1LIBPERF-SAMPLING(7) libperf Manual LIBPERF-SAMPLING(7)
2
6 libperf-sampling - sampling interface
7
9 The sampling interface provides API to measure and get count for
10 specific perf events.
11 The following test tries to explain count on sampling.c example.
13 It is by no means complete guide to sampling, but shows libperf basic
15 API for sampling.
16 The sampling.c comes with libperf package and can be compiled and run
18 like:
19 $ gcc -o sampling sampling.c -lperf
21 $ sudo ./sampling
22 cpu 0, pid 0, tid 0, ip ffffffffad06c4e6, period 1
23 cpu 0, pid 4465, tid 4469, ip ffffffffad118748, period 18322959
24 cpu 0, pid 0, tid 0, ip ffffffffad115722, period 33544846
25 cpu 0, pid 4465, tid 4470, ip 7f84fe0cdad6, period 23687474
26 cpu 0, pid 0, tid 0, ip ffffffffad9e0349, period 34255790
27 cpu 0, pid 4465, tid 4469, ip ffffffffad136581, period 38664069
28 cpu 0, pid 0, tid 0, ip ffffffffad9e55e2, period 21922384
29 cpu 0, pid 4465, tid 4470, ip 7f84fe0ebebf, period 17655175
30 ...
31 It requires root access, because it uses hardware cycles event.
33 The sampling.c example profiles/samples all CPUs with hardware cycles,
35 in a nutshell it:
36 • creates events
38 • adds them to the event list
40 • opens and enables events through the event list
42 • sleeps for 3 seconds
44 • disables events
46 • reads and displays recorded samples
48 • destroys the event list
50 The first thing you need to do before using libperf is to call init
52 function:
53 12 static int libperf_print(enum libperf_print_level level,
55 13 const char *fmt, va_list ap)
56 14 {
57 15 return vfprintf(stderr, fmt, ap);
58 16 }
59 23 int main(int argc, char **argv)
61 24 {
62 ...
63 40 libperf_init(libperf_print);
64 It will setup the library and sets function for debug output from
66 library.
67 The libperf_print callback will receive any message with its debug
69 level, defined as:
70 enum libperf_print_level {
72 LIBPERF_ERR,
73 LIBPERF_WARN,
74 LIBPERF_INFO,
75 LIBPERF_DEBUG,
76 LIBPERF_DEBUG2,
77 LIBPERF_DEBUG3,
78 };
79 Once the setup is complete we start by defining cycles event using the
81 struct perf_event_attr:
82 29 struct perf_event_attr attr = {
84 30 .type = PERF_TYPE_HARDWARE,
85 31 .config = PERF_COUNT_HW_CPU_CYCLES,
86 32 .disabled = 1,
87 33 .freq = 1,
88 34 .sample_freq = 10,
89 35 .sample_type = PERF_SAMPLE_IP|PERF_SAMPLE_TID|PERF_SAMPLE_CPU|PERF_SAMPLE_PERIOD,
90 36 };
91 Next step is to prepare CPUs map.
93 In this case we will monitor all the available CPUs:
95 42 cpus = perf_cpu_map__new(NULL);
97 43 if (!cpus) {
98 44 fprintf(stderr, "failed to create cpus\n");
99 45 return -1;
100 46 }
101 Now we create libperf’s event list, which will serve as holder for the
103 cycles event:
104 48 evlist = perf_evlist__new();
106 49 if (!evlist) {
107 50 fprintf(stderr, "failed to create evlist\n");
108 51 goto out_cpus;
109 52 }
110 We create libperf’s event for the cycles attribute we defined earlier
112 and add it to the list:
113 54 evsel = perf_evsel__new(&attr);
115 55 if (!evsel) {
116 56 fprintf(stderr, "failed to create cycles\n");
117 57 goto out_cpus;
118 58 }
119 59
120 60 perf_evlist__add(evlist, evsel);
121 Configure event list with the cpus map and open event:
123 62 perf_evlist__set_maps(evlist, cpus, NULL);
125 63
126 64 err = perf_evlist__open(evlist);
127 65 if (err) {
128 66 fprintf(stderr, "failed to open evlist\n");
129 67 goto out_evlist;
130 68 }
131 Once the events list is open, we can create memory maps AKA perf ring
133 buffers:
134 70 err = perf_evlist__mmap(evlist, 4);
136 71 if (err) {
137 72 fprintf(stderr, "failed to mmap evlist\n");
138 73 goto out_evlist;
139 74 }
140 The event is created as disabled (note the disabled = 1 assignment
142 above), so we need to enable the events list explicitly.
143 From this moment the cycles event is sampling.
145 We will sleep for 3 seconds while the ring buffers get data from all
147 CPUs, then we disable the events list.
148 76 perf_evlist__enable(evlist);
150 77 sleep(3);
151 78 perf_evlist__disable(evlist);
152 Following code walks through the ring buffers and reads stored
154 events/samples:
155 80 perf_evlist__for_each_mmap(evlist, map, false) {
157 81 if (perf_mmap__read_init(map) < 0)
158 82 continue;
159 83
160 84 while ((event = perf_mmap__read_event(map)) != NULL) {
161 /* process event */
163 108 perf_mmap__consume(map);
165 109 }
166 110 perf_mmap__read_done(map);
167 111 }
168 Each sample needs to get parsed:
170 85 int cpu, pid, tid;
172 86 __u64 ip, period, *array;
173 87 union u64_swap u;
174 88
175 89 array = event->sample.array;
176 90
177 91 ip = *array;
178 92 array++;
179 93
180 94 u.val64 = *array;
181 95 pid = u.val32[0];
182 96 tid = u.val32[1];
183 97 array++;
184 98
185 99 u.val64 = *array;
186 100 cpu = u.val32[0];
187 101 array++;
188 102
189 103 period = *array;
190 104
191 105 fprintf(stdout, "cpu %3d, pid %6d, tid %6d, ip %20llx, period %20llu\n",
192 106 cpu, pid, tid, ip, period);
193 And finally cleanup.
195 We close the whole events list (both events) and remove it together
197 with the threads map:
198 113 out_evlist:
200 114 perf_evlist__delete(evlist);
201 115 out_cpus:
202 116 perf_cpu_map__put(cpus);
203 117 return err;
204 118 }
205
207 Report bugs to <linux-perf-users@vger.kernel.org[1]>.
208
210 libperf is Free Software licensed under the GNU LGPL 2.1
211
213 https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
214
216 libperf(3), libperf-counting(7)
217
219 1. linux-perf-users@vger.kernel.org
220 mailto:linux-perf-users@vger.kernel.org
221libperf 06/14/2022 LIBPERF-SAMPLING(7)