[tip: perf/core] perf/core: Fix endless multiplex timer

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the perf/core branch of tip:

Commit-ID:     90c91dfb86d0ff545bd329d3ddd72c147e2ae198
Gitweb:        https://git.kernel.org/tip/90c91dfb86d0ff545bd329d3ddd72c147e2ae198
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Thu, 05 Mar 2020 13:38:51 +01:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 20 Mar 2020 13:06:22 +01:00

perf/core: Fix endless multiplex timer

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

Fixes: fd7d55172d1e ("perf/cgroups: Don't rotate events for cgroups unnecessarily")
Reported-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Reported-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
Tested-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20200305123851.GX2596@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
 kernel/events/core.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index ccf8d4f..b5a68d2 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2291,6 +2291,7 @@ __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;
@@ -3188,12 +3189,6 @@ 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)
@@ -3203,6 +3198,13 @@ 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);
 }
@@ -3985,6 +3987,12 @@ 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;
 }