perf record: Enable off-cpu analysis with BPF

Add --off-cpu option to enable the off-cpu profiling with BPF.  It'd
use a bpf_output event and rename it to "offcpu-time".  Samples will
be synthesized at the end of the record session using data from a BPF
map which contains the aggregated off-cpu time at context switches.
So it needs root privilege to get the off-cpu profiling.

Each sample will have a separate user stacktrace so it will skip
kernel threads.  The sample ip will be set from the stacktrace and
other sample data will be updated accordingly.  Currently it only
handles some basic sample types.

The sample timestamp is set to a dummy value just not to bother with
other events during the sorting.  So it has a very big initial value
and increase it on processing each samples.

Good thing is that it can be used together with regular profiling like
cpu cycles.  If you don't want to that, you can use a dummy event to
enable off-cpu profiling only.

Example output:
  $ sudo perf record --off-cpu perf bench sched messaging -l 1000

  $ sudo perf report --stdio --call-graph=no
  # Total Lost Samples: 0
  #
  # Samples: 41K of event 'cycles'
  # Event count (approx.): 42137343851
  ...

  # Samples: 1K of event 'offcpu-time'
  # Event count (approx.): 587990831640
  #
  # Children      Self  Command          Shared Object       Symbol
  # ........  ........  ...............  ..................  .........................
  #
      81.66%     0.00%  sched-messaging  libc-2.33.so        [.] __libc_start_main
      81.66%     0.00%  sched-messaging  perf                [.] cmd_bench
      81.66%     0.00%  sched-messaging  perf                [.] main
      81.66%     0.00%  sched-messaging  perf                [.] run_builtin
      81.43%     0.00%  sched-messaging  perf                [.] bench_sched_messaging
      40.86%    40.86%  sched-messaging  libpthread-2.33.so  [.] __read
      37.66%    37.66%  sched-messaging  libpthread-2.33.so  [.] __write
       2.91%     2.91%  sched-messaging  libc-2.33.so        [.] __poll
  ...

As you can see it spent most of off-cpu time in read and write in
bench_sched_messaging().  The --call-graph=no was added just to make
the output concise here.

It uses perf hooks facility to control BPF program during the record
session rather than adding new BPF/off-cpu specific calls.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Ian Rogers <irogers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Blake Jones <blakejones@google.com>
Cc: Hao Luo <haoluo@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: bpf@vger.kernel.org
Link: https://lore.kernel.org/r/20220518224725.742882-3-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

1 files changed, 204 insertions, 0 deletions

diff --git a/tools/perf/util/bpf_off_cpu.c b/tools/perf/util/bpf_off_cpu.c
new file mode 100644
index 000000000000..9ed7aca3f4ac
--- /dev/null
+++ b/tools/perf/util/bpf_off_cpu.c
@@ -0,0 +1,204 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "util/bpf_counter.h"
+#include "util/debug.h"
+#include "util/evsel.h"
+#include "util/evlist.h"
+#include "util/off_cpu.h"
+#include "util/perf-hooks.h"
+#include "util/session.h"
+#include <bpf/bpf.h>
+
+#include "bpf_skel/off_cpu.skel.h"
+
+#define MAX_STACKS  32
+/* we don't need actual timestamp, just want to put the samples at last */
+#define OFF_CPU_TIMESTAMP  (~0ull << 32)
+
+static struct off_cpu_bpf *skel;
+
+struct off_cpu_key {
+	u32 pid;
+	u32 tgid;
+	u32 stack_id;
+	u32 state;
+};
+
+union off_cpu_data {
+	struct perf_event_header hdr;
+	u64 array[1024 / sizeof(u64)];
+};
+
+static int off_cpu_config(struct evlist *evlist)
+{
+	struct evsel *evsel;
+	struct perf_event_attr attr = {
+		.type	= PERF_TYPE_SOFTWARE,
+		.config = PERF_COUNT_SW_BPF_OUTPUT,
+		.size	= sizeof(attr), /* to capture ABI version */
+	};
+	char *evname = strdup(OFFCPU_EVENT);
+
+	if (evname == NULL)
+		return -ENOMEM;
+
+	evsel = evsel__new(&attr);
+	if (!evsel) {
+		free(evname);
+		return -ENOMEM;
+	}
+
+	evsel->core.attr.freq = 1;
+	evsel->core.attr.sample_period = 1;
+	/* off-cpu analysis depends on stack trace */
+	evsel->core.attr.sample_type = PERF_SAMPLE_CALLCHAIN;
+
+	evlist__add(evlist, evsel);
+
+	free(evsel->name);
+	evsel->name = evname;
+
+	return 0;
+}
+
+static void off_cpu_start(void *arg __maybe_unused)
+{
+	skel->bss->enabled = 1;
+}
+
+static void off_cpu_finish(void *arg __maybe_unused)
+{
+	skel->bss->enabled = 0;
+	off_cpu_bpf__destroy(skel);
+}
+
+int off_cpu_prepare(struct evlist *evlist)
+{
+	int err;
+
+	if (off_cpu_config(evlist) < 0) {
+		pr_err("Failed to config off-cpu BPF event\n");
+		return -1;
+	}
+
+	set_max_rlimit();
+
+	skel = off_cpu_bpf__open_and_load();
+	if (!skel) {
+		pr_err("Failed to open off-cpu BPF skeleton\n");
+		return -1;
+	}
+
+	err = off_cpu_bpf__attach(skel);
+	if (err) {
+		pr_err("Failed to attach off-cpu BPF skeleton\n");
+		goto out;
+	}
+
+	if (perf_hooks__set_hook("record_start", off_cpu_start, NULL) ||
+	    perf_hooks__set_hook("record_end", off_cpu_finish, NULL)) {
+		pr_err("Failed to attach off-cpu skeleton\n");
+		goto out;
+	}
+
+	return 0;
+
+out:
+	off_cpu_bpf__destroy(skel);
+	return -1;
+}
+
+int off_cpu_write(struct perf_session *session)
+{
+	int bytes = 0, size;
+	int fd, stack;
+	u64 sample_type, val, sid = 0;
+	struct evsel *evsel;
+	struct perf_data_file *file = &session->data->file;
+	struct off_cpu_key prev, key;
+	union off_cpu_data data = {
+		.hdr = {
+			.type = PERF_RECORD_SAMPLE,
+			.misc = PERF_RECORD_MISC_USER,
+		},
+	};
+	u64 tstamp = OFF_CPU_TIMESTAMP;
+
+	skel->bss->enabled = 0;
+
+	evsel = evlist__find_evsel_by_str(session->evlist, OFFCPU_EVENT);
+	if (evsel == NULL) {
+		pr_err("%s evsel not found\n", OFFCPU_EVENT);
+		return 0;
+	}
+
+	sample_type = evsel->core.attr.sample_type;
+
+	if (sample_type & (PERF_SAMPLE_ID | PERF_SAMPLE_IDENTIFIER)) {
+		if (evsel->core.id)
+			sid = evsel->core.id[0];
+	}
+
+	fd = bpf_map__fd(skel->maps.off_cpu);
+	stack = bpf_map__fd(skel->maps.stacks);
+	memset(&prev, 0, sizeof(prev));
+
+	while (!bpf_map_get_next_key(fd, &prev, &key)) {
+		int n = 1;  /* start from perf_event_header */
+		int ip_pos = -1;
+
+		bpf_map_lookup_elem(fd, &key, &val);
+
+		if (sample_type & PERF_SAMPLE_IDENTIFIER)
+			data.array[n++] = sid;
+		if (sample_type & PERF_SAMPLE_IP) {
+			ip_pos = n;
+			data.array[n++] = 0;  /* will be updated */
+		}
+		if (sample_type & PERF_SAMPLE_TID)
+			data.array[n++] = (u64)key.pid << 32 | key.tgid;
+		if (sample_type & PERF_SAMPLE_TIME)
+			data.array[n++] = tstamp;
+		if (sample_type & PERF_SAMPLE_ID)
+			data.array[n++] = sid;
+		if (sample_type & PERF_SAMPLE_CPU)
+			data.array[n++] = 0;
+		if (sample_type & PERF_SAMPLE_PERIOD)
+			data.array[n++] = val;
+		if (sample_type & PERF_SAMPLE_CALLCHAIN) {
+			int len = 0;
+
+			/* data.array[n] is callchain->nr (updated later) */
+			data.array[n + 1] = PERF_CONTEXT_USER;
+			data.array[n + 2] = 0;
+
+			bpf_map_lookup_elem(stack, &key.stack_id, &data.array[n + 2]);
+			while (data.array[n + 2 + len])
+				len++;
+
+			/* update length of callchain */
+			data.array[n] = len + 1;
+
+			/* update sample ip with the first callchain entry */
+			if (ip_pos >= 0)
+				data.array[ip_pos] = data.array[n + 2];
+
+			/* calculate sample callchain data array length */
+			n += len + 2;
+		}
+		/* TODO: handle more sample types */
+
+		size = n * sizeof(u64);
+		data.hdr.size = size;
+		bytes += size;
+
+		if (perf_data_file__write(file, &data, size) < 0) {
+			pr_err("failed to write perf data, error: %m\n");
+			return bytes;
+		}
+
+		prev = key;
+		/* increase dummy timestamp to sort later samples */
+		tstamp++;
+	}
+	return bytes;
+}