Re: [PATCH] perf/core: fix corner case in perf_rotate_context()

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Oct 02, 2019 at 11:43:17PM -0700, Song Liu wrote:
> This is a rare corner case, but it does happen:
> 
> In perf_rotate_context(), when the first cpu flexible event fail to
> schedule, cpu_rotate is 1, while cpu_event is NULL.

You're failing to explain how we get here in the first place.

> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 4655adbbae10..50021735f367 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3775,6 +3775,13 @@ static void rotate_ctx(struct perf_event_context *ctx, struct perf_event *event)
>  	if (ctx->rotate_disable)
>  		return;
>  
> +	/* if no event specified, try to rotate the first event */
> +	if (!event)
> +		event = rb_entry_safe(rb_first(&ctx->flexible_groups.tree),
> +				      typeof(*event), group_node);
> +	if (!event)
> +		return;
> +
>  	perf_event_groups_delete(&ctx->flexible_groups, event);
>  	perf_event_groups_insert(&ctx->flexible_groups, event);
>  }

I don't think that is a very nice solution; would not something simpler
like this be sufficient? (although possible we should rename that
function now).

And it needs more comments.

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 4655adbbae10..772a9854eb3a 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3782,8 +3797,17 @@ static void rotate_ctx(struct perf_event_context *ctx, struct perf_event *event)
 static inline struct perf_event *
 ctx_first_active(struct perf_event_context *ctx)
 {
-	return list_first_entry_or_null(&ctx->flexible_active,
-					struct perf_event, active_list);
+	struct perf_event *event;
+
+	event = list_first_entry_or_null(&ctx->flexible_active,
+					 struct perf_event, active_list);
+
+	if (!event) {
+		event = rb_entry_safe(rb_first(&ctx->flexible_groups.tree),
+				      typeof(*event), group_node);
+	}
+
+	return event;
 }
 
 static bool perf_rotate_context(struct perf_cpu_context *cpuctx)