Re: [PATCH] perf/core: Fix endless multiplex timer

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Mar 04, 2020 at 09:20:42AM -0500, Liang, Kan wrote:
> 
> NMI watchdog is pinned event.
> ctx_event_to_rotate() will only pick an event from the flexible_groups.
> So the cpu_ctx_sched_out() in perf_rotate_context() will never be called.

Surely that's fixable; same principle.

---
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 3f1f77de7247..595fb3decd43 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2182,6 +2182,7 @@ __perf_remove_from_context(struct perf_event *event,
 
 	if (!ctx->nr_events && ctx->is_active) {
 		ctx->is_active = 0;
+		ctx->rotate_necessary = 0;
 		if (ctx->task) {
 			WARN_ON_ONCE(cpuctx->task_ctx != ctx);
 			cpuctx->task_ctx = NULL;
@@ -3077,12 +3078,6 @@ static void ctx_sched_out(struct perf_event_context *ctx,
 	if (!ctx->nr_active || !(is_active & EVENT_ALL))
 		return;
 
-	/*
-	 * If we had been multiplexing, no rotations are necessary, now no events
-	 * are active.
-	 */
-	ctx->rotate_necessary = 0;
-
 	perf_pmu_disable(ctx->pmu);
 	if (is_active & EVENT_PINNED) {
 		list_for_each_entry_safe(event, tmp, &ctx->pinned_active, active_list)
@@ -3092,6 +3087,13 @@ static void ctx_sched_out(struct perf_event_context *ctx,
 	if (is_active & EVENT_FLEXIBLE) {
 		list_for_each_entry_safe(event, tmp, &ctx->flexible_active, active_list)
 			group_sched_out(event, cpuctx, ctx);
+
+		/*
+		 * Since we cleared EVENT_FLEXIBLE, also clear
+		 * rotate_necessary, is will be reset by
+		 * ctx_flexible_sched_in() when needed.
+		 */
+		ctx->rotate_necessary = 0;
 	}
 	perf_pmu_enable(ctx->pmu);
 }
@@ -3841,6 +3843,12 @@ ctx_event_to_rotate(struct perf_event_context *ctx)
 				      typeof(*event), group_node);
 	}
 
+	/*
+	 * Unconditionally clear rotate_necessary; if ctx_flexible_sched_in()
+	 * finds there are unschedulable events, it will set it again.
+	 */
+	ctx->rotate_necessary = 0;
+
 	return event;
 }