linux-kernel - Re: [PATCH v23 5/9] sched: Add logic to zap balance callbacks if we pick again

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Message-ID: <90e556a4-59d3-4be4-b4a8-5e7ef4b523d1@amd.com>
Date: Thu, 30 Oct 2025 13:38:23 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: John Stultz <jstultz@...gle.com>, LKML <linux-kernel@...r.kernel.org>
CC: Joel Fernandes <joelagnelf@...dia.com>, Qais Yousef <qyousef@...alina.io>,
	Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, "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>, Thomas Gleixner <tglx@...utronix.de>, "Daniel
 Lezcano" <daniel.lezcano@...aro.org>, Suleiman Souhlal <suleiman@...gle.com>,
	kuyo chang <kuyo.chang@...iatek.com>, hupu <hupu.gm@...il.com>,
	<kernel-team@...roid.com>
Subject: Re: [PATCH v23 5/9] sched: Add logic to zap balance callbacks if we
 pick again

Hello John,

On 10/30/2025 5:48 AM, John Stultz wrote:
> +#ifdef CONFIG_SCHED_PROXY_EXEC
> +/*
> + * Only called from __schedule context
> + *
> + * There are some cases where we are going to re-do the action
> + * that added the balance callbacks. We may not be in a state
> + * where we can run them, so just zap them so they can be
> + * properly re-added on the next time around. This is similar
> + * handling to running the callbacks, except we just don't call
> + * them.
> + */
> +static void zap_balance_callbacks(struct rq *rq)
> +{
> +	struct balance_callback *next, *head;
> +	bool found = false;
> +
> +	lockdep_assert_rq_held(rq);
> +
> +	head = rq->balance_callback;
> +	while (head) {
> +		if (head == &balance_push_callback)
> +			found = true;
> +		next = head->next;
> +		head->next = NULL;
> +		head = next;
> +	}
> +	rq->balance_callback = found ? &balance_push_callback : NULL;
> +}

There is nothing proxy-exec specific in this function. Do we need to
keep it behind CONFIG_SCHED_PROXY_EXEC?

I believe compiler will optimize out the dead code and having
zap_balance_callbacks() unconditionally shouldn't have make any
difference to the size of generated binary for
!CONFIG_SCHED_PROXY_EXEC case.

Apart from that nit. feel free to include:

Reviewed-by: K Prateek Nayak <kprateek.nayak@....com>

> +#else
> +static inline void zap_balance_callbacks(struct rq *rq) {}
> +#endif
> +
>  static void do_balance_callbacks(struct rq *rq, struct balance_callback *head)
>  {
>  	void (*func)(struct rq *rq);
> @@ -6901,10 +6933,15 @@ static void __sched notrace __schedule(int sched_mode)
>  	rq_set_donor(rq, next);
>  	if (unlikely(task_is_blocked(next))) {
>  		next = find_proxy_task(rq, next, &rf);
> -		if (!next)
> +		if (!next) {
> +			/* zap the balance_callbacks before picking again */
> +			zap_balance_callbacks(rq);
>  			goto pick_again;
> -		if (next == rq->idle)
> +		}
> +		if (next == rq->idle) {
> +			zap_balance_callbacks(rq);

Also I would have preferred to have that zap_balance_callbacks() in
proxy_resched_idle() but this is okay too.

-- 
Thanks and Regards,
Prateek

>  			goto keep_resched;
> +		}
>  	}
>  picked:
>  	clear_tsk_need_resched(prev);