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>

1 files changed, 12 insertions, 4 deletions

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 3cfabdf7b942..07ac8596a728 100644
--- 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;