| libperf-sampling(7) |
| =================== |
| |
| NAME |
| ---- |
| libperf-sampling - sampling interface |
| |
| |
| DESCRIPTION |
| ----------- |
| The sampling interface provides API to measure and get count for specific perf events. |
| |
| The following test tries to explain count on `sampling.c` example. |
| |
| It is by no means complete guide to sampling, but shows libperf basic API for sampling. |
| |
| The `sampling.c` comes with libperf package and can be compiled and run like: |
| |
| [source,bash] |
| -- |
| $ gcc -o sampling sampling.c -lperf |
| $ sudo ./sampling |
| cpu 0, pid 0, tid 0, ip ffffffffad06c4e6, period 1 |
| cpu 0, pid 4465, tid 4469, ip ffffffffad118748, period 18322959 |
| cpu 0, pid 0, tid 0, ip ffffffffad115722, period 33544846 |
| cpu 0, pid 4465, tid 4470, ip 7f84fe0cdad6, period 23687474 |
| cpu 0, pid 0, tid 0, ip ffffffffad9e0349, period 34255790 |
| cpu 0, pid 4465, tid 4469, ip ffffffffad136581, period 38664069 |
| cpu 0, pid 0, tid 0, ip ffffffffad9e55e2, period 21922384 |
| cpu 0, pid 4465, tid 4470, ip 7f84fe0ebebf, period 17655175 |
| ... |
| -- |
| |
| It requires root access, because it uses hardware cycles event. |
| |
| The `sampling.c` example profiles/samples all CPUs with hardware cycles, in a |
| nutshell it: |
| |
| - creates events |
| - adds them to the event list |
| - opens and enables events through the event list |
| - sleeps for 3 seconds |
| - disables events |
| - reads and displays recorded samples |
| - destroys the event list |
| |
| The first thing you need to do before using libperf is to call init function: |
| |
| [source,c] |
| -- |
| 12 static int libperf_print(enum libperf_print_level level, |
| 13 const char *fmt, va_list ap) |
| 14 { |
| 15 return vfprintf(stderr, fmt, ap); |
| 16 } |
| |
| 23 int main(int argc, char **argv) |
| 24 { |
| ... |
| 40 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: |
| |
| [source,c] |
| -- |
| 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 cycles event using the `struct perf_event_attr`: |
| |
| [source,c] |
| -- |
| 29 struct perf_event_attr attr = { |
| 30 .type = PERF_TYPE_HARDWARE, |
| 31 .config = PERF_COUNT_HW_CPU_CYCLES, |
| 32 .disabled = 1, |
| 33 .freq = 1, |
| 34 .sample_freq = 10, |
| 35 .sample_type = PERF_SAMPLE_IP|PERF_SAMPLE_TID|PERF_SAMPLE_CPU|PERF_SAMPLE_PERIOD, |
| 36 }; |
| -- |
| |
| Next step is to prepare CPUs map. |
| |
| In this case we will monitor all the available CPUs: |
| |
| [source,c] |
| -- |
| 42 cpus = perf_cpu_map__new(NULL); |
| 43 if (!cpus) { |
| 44 fprintf(stderr, "failed to create cpus\n"); |
| 45 return -1; |
| 46 } |
| -- |
| |
| Now we create libperf's event list, which will serve as holder for the cycles event: |
| |
| [source,c] |
| -- |
| 48 evlist = perf_evlist__new(); |
| 49 if (!evlist) { |
| 50 fprintf(stderr, "failed to create evlist\n"); |
| 51 goto out_cpus; |
| 52 } |
| -- |
| |
| We create libperf's event for the cycles attribute we defined earlier and add it to the list: |
| |
| [source,c] |
| -- |
| 54 evsel = perf_evsel__new(&attr); |
| 55 if (!evsel) { |
| 56 fprintf(stderr, "failed to create cycles\n"); |
| 57 goto out_cpus; |
| 58 } |
| 59 |
| 60 perf_evlist__add(evlist, evsel); |
| -- |
| |
| Configure event list with the cpus map and open event: |
| |
| [source,c] |
| -- |
| 62 perf_evlist__set_maps(evlist, cpus, NULL); |
| 63 |
| 64 err = perf_evlist__open(evlist); |
| 65 if (err) { |
| 66 fprintf(stderr, "failed to open evlist\n"); |
| 67 goto out_evlist; |
| 68 } |
| -- |
| |
| Once the events list is open, we can create memory maps AKA perf ring buffers: |
| |
| [source,c] |
| -- |
| 70 err = perf_evlist__mmap(evlist, 4); |
| 71 if (err) { |
| 72 fprintf(stderr, "failed to mmap evlist\n"); |
| 73 goto out_evlist; |
| 74 } |
| -- |
| |
| The event is created as disabled (note the `disabled = 1` assignment above), |
| so we need to enable the events list explicitly. |
| |
| From this moment the cycles event is sampling. |
| |
| We will sleep for 3 seconds while the ring buffers get data from all CPUs, then we disable the events list. |
| |
| [source,c] |
| -- |
| 76 perf_evlist__enable(evlist); |
| 77 sleep(3); |
| 78 perf_evlist__disable(evlist); |
| -- |
| |
| Following code walks through the ring buffers and reads stored events/samples: |
| |
| [source,c] |
| -- |
| 80 perf_evlist__for_each_mmap(evlist, map, false) { |
| 81 if (perf_mmap__read_init(map) < 0) |
| 82 continue; |
| 83 |
| 84 while ((event = perf_mmap__read_event(map)) != NULL) { |
| |
| /* process event */ |
| |
| 108 perf_mmap__consume(map); |
| 109 } |
| 110 perf_mmap__read_done(map); |
| 111 } |
| |
| -- |
| |
| Each sample needs to get parsed: |
| |
| [source,c] |
| -- |
| 85 int cpu, pid, tid; |
| 86 __u64 ip, period, *array; |
| 87 union u64_swap u; |
| 88 |
| 89 array = event->sample.array; |
| 90 |
| 91 ip = *array; |
| 92 array++; |
| 93 |
| 94 u.val64 = *array; |
| 95 pid = u.val32[0]; |
| 96 tid = u.val32[1]; |
| 97 array++; |
| 98 |
| 99 u.val64 = *array; |
| 100 cpu = u.val32[0]; |
| 101 array++; |
| 102 |
| 103 period = *array; |
| 104 |
| 105 fprintf(stdout, "cpu %3d, pid %6d, tid %6d, ip %20llx, period %20llu\n", |
| 106 cpu, pid, tid, ip, period); |
| -- |
| |
| And finally cleanup. |
| |
| We close the whole events list (both events) and remove it together with the threads map: |
| |
| [source,c] |
| -- |
| 113 out_evlist: |
| 114 perf_evlist__delete(evlist); |
| 115 out_cpus: |
| 116 perf_cpu_map__put(cpus); |
| 117 return err; |
| 118 } |
| -- |
| |
| REPORTING BUGS |
| -------------- |
| Report bugs to <linux-perf-users@vger.kernel.org>. |
| |
| LICENSE |
| ------- |
| libperf is Free Software licensed under the GNU LGPL 2.1 |
| |
| RESOURCES |
| --------- |
| https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git |
| |
| SEE ALSO |
| -------- |
| libperf(3), libperf-counting(7) |
