[PATCH] perf cgroups: Don't rotate events for cgroups unnecessarily

[Ian Rogers]

Currently perf_rotate_context assumes that if the context's nr_events !=
nr_active a rotation is necessary for perf event multiplexing. With
cgroups, nr_events is the total count of events for all cgroups and
nr_active will not include events in a cgroup other than the current
task's. This makes rotation appear necessary for cgroups when it is not.

Add a perf_event_context flag that is set when rotation is necessary.
Clear the flag during sched_out and set it when a flexible sched_in
fails due to resources.

Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
---
 include/linux/perf_event.h |  5 +++++
 kernel/events/core.c       | 42 +++++++++++++++++++++++---------------
 2 files changed, 30 insertions(+), 17 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 15a82ff0aefe..7ab6c251aa3d 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -747,6 +747,11 @@ struct perf_event_context {
 	int				nr_stat;
 	int				nr_freq;
 	int				rotate_disable;
+	/*
+	 * Set when nr_events != nr_active, except tolerant to events not
+	 * needing to be active due to scheduling constraints, such as cgroups.
+	 */
+	int				rotate_necessary;
 	refcount_t			refcount;
 	struct task_struct		*task;
 
diff --git a/kernel/events/core.c b/kernel/events/core.c
index abbd4b3b96c2..41ae424b9f1d 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -2952,6 +2952,12 @@ 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)
@@ -3325,6 +3331,15 @@ static int flexible_sched_in(struct perf_event *event, void *data)
 			sid->can_add_hw = 0;
 	}
 
+	/*
+	 * If the group wasn't scheduled then set that multiplexing is necessary
+	 * for the context. Note, this won't be set if the event wasn't
+	 * scheduled due to event_filter_match failing due to the earlier
+	 * return.
+	 */
+	if (event->state == PERF_EVENT_STATE_INACTIVE)
+		sid->ctx->rotate_necessary = 1;
+
 	return 0;
 }
 
@@ -3690,24 +3705,17 @@ ctx_first_active(struct perf_event_context *ctx)
 static bool perf_rotate_context(struct perf_cpu_context *cpuctx)
 {
 	struct perf_event *cpu_event = NULL, *task_event = NULL;
-	bool cpu_rotate = false, task_rotate = false;
-	struct perf_event_context *ctx = NULL;
+	struct perf_event_context *task_ctx = NULL;
+	int cpu_rotate, task_rotate;
 
 	/*
 	 * Since we run this from IRQ context, nobody can install new
 	 * events, thus the event count values are stable.
 	 */
 
-	if (cpuctx->ctx.nr_events) {
-		if (cpuctx->ctx.nr_events != cpuctx->ctx.nr_active)
-			cpu_rotate = true;
-	}
-
-	ctx = cpuctx->task_ctx;
-	if (ctx && ctx->nr_events) {
-		if (ctx->nr_events != ctx->nr_active)
-			task_rotate = true;
-	}
+	cpu_rotate = cpuctx->ctx.rotate_necessary;
+	task_ctx = cpuctx->task_ctx;
+	task_rotate = task_ctx ? task_ctx->rotate_necessary : 0;
 
 	if (!(cpu_rotate || task_rotate))
 		return false;
@@ -3716,7 +3724,7 @@ static bool perf_rotate_context(struct perf_cpu_context *cpuctx)
 	perf_pmu_disable(cpuctx->ctx.pmu);
 
 	if (task_rotate)
-		task_event = ctx_first_active(ctx);
+		task_event = ctx_first_active(task_ctx);
 	if (cpu_rotate)
 		cpu_event = ctx_first_active(&cpuctx->ctx);
 
@@ -3724,17 +3732,17 @@ static bool perf_rotate_context(struct perf_cpu_context *cpuctx)
 	 * As per the order given at ctx_resched() first 'pop' task flexible
 	 * and then, if needed CPU flexible.
 	 */
-	if (task_event || (ctx && cpu_event))
-		ctx_sched_out(ctx, cpuctx, EVENT_FLEXIBLE);
+	if (task_event || (task_ctx && cpu_event))
+		ctx_sched_out(task_ctx, cpuctx, EVENT_FLEXIBLE);
 	if (cpu_event)
 		cpu_ctx_sched_out(cpuctx, EVENT_FLEXIBLE);
 
 	if (task_event)
-		rotate_ctx(ctx, task_event);
+		rotate_ctx(task_ctx, task_event);
 	if (cpu_event)
 		rotate_ctx(&cpuctx->ctx, cpu_event);
 
-	perf_event_sched_in(cpuctx, ctx, current);
+	perf_event_sched_in(cpuctx, task_ctx, current);
 
 	perf_pmu_enable(cpuctx->ctx.pmu);
 	perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
-- 
2.22.0.rc1.257.g3120a18244-goog