linux-kernel - [PATCH v2] sched/fair: Preserve PLACE_DEADLINE

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Date:   Wed,  4 Oct 2023 09:09:08 -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 v2] sched/fair: Preserve PLACE_DEADLINE_INITIAL deadline

An entity is supposed to get an earlier deadline with
PLACE_DEADLINE_INITIAL when it's forked, but the deadline gets
overwritten soon after in enqueue_entity() the first time a forked
entity is woken so that PLACE_DEADLINE_INITIAL is effectively a no-op.

Placing in task_fork_fair() seems unnecessary since none of the values
that get set (slice, vruntime, deadline) are used before they're set
again at enqueue time, so get rid of that (and with it all of
task_fork_fair()) and just pass ENQUEUE_INITIAL to enqueue_entity() via
wake_up_new_task().

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

v2
 - place_entity() seems like the only reason for task_fork_fair() to exist
   after the recent removal of sysctl_sched_child_runs_first, so take out
   the whole function.

Still based on today's peterz/sched/eevdf

 kernel/sched/core.c |  2 +-
 kernel/sched/fair.c | 24 ------------------------
 2 files changed, 1 insertion(+), 25 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 779cdc7969c81..500e2dbfd41dd 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4854,7 +4854,7 @@ void wake_up_new_task(struct task_struct *p)
 	update_rq_clock(rq);
 	post_init_entity_util_avg(p);
 
-	activate_task(rq, p, ENQUEUE_NOCLOCK);
+	activate_task(rq, p, ENQUEUE_INITIAL | ENQUEUE_NOCLOCK);
 	trace_sched_wakeup_new(p);
 	wakeup_preempt(rq, p, WF_FORK);
 #ifdef CONFIG_SMP
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a0b4dac2662c9..3827b302eeb9b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -12427,29 +12427,6 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)
 	task_tick_core(rq, curr);
 }
 
-/*
- * called on fork with the child task as argument from the parent's context
- *  - child not yet on the tasklist
- *  - preemption disabled
- */
-static void task_fork_fair(struct task_struct *p)
-{
-	struct sched_entity *se = &p->se, *curr;
-	struct cfs_rq *cfs_rq;
-	struct rq *rq = this_rq();
-	struct rq_flags rf;
-
-	rq_lock(rq, &rf);
-	update_rq_clock(rq);
-
-	cfs_rq = task_cfs_rq(current);
-	curr = cfs_rq->curr;
-	if (curr)
-		update_curr(cfs_rq);
-	place_entity(cfs_rq, se, ENQUEUE_INITIAL);
-	rq_unlock(rq, &rf);
-}
-
 /*
  * Priority of the task has changed. Check to see if we preempt
  * the current task.
@@ -12953,7 +12930,6 @@ DEFINE_SCHED_CLASS(fair) = {
 #endif
 
 	.task_tick		= task_tick_fair,
-	.task_fork		= task_fork_fair,
 
 	.prio_changed		= prio_changed_fair,
 	.switched_from		= switched_from_fair,
-- 
2.41.0