linux-kernel - Re: [PATCH 2/2] sched/fair: Reimplement NEXT

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Message-ID: <20251028154734.GS4068168@noisy.programming.kicks-ass.net>
Date: Tue, 28 Oct 2025 16:47:34 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Mel Gorman <mgorman@...hsingularity.net>
Cc: linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Valentin Schneider <vschneid@...hat.com>,
	Chris Mason <clm@...a.com>
Subject: Re: [PATCH 2/2] sched/fair: Reimplement NEXT_BUDDY to align with
 EEVDF goals

On Tue, Oct 28, 2025 at 04:33:51PM +0100, Peter Zijlstra wrote:
> On Mon, Oct 27, 2025 at 01:39:15PM +0000, Mel Gorman wrote:
> > @@ -8783,7 +8862,7 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
> >  		 * When non-idle entity preempt an idle entity,
> >  		 * don't give idle entity slice protection.
> >  		 */
> > -		do_preempt_short = true;
> > +		do_preempt_short = PREEMPT_WAKEUP_NEXT;
> >  		goto preempt;
> >  	}
> 
> I'm confused, should this not be WAKEUP_RESCHED?

It doesn't matter, you cannot end up with !do_preempt_short at preempt:,
so that condition is always true and can thus be deleted, at which point
the value of do_preempt_short is irrelevant and doesn't need to be set.

A little something like so perhaps...

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8727,7 +8727,7 @@ static void set_next_buddy(struct sched_
 }
 
 enum preempt_wakeup_action {
-	PREEMPT_WAKEUP_NONE,		/* No action on the buddy */
+	PREEMPT_WAKEUP_NONE = 0,	/* No action on the buddy */
 	PREEMPT_WAKEUP_NEXT,		/* Check next is most eligible
 					 * before rescheduling.
 					 */
@@ -8814,7 +8814,7 @@ __do_preempt_buddy(struct rq *rq, struct
  */
 static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int wake_flags)
 {
-	enum preempt_wakeup_action do_preempt_short = PREEMPT_WAKEUP_NONE;
+	enum preempt_wakeup_action preempt_action = PREEMPT_WAKEUP_NONE;
 	struct task_struct *donor = rq->donor;
 	struct sched_entity *se = &donor->se, *pse = &p->se;
 	struct cfs_rq *cfs_rq = task_cfs_rq(donor);
@@ -8863,7 +8863,6 @@ static void check_preempt_wakeup_fair(st
 		 * When non-idle entity preempt an idle entity,
 		 * don't give idle entity slice protection.
 		 */
-		do_preempt_short = PREEMPT_WAKEUP_NEXT;
 		goto preempt;
 	}
 
@@ -8883,17 +8882,17 @@ static void check_preempt_wakeup_fair(st
 	 * current's slice protection in order to allow preemption.
 	 */
 	if (sched_feat(PREEMPT_SHORT) && (pse->slice < se->slice)) {
-		do_preempt_short = PREEMPT_WAKEUP_NEXT;
+		preempt_action = PREEMPT_WAKEUP_NEXT;
 	} else {
 		/*
 		 * If @p potentially is completing work required by current then
 		 * consider preemption.
 		 */
-		do_preempt_short = __do_preempt_buddy(rq, cfs_rq, wake_flags,
+		preempt_action = __do_preempt_buddy(rq, cfs_rq, wake_flags,
 						      pse, se);
 	}
 
-	switch (do_preempt_short) {
+	switch (preempt_action) {
 	case PREEMPT_WAKEUP_NONE:
 		return;
 	case PREEMPT_WAKEUP_RESCHED:
@@ -8905,18 +8904,16 @@ static void check_preempt_wakeup_fair(st
 	/*
 	 * If @p has become the most eligible task, force preemption.
 	 */
-	if (__pick_eevdf(cfs_rq, !do_preempt_short) == pse)
+	if (__pick_eevdf(cfs_rq, false) == pse)
 		goto preempt;
 
-	if (sched_feat(RUN_TO_PARITY) && do_preempt_short != PREEMPT_WAKEUP_NONE)
+	if (sched_feat(RUN_TO_PARITY))
 		update_protect_slice(cfs_rq, se);
 
 	return;
 
 preempt:
-	if (do_preempt_short)
-		cancel_protect_slice(se);
-
+	cancel_protect_slice(se);
 	resched_curr_lazy(rq);
 }