linux-kernel - Re: [PATCH v6] sched: do not call __put_task_struct() on rt if pi_blocked

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Message-ID: <20250728201441.GA4690@redhat.com>
Date: Mon, 28 Jul 2025 22:14:41 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Clark Williams <clrkwllms@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>, Tejun Heo <tj@...nel.org>,
	David Vernet <dvernet@...a.com>, Barret Rhoden <brho@...gle.com>,
	Josh Don <joshdon@...gle.com>, Crystal Wood <crwood@...hat.com>,
	linux-kernel@...r.kernel.org, linux-rt-devel@...ts.linux.dev,
	Juri Lelli <juri.lelli@...hat.com>, Ben Segall <bsegall@...gle.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Ingo Molnar <mingo@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Wander Lairson Costa <wander@...hat.com>, lclaudio00@...il.com
Subject: Re: [PATCH v6] sched: do not call __put_task_struct() on rt if
 pi_blocked_on is set

On 07/07, Luis Claudio R. Goncalves wrote:
>
> Instead of adding more complex conditions to decide when to directly
> call __put_task_struct() and when to defer the call, unconditionally
> resort to the deferred call on PREEMPT_RT to simplify the code.
                              ^^^^^^^^^^^^^
Confused... with this patch put_task_struct() always uses the deferred
call, regardless of PREEMPT_RT?

Oleg.

> --- a/include/linux/sched/task.h
> +++ b/include/linux/sched/task.h
> @@ -135,24 +135,17 @@ static inline void put_task_struct(struct task_struct *t)
>  		return;
>  
>  	/*
> -	 * In !RT, it is always safe to call __put_task_struct().
> -	 * Under RT, we can only call it in preemptible context.
> -	 */
> -	if (!IS_ENABLED(CONFIG_PREEMPT_RT) || preemptible()) {
> -		static DEFINE_WAIT_OVERRIDE_MAP(put_task_map, LD_WAIT_SLEEP);
> -
> -		lock_map_acquire_try(&put_task_map);
> -		__put_task_struct(t);
> -		lock_map_release(&put_task_map);
> -		return;
> -	}
> -
> -	/*
> -	 * under PREEMPT_RT, we can't call put_task_struct
> +	 * Under PREEMPT_RT, we can't call __put_task_struct
>  	 * in atomic context because it will indirectly
> -	 * acquire sleeping locks.
> +	 * acquire sleeping locks. The same is true if the
> +	 * current process has a mutex enqueued (blocked on
> +	 * a PI chain).
> +	 *
> +	 * In !RT, it is always safe to call __put_task_struct().
> +	 * Though, in order to simplify the code, resort to the
> +	 * deferred call too.
>  	 *
> -	 * call_rcu() will schedule delayed_put_task_struct_rcu()
> +	 * call_rcu() will schedule __put_task_struct_rcu_cb()
>  	 * to be called in process context.
>  	 *
>  	 * __put_task_struct() is called when
> @@ -165,7 +158,7 @@ static inline void put_task_struct(struct task_struct *t)
>  	 *
>  	 * delayed_free_task() also uses ->rcu, but it is only called
>  	 * when it fails to fork a process. Therefore, there is no
> -	 * way it can conflict with put_task_struct().
> +	 * way it can conflict with __put_task_struct().
>  	 */
>  	call_rcu(&t->rcu, __put_task_struct_rcu_cb);
>  }
> -- 
> 2.50.0
>