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

[Jiri Olsa]

On Tue, Aug 30, 2016 at 09:26:03AM +0200, Peter Zijlstra wrote:
> 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..

yep, it works in my tests

also I thought there's no group time update in __perf_event_read,
so I was hunting that but then I noticed we do that after during
the read..  and meanwhile came to patch below ;-)

jirka


---
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 305dbd28ea86..c637496251fe 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3440,6 +3440,26 @@ static int find_cpu_to_read(struct perf_event *event, 
int local_cpu)
        return event_cpu;
 }
 
+static void read_update_times(struct perf_event *event, bool group)
+{
+       struct perf_event_context *ctx = event->ctx;
+
+       /*
+        * may read while context is not active
+        * (e.g., thread is blocked), in that case
+        * we cannot update context time
+        */
+       if (ctx->is_active) {
+               update_context_time(ctx);
+               update_cgrp_time_from_event(event);
+       }
+
+       if (group)
+               update_group_times(event);
+       else
+               update_event_times(event);
+}
+
 /*
  * Cross CPU call to read the hardware event
  */
@@ -3462,12 +3482,9 @@ static void __perf_event_read(void *info)
                return;
 
        raw_spin_lock(&ctx->lock);
-       if (ctx->is_active) {
-               update_context_time(ctx);
-               update_cgrp_time_from_event(event);
-       }
 
-       update_event_times(event);
+       read_update_times(event, data->group);
+
        if (event->state != PERF_EVENT_STATE_ACTIVE)
                goto unlock;
 
@@ -3482,7 +3499,6 @@ static void __perf_event_read(void *info)
        pmu->read(event);
 
        list_for_each_entry(sub, &event->sibling_list, group_entry) {
-               update_event_times(sub);
                if (sub->state == PERF_EVENT_STATE_ACTIVE) {
                        /*
                         * Use sibling's PMU rather than @event's since
@@ -3596,19 +3612,7 @@ static int perf_event_read(struct perf_event *event, 
bool group)
                unsigned long flags;
 
                raw_spin_lock_irqsave(&ctx->lock, flags);
-               /*
-                * may read while context is not active
-                * (e.g., thread is blocked), in that case
-                * we cannot update context time
-                */
-               if (ctx->is_active) {
-                       update_context_time(ctx);
-                       update_cgrp_time_from_event(event);
-               }
-               if (group)
-                       update_group_times(event);
-               else
-                       update_event_times(event);
+               read_update_times(event, group);
                raw_spin_unlock_irqrestore(&ctx->lock, flags);
        }