perf record --off-cpu: Parse off-cpu event

Parse the off-cpu event using parse_event(), as bpf-output.

Call evlist__enable_evsel() on off-cpu event. This fixes the inability
to collect direct off-cpu samples on a workload, as reported by Arnaldo
Carvalho de Melo <[email protected]>.

The reason being, workload sets enable_on_exec instead of calling
evlist__enable(), but off-cpu event does not attach to an executable and
execve won't be called, so the fds from perf_event_open() are not
enabled.

no-inherit should be set to 1, here's the reason:

We update the BPF perf_event map for direct off-cpu sample dumping (in
following patches), it executes as follows:

bpf_map_update_value()
 bpf_fd_array_map_update_elem()
  perf_event_fd_array_get_ptr()
   perf_event_read_local()

In perf_event_read_local(), there is:

int perf_event_read_local(struct perf_event *event, u64 *value,
			  u64 *enabled, u64 *running)
{
...
	/*
	 * It must not be an event with inherit set, we cannot read
	 * all child counters from atomic context.
	 */
	if (event->attr.inherit) {
		ret = -EOPNOTSUPP;
		goto out;
	}

Which means no-inherit has to be true for updating the BPF perf_event
map.

Moreover, for bpf-output events, we primarily want a system-wide event
instead of a per-task event.

The reason is that in BPF's bpf_perf_event_output(), BPF uses the CPU
index to retrieve the perf_event file descriptor it outputs to.

Making a bpf-output event system-wide naturally satisfies this
requirement by mapping CPU appropriately.

Suggested-by: Namhyung Kim <[email protected]>
Reviewed-by: Ian Rogers <[email protected]>
Signed-off-by: Howard Chu <[email protected]>
Tested-by: Arnaldo Carvalho de Melo <[email protected]>
Tested-by: Gautam Menghani <[email protected]>
Tested-by: Ian Rogers <[email protected]>
Acked-by: Namhyung Kim <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: James Clark <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

0 files changed, 0 insertions, 0 deletions