linux-kernel - [PATCH v5 1/8] perf evsel: Set off-cpu BPF output to system-wide

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Message-ID: <20240927185340.658143-2-howardchu95@gmail.com>
Date: Fri, 27 Sep 2024 11:53:33 -0700
From: Howard Chu <howardchu95@...il.com>
To: peterz@...radead.org
Cc: mingo@...hat.com,
	acme@...nel.org,
	namhyung@...nel.org,
	mark.rutland@....com,
	alexander.shishkin@...ux.intel.com,
	jolsa@...nel.org,
	irogers@...gle.com,
	adrian.hunter@...el.com,
	kan.liang@...ux.intel.com,
	linux-perf-users@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Howard Chu <howardchu95@...il.com>
Subject: [PATCH v5 1/8] perf evsel: Set off-cpu BPF output to system-wide

pid = -1 for off-cpu's bpf-output event.

This makes 'perf record -p <PID> --off-cpu', and 'perf record --off-cpu
<workload>' work. Otherwise bpf-output cannot be collected.

The reason (conjecture): say if we open perf_event on pid = 11451, then
in BPF, we call bpf_perf_event_output() when a direct sample is ready to
be dumped. But currently the perf_event of pid 11451 is not __fully__
sched_in yet, so in kernel/trace/bpf_trace.c's
__bpf_perf_event_output(), there will be event->oncpu != cpu, thus
return -EOPNOTSUPP, direct off-cpu sample output failed.

if (unlikely(event->oncpu != cpu))
		return -EOPNOTSUPP;

So I'm making it pid = -1, everybody can do bpf_perf_event_output()

P.S. In perf trace this is not necessary, because it uses syscall
tracepoints, instead of sched_switch.

Signed-off-by: Howard Chu <howardchu95@...il.com>
---
 tools/perf/util/evsel.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index edfb376f0611..500ca62669cb 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -2368,6 +2368,9 @@ static int evsel__open_cpu(struct evsel *evsel, struct perf_cpu_map *cpus,

 			test_attr__ready();

+			if (evsel__is_offcpu_event(evsel))
+				pid = -1;
+
 			/* Debug message used by test scripts */
 			pr_debug2_peo("sys_perf_event_open: pid %d  cpu %d  group_fd %d  flags %#lx",
 				pid, perf_cpu_map__cpu(cpus, idx).cpu, group_fd, evsel->open_flags);
-- 
2.43.0