libperf: Add counting example

Current libperf man pages mention file counting.c "coming with libperf package",
however, the file is missing. Add the file then.

Fixes: 81de3bf3 ("libperf: Add man pages")
Signed-off-by: Michael Petlan <mpetlan@redhat.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
LPU-Reference: 20200227194424.28210-1-mpetlan@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/perf/Documentation/examples/counting.c

+#include <linux/perf_event.h>
+#include <perf/evlist.h>
+#include <perf/evsel.h>
+#include <perf/cpumap.h>
+#include <perf/threadmap.h>
+#include <perf/mmap.h>
+#include <perf/core.h>
+#include <perf/event.h>
+#include <stdio.h>
+#include <unistd.h>
+
+static int libperf_print(enum libperf_print_level level,
+                         const char *fmt, va_list ap)
+{
+	return vfprintf(stderr, fmt, ap);
+}
+
+int main(int argc, char **argv)
+{
+	int count = 100000, err = 0;
+	struct perf_evlist *evlist;
+	struct perf_evsel *evsel;
+	struct perf_thread_map *threads;
+	struct perf_counts_values counts;
+
+	struct perf_event_attr attr1 = {
+		.type        = PERF_TYPE_SOFTWARE,
+		.config      = PERF_COUNT_SW_CPU_CLOCK,
+		.read_format = PERF_FORMAT_TOTAL_TIME_ENABLED|PERF_FORMAT_TOTAL_TIME_RUNNING,
+		.disabled    = 1,
+	};
+	struct perf_event_attr attr2 = {
+		.type        = PERF_TYPE_SOFTWARE,
+		.config      = PERF_COUNT_SW_TASK_CLOCK,
+		.read_format = PERF_FORMAT_TOTAL_TIME_ENABLED|PERF_FORMAT_TOTAL_TIME_RUNNING,
+		.disabled    = 1,
+	};
+
+	libperf_init(libperf_print);
+	threads = perf_thread_map__new_dummy();
+	if (!threads) {
+		fprintf(stderr, "failed to create threads\n");
+		return -1;
+	}
+	perf_thread_map__set_pid(threads, 0, 0);
+	evlist = perf_evlist__new();
+	if (!evlist) {
+		fprintf(stderr, "failed to create evlist\n");
+		goto out_threads;
+	}
+	evsel = perf_evsel__new(&attr1);
+	if (!evsel) {
+		fprintf(stderr, "failed to create evsel1\n");
+		goto out_evlist;
+	}
+	perf_evlist__add(evlist, evsel);
+	evsel = perf_evsel__new(&attr2);
+	if (!evsel) {
+		fprintf(stderr, "failed to create evsel2\n");
+		goto out_evlist;
+	}
+	perf_evlist__add(evlist, evsel);
+	perf_evlist__set_maps(evlist, NULL, threads);
+	err = perf_evlist__open(evlist);
+	if (err) {
+		fprintf(stderr, "failed to open evsel\n");
+		goto out_evlist;
+	}
+	perf_evlist__enable(evlist);
+	while (count--);
+	perf_evlist__disable(evlist);
+	perf_evlist__for_each_evsel(evlist, evsel) {
+		perf_evsel__read(evsel, 0, 0, &counts);
+		fprintf(stdout, "count %llu, enabled %llu, run %llu\n",
+				counts.val, counts.ena, counts.run);
+	}
+	perf_evlist__close(evlist);
+out_evlist:
+	perf_evlist__delete(evlist);
+out_threads:
+	perf_thread_map__put(threads);
+	return err;
+}