linux-kernel - [PATCH] perf/core: Avoid calling perf_mux_hrtimer_restart multiple times when scheduling event groups

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Message-Id: <20210930131547.1923-1-atrajeev@linux.vnet.ibm.com>
Date:   Thu, 30 Sep 2021 18:45:47 +0530
From:   Athira Rajeev <atrajeev@...ux.vnet.ibm.com>
To:     peterz@...radead.org, acme@...nel.org, jolsa@...nel.org
Cc:     linux-kernel@...r.kernel.org, songliubraving@...com,
        maddy@...ux.vnet.ibm.com, kjain@...ux.ibm.com, mingo@...hat.com,
        eranian@...gle.com
Subject: [PATCH] perf/core: Avoid calling perf_mux_hrtimer_restart multiple times when scheduling event groups

Perf uses multiplexing if there are more events to be scheduled than the
available counters. With multiplexing, event groups will be rotated at
specified interval of time which is set using "hrtimer". During event
scheduling, if any of the event group fails to schedule, multiplexing
gets enabled by setting "rotate_necessary" for that event context and
starting the hrtimer by calling "perf_mux_hrtimer_restart".

Patch adds an optimisation to avoid calling "perf_mux_hrtimer_restart"
multiple times if already rotate_necessary is set for that context.
Even though "perf_mux_hrtimer_restart" will just return if hrtimer is
already active, this could avoid the overhead of calling this function
multiple times if there are many event groups. Patch adds the check to
avoid calling perf_mux_hrtimer_restart() for each event group on
every schedule.

Signed-off-by: Athira Rajeev <atrajeev@...ux.vnet.ibm.com>
---
 kernel/events/core.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 0c000cb01eeb..26eae79bd723 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3731,8 +3731,10 @@ static int merge_sched_in(struct perf_event *event, void *data)
 		}

 		*can_add_hw = 0;
-		ctx->rotate_necessary = 1;
-		perf_mux_hrtimer_restart(cpuctx);
+		if (!ctx->rotate_necessary) {
+			ctx->rotate_necessary = 1;
+			perf_mux_hrtimer_restart(cpuctx);
+		}
 	}

 	return 0;
-- 
2.30.1 (Apple Git-130)