linux-kernel - [PATCH v2 2/2] sched/rt: Fix Deadline utilization tracking during policy change

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Message-Id: <1624271872-211872-3-git-send-email-vincent.donnefort@arm.com>
Date:   Mon, 21 Jun 2021 11:37:52 +0100
From:   Vincent Donnefort <vincent.donnefort@....com>
To:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com
Cc:     vincent.guittot@...aro.org, dietmar.eggemann@....com,
        valentin.schneider@....com, rostedt@...dmis.org,
        linux-kernel@...r.kernel.org,
        Vincent Donnefort <vincent.donnefort@....com>
Subject: [PATCH v2 2/2] sched/rt: Fix Deadline utilization tracking during policy change

DL keeps track of the utilization on a per-rq basis with the structure
avg_dl. This utilization is updated during task_tick_dl(),
put_prev_task_dl() and set_next_task_dl(). However, when the current
running task changes its policy, set_next_task_dl() which would usually
take care of updating the utilization when the rq starts running DL
tasks, will not see a such change, leaving the avg_dl structure outdated.
When that very same task will be dequeued later, put_prev_task_dl() will
then update the utilization, based on a wrong last_update_time, leading to
a huge spike in the DL utilization signal.

The signal would eventually recover from this issue after few ms. Even
if no DL tasks are run, avg_dl is also updated in
__update_blocked_others(). But as the CPU capacity depends partly on the
avg_dl, this issue has nonetheless a significant impact on the scheduler.

Fix this issue by ensuring a load update when a running task changes
its policy to DL.

Fixes: 3727e0e ("sched/dl: Add dl_rq utilization tracking")
Signed-off-by: Vincent Donnefort <vincent.donnefort@....com>
Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 3829c5a..915227a 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2497,6 +2497,8 @@ static void switched_to_dl(struct rq *rq, struct task_struct *p)
 			check_preempt_curr_dl(rq, p, 0);
 		else
 			resched_curr(rq);
+	} else {
+		update_dl_rq_load_avg(rq_clock_pelt(rq), rq, 0);
 	}
 }

-- 
2.7.4