perf/core: Remove WARN from perf_event_read()

This effectively reverts commit:

  71e7bc2bab77 ("perf/core: Check return value of the perf_event_read() IPI")

... and puts in a comment explaining why we ignore the return value.

Reported-by: Vegard Nossum <vegard.nossum@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: David Carrillo-Cisneros <davidcc@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 71e7bc2bab77 ("perf/core: Check return value of the perf_event_read() IPI")
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 3cfabdf..07ac859 100644 (file)
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3549,10 +3549,18 @@ static int perf_event_read(struct perf_event *event, bool group)
                        .group = group,
                        .ret = 0,
                };
-               ret = smp_call_function_single(event->oncpu, __perf_event_read, &data, 1);
-               /* The event must have been read from an online CPU: */
-               WARN_ON_ONCE(ret);
-               ret = ret ? : data.ret;
+               /*
+                * Purposely ignore the smp_call_function_single() return
+                * value.
+                *
+                * If event->oncpu isn't a valid CPU it means the event got
+                * scheduled out and that will have updated the event count.
+                *
+                * Therefore, either way, we'll have an up-to-date event count
+                * after this.
+                */
+               (void)smp_call_function_single(event->oncpu, __perf_event_read, &data, 1);
+               ret = data.ret;
        } else if (event->state == PERF_EVENT_STATE_INACTIVE) {
                struct perf_event_context *ctx = event->ctx;
                unsigned long flags;