linux-kernel - Re: [RFC PATCH v15 7/7] sched: Start blocked_on chain processing in find_proxy

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Message-ID: <20250317164725.GC6888@noisy.programming.kicks-ass.net>
Date: Mon, 17 Mar 2025 17:47:25 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: John Stultz <jstultz@...gle.com>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Joel Fernandes <joelagnelf@...dia.com>,
	Qais Yousef <qyousef@...alina.io>, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Valentin Schneider <vschneid@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>,
	Zimuzo Ezeozue <zezeozue@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>,
	Boqun Feng <boqun.feng@...il.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Metin Kaya <Metin.Kaya@....com>,
	Xuewen Yan <xuewen.yan94@...il.com>,
	K Prateek Nayak <kprateek.nayak@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Suleiman Souhlal <suleiman@...gle.com>, kernel-team@...roid.com,
	Valentin Schneider <valentin.schneider@....com>,
	Connor O'Brien <connoro@...gle.com>
Subject: Re: [RFC PATCH v15 7/7] sched: Start blocked_on chain processing in
 find_proxy_task()

On Wed, Mar 12, 2025 at 03:11:37PM -0700, John Stultz wrote:
> @@ -6791,6 +6890,7 @@ static void __sched notrace __schedule(int sched_mode)
>  	struct rq_flags rf;
>  	struct rq *rq;
>  	int cpu;
> +	bool preserve_need_resched = false;
>  
>  	cpu = smp_processor_id();
>  	rq = cpu_rq(cpu);
> @@ -6858,9 +6958,12 @@ static void __sched notrace __schedule(int sched_mode)
>  		next = find_proxy_task(rq, next, &rf);
>  		if (!next)
>  			goto pick_again;
> +		if (next == rq->idle)
> +			preserve_need_resched = true;
>  	}
>  picked:
> -	clear_tsk_need_resched(prev);
> +	if (!preserve_need_resched)
> +		clear_tsk_need_resched(prev);
>  	clear_preempt_need_resched();
>  #ifdef CONFIG_SCHED_DEBUG
>  	rq->last_seen_need_resched_ns = 0;

I would really rather this was done like so:


--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6957,12 +6944,12 @@ static void __sched notrace __schedule(i
 		if (!next)
 			goto pick_again;
 		if (next == rq->idle)
-			preserve_need_resched = true;
+			goto keep_resched;
 	}
 picked:
-	if (!preserve_need_resched)
-		clear_tsk_need_resched(prev);
+	clear_tsk_need_resched(prev);
 	clear_preempt_need_resched();
+keep_resched:
 #ifdef CONFIG_SCHED_DEBUG
 	rq->last_seen_need_resched_ns = 0;
 #endif