Re: [tip:perf/core] perf/core: Check return value of the perf_event_read() IPI

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Aug 30, 2016 at 08:47:24AM +0200, Peter Zijlstra wrote:

> If oncpu is not valid, the sched_out that made it invalid will have
> updated the event count and we're good.
> 
> All I'll leave is an explicit comment that we've ignored the
> smp_call_function_single() return value on purpose.

Something like so..

---
 kernel/events/core.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 3f07e6cfc1b6..a35cbc382b2c 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3576,12 +3576,21 @@ static int perf_event_read(struct perf_event *event, bool group)
 
 		local_cpu = get_cpu();
 		cpu_to_read = find_cpu_to_read(event, local_cpu);
+
+		/*
+		 * 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(cpu_to_read, __perf_event_read, &data, 1);
 		put_cpu();
 
-		ret = smp_call_function_single(cpu_to_read, __perf_event_read, &data, 1);
-		/* The event must have been read from an online CPU: */
-		WARN_ON_ONCE(ret);
-		ret = ret ? : data.ret;
+		ret = data.ret;
 	} else if (event->state == PERF_EVENT_STATE_INACTIVE) {
 		struct perf_event_context *ctx = event->ctx;
 		unsigned long flags;