linux-kernel - [PATCH] timers/nohz: Update nohz load even if tick already stopped

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Message-Id: <20191028150716.22890-1-frederic@kernel.org>
Date:   Mon, 28 Oct 2019 16:07:16 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, Scott Wood <swood@...hat.com>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: [PATCH] timers/nohz: Update nohz load even if tick already stopped

From: Scott Wood <swood@...hat.com>

The way loadavg is tracked during nohz only pays attention to the load
upon entering nohz. This can be particularly noticeable if nohz is
entered while non-idle, and then the cpu goes idle and stays that way for
a long time. We've had reports of a loadavg near 150 on a mostly idle
system.

Calling calc_load_nohz_start() regardless of whether the tick is already
stopped addresses the issue when going idle. Tracking load changes when
not going idle (e.g. multiple SCHED_FIFO tasks coming and going) is not
addressed by this patch.

Signed-off-by: Scott Wood <swood@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
---
 kernel/time/tick-sched.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index c2748232f607..f55ddeb652a3 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -763,6 +763,9 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
 		ts->do_timer_last = 0;
 	}
 
+	/* Even if the tick was already stopped, load may have changed */
+	calc_load_nohz_start();
+
 	/* Skip reprogram of event if its not changed */
 	if (ts->tick_stopped && (expires == ts->next_tick)) {
 		/* Sanity check: make sure clockevent is actually programmed */
@@ -783,7 +786,6 @@ static void tick_nohz_stop_tick(struct tick_sched *ts, int cpu)
 	 * the scheduler tick in nohz_restart_sched_tick.
 	 */
 	if (!ts->tick_stopped) {
-		calc_load_nohz_start();
 		quiet_vmstat();
 
 		ts->last_tick = hrtimer_get_expires(&ts->sched_timer);
-- 
2.23.0