linux-kernel - [V2] perf/core: Avoid calling perf_mux_hrtimer_restart multiple times when scheduling event groups

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Message-Id: <20211015070403.4809-1-atrajeev@linux.vnet.ibm.com>
Date:   Fri, 15 Oct 2021 12:34:03 +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: [V2] 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>
---
Changelog:
>From v1 -> v2:
 - Rebased on top of latest kernel source.

 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 f23ca260307f..9e1a06646ad0 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -3753,8 +3753,10 @@ static int merge_sched_in(struct perf_event *event, void *data)
 			perf_cgroup_event_disable(event, ctx);
 			perf_event_set_state(event, PERF_EVENT_STATE_ERROR);
 		} else {
-			ctx->rotate_necessary = 1;
-			perf_mux_hrtimer_restart(cpuctx);
+			if (!ctx->rotate_necessary) {
+				ctx->rotate_necessary = 1;
+				perf_mux_hrtimer_restart(cpuctx);
+			}
 			group_update_userpage(event);
 		}
 	}
-- 
2.27.0