linux-kernel - [PATCH v2 09/10] sched/fair: stop load tracking when task switched_from

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Message-Id: <20220713040430.25778-10-zhouchengming@bytedance.com>
Date:   Wed, 13 Jul 2022 12:04:29 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     mingo@...hat.com, peterz@...radead.org, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        vschneid@...hat.com
Cc:     linux-kernel@...r.kernel.org,
        Chengming Zhou <zhouchengming@...edance.com>
Subject: [PATCH v2 09/10] sched/fair: stop load tracking when task switched_from_fair()

The same reason as the previous commit, if we don't reset the
sched_avg last_update_time to 0, after a while in switched_to_fair():

switched_to_fair
  attach_task_cfs_rq
    attach_entity_cfs_rq
      update_load_avg
        __update_load_avg_se(now, cfs_rq, se)

The delta (now - sa->last_update_time) will wrongly contribute/decay
sched_avg depends on the task running/runnable status at that time.

This patch reset it's sched_avg last_update_time to 0, stop load
tracking for !fair task, later in switched_to_fair() ->
update_load_avg(), we can use its saved sched_avg.

Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
---
 kernel/sched/fair.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 50f65a2ede32..576028f5a09e 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11552,6 +11552,11 @@ static void attach_task_cfs_rq(struct task_struct *p)
 static void switched_from_fair(struct rq *rq, struct task_struct *p)
 {
 	detach_task_cfs_rq(p);
+
+#ifdef CONFIG_SMP
+	/* Stop load tracking for !fair task */
+	p->se.avg.last_update_time = 0;
+#endif
 }
 
 static void switched_to_fair(struct rq *rq, struct task_struct *p)
-- 
2.36.1