perf/core: Fix endless multiplex timer

commit 90c91df upstream.

Kan and Andi reported that we fail to kill rotation when the flexible
events go empty, but the context does not. XXX moar

Fixes: fd7d551 ("perf/cgroups: Don't rotate events for cgroups unnecessarily")
Reported-by: Andi Kleen <ak@linux.intel.com>
Reported-by: Kan Liang <kan.liang@linux.intel.com>
Tested-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200305123851.GX2596@hirez.programming.kicks-ass.net
Cc: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

kernel/events/core.c

@@ -2171,6 +2171,7 @@ __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;
@@ -3047,12 +3048,6 @@ 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)
@@ -3062,6 +3057,13 @@ 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);
 }
@@ -3800,6 +3802,12 @@ 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;
 }