linux-kernel - Re: [PATCH v2] sched: Remove a preempt-disable section in rt_mutex

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Message-ID: <20250611090306.GA2273038@noisy.programming.kicks-ass.net>
Date: Wed, 11 Jun 2025 11:03:06 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: linux-kernel@...r.kernel.org, Ben Segall <bsegall@...gle.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
	Mel Gorman <mgorman@...e.de>, Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleinxer <tglx@...utronix.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH v2] sched: Remove a preempt-disable section in
 rt_mutex_setprio()

On Tue, Jun 10, 2025 at 04:47:00PM +0200, Sebastian Andrzej Siewior wrote:
> rt_mutex_setprio() has only one caller: rt_mutex_adjust_prio(). It
> expects that task_struct::pi_lock and rt_mutex_base::wait_lock are held.
> Both locks are raw_spinlock_t and are acquired with disabled interrupts.
> 
> Nevertheless rt_mutex_setprio() disables preemption while invoking
> __balance_callbacks() and raw_spin_rq_unlock(). Even if one of the
> balance callbacks unlocks the rq then it must not enable interrupts
> because rt_mutex_base::wait_lock is still locked.
> Therefore interrupts should remain disabled and disabling preemption is
> not needed.
> 
> Commit 4c9a4bc89a9cc ("sched: Allow balance callbacks for check_class_changed()")
> adds a preempt-disable section to rt_mutex_setprio() and
> __sched_setscheduler(). In __sched_setscheduler() the preemption is
> disabled before rq is unlocked and interrupts enabled but I don't see
> why it makes a difference in rt_mutex_setprio().
> 
> Remove the preempt_disable() section from rt_mutex_setprio().
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
> 
> v1…v2: https://lore.kernel.org/all/20250513142954.ZM5QSQNc@linutronix.de/
>   - Repost without RFC
> 
>  kernel/sched/core.c | 4 ----
>  1 file changed, 4 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index dce50fa57471d..362d8ab888748 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7292,14 +7292,10 @@ void rt_mutex_setprio(struct task_struct *p, struct task_struct *pi_task)
>  
>  	check_class_changed(rq, p, prev_class, oldprio);
>  out_unlock:
> -	/* Avoid rq from going away on us: */
> -	preempt_disable();

Perhaps add:

	/* IRQs are still disabled */

or something to that effect such that it is obvious from reading the
code that dropping the lock will not enable preemption?

>  
>  	rq_unpin_lock(rq, &rf);
>  	__balance_callbacks(rq);
>  	raw_spin_rq_unlock(rq);
> -
> -	preempt_enable();
>  }
>  #endif
>  
> -- 
> 2.49.0
>