linux-kernel - [PATCH 1/2] perf: Don't re-schedule cpu flexible events needlessly

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Date:   Thu, 19 Jan 2017 18:43:29 +0200
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        vince@...ter.net, eranian@...gle.com,
        Arnaldo Carvalho de Melo <acme@...radead.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Subject: [PATCH 1/2] perf: Don't re-schedule cpu flexible events needlessly

In the sched-in path, we first remove cpu's flexible events in order to
give priority to task's pinned events. However, this step can be safely
skipped if the task doesn't have its own pinned events.

This patch implements this skipping.

Signed-off-by: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Reported-by: Adrian Hunter <adrian.hunter@...el.com>
---
 kernel/events/core.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index dedfd3078c..c196b9537f 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3137,8 +3137,12 @@ static void perf_event_context_sched_in(struct perf_event_context *ctx,
 	 * We want to keep the following priority order:
 	 * cpu pinned (that don't need to move), task pinned,
 	 * cpu flexible, task flexible.
+	 *
+	 * However, if task's ctx is not carrying any pinned
+	 * events, no need to flip the cpuctx's events around.
 	 */
-	cpu_ctx_sched_out(cpuctx, EVENT_FLEXIBLE);
+	if (!list_empty(&ctx->pinned_groups))
+		cpu_ctx_sched_out(cpuctx, EVENT_FLEXIBLE);
 	perf_event_sched_in(cpuctx, ctx, task);
 	perf_pmu_enable(ctx->pmu);
 	perf_ctx_unlock(cpuctx, ctx);
-- 
2.11.0