69         err = perf_evlist__open(evlist);
 70         if (err) {
 71                 fprintf(stderr, "failed to open evsel\n");
 72                 goto out_evlist;
 73         }

Both events are created as disabled (note the disabled = 1 assignment above), so we need to enable the whole list explicitly (both events).

From this moment events are counting and we can do our workload.

When we are done we disable the events list.

 75         perf_evlist__enable(evlist);
 76
 77         while (count--);
 78
 79         perf_evlist__disable(evlist);

Now we need to get the counts from events, following code iterates through the events list and read counts:

 81         perf_evlist__for_each_evsel(evlist, evsel) {
 82                 perf_evsel__read(evsel, 0, 0, &counts);
 83                 fprintf(stdout, "count %llu, enabled %llu, run %llu\n",
 84                         counts.val, counts.ena, counts.run);
 85         }

And finally cleanup.

We close the whole events list (both events) and remove it together with the threads map:

 87         perf_evlist__close(evlist);
 88
 89 out_evlist:
 90         perf_evlist__delete(evlist);
 91 out_threads:
 92         perf_thread_map__put(threads);
 93         return err;
 94 }