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)
