linux-kernel - [PATCH] sched/fair: Always update_curr() before placing at enqueue

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Date:   Fri,  6 Oct 2023 12:48:26 -0400
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     peterz@...radead.org
Cc:     bristot@...hat.com, bsegall@...gle.com, chris.hyser@...cle.com,
        corbet@....net, dietmar.eggemann@....com, efault@....de,
        joel@...lfernandes.org, joshdon@...gle.com, juri.lelli@...hat.com,
        kprateek.nayak@....com, linux-kernel@...r.kernel.org,
        mgorman@...e.de, mingo@...nel.org, patrick.bellasi@...bug.net,
        pavel@....cz, pjt@...gle.com, qperret@...gle.com,
        qyousef@...alina.io, rostedt@...dmis.org, tglx@...utronix.de,
        tim.c.chen@...ux.intel.com, timj@....org,
        vincent.guittot@...aro.org, youssefesmat@...omium.org,
        yu.c.chen@...el.com, daniel.m.jordan@...cle.com
Subject: [PATCH] sched/fair: Always update_curr() before placing at enqueue

Placing wants current's vruntime and the cfs_rq's min_vruntime up to
date so that avg_runtime() is too, and similarly it wants the entity to
be re-weighted and lag adjusted so vslice and vlag are fresh, so always
do update_curr() and update_cfs_group() beforehand.

There doesn't seem to be a reason to treat the 'curr' case specially
after e8f331bcc270 since vruntime doesn't get normalized anymore.

Fixes: e8f331bcc270 ("sched/smp: Use lag to simplify cross-runqueue placement")
Signed-off-by: Daniel Jordan <daniel.m.jordan@...cle.com>
---

Not sure what the XXX above place_entity() is for, maybe it can go away?

Based on tip/sched/core.

 kernel/sched/fair.c | 14 ++------------
 1 file changed, 2 insertions(+), 12 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 04fbcbda97d5f..db2ca9bf9cc49 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5047,15 +5047,6 @@ static inline bool cfs_bandwidth_used(void);
 static void
 enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 {
-	bool curr = cfs_rq->curr == se;
-
-	/*
-	 * If we're the current task, we must renormalise before calling
-	 * update_curr().
-	 */
-	if (curr)
-		place_entity(cfs_rq, se, flags);
-
 	update_curr(cfs_rq);
 
 	/*
@@ -5080,8 +5071,7 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 	 * XXX now that the entity has been re-weighted, and it's lag adjusted,
 	 * we can place the entity.
 	 */
-	if (!curr)
-		place_entity(cfs_rq, se, flags);
+	place_entity(cfs_rq, se, flags);
 
 	account_entity_enqueue(cfs_rq, se);
 
@@ -5091,7 +5081,7 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 
 	check_schedstat_required();
 	update_stats_enqueue_fair(cfs_rq, se, flags);
-	if (!curr)
+	if (cfs_rq->curr != se)
 		__enqueue_entity(cfs_rq, se);
 	se->on_rq = 1;
 
-- 
2.41.0