linux-kernel - Re: [RESEND PATCH] sched: restore the behavior of put_task

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Message-ID: <20250811104033.GA5250@redhat.com>
Date: Mon, 11 Aug 2025 12:40:34 +0200
From: Oleg Nesterov <oleg@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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>
Subject: Re: [RESEND PATCH] sched: restore the behavior of put_task_struct()
 for non-rt

On 08/11, Sebastian Andrzej Siewior wrote:
>
> I don't want to drag this but this comment is obvious for anyone who is
> fluent in C. It is just a statement with no explanation.
> An important note would be that the atomic context restriction only
> apply to PREEMPT_RT and therefore we have this context override for
> lockdep below. The other question would be why don't we do this
> unconditionally regardless of PREEMPT_RT. The only reason I could find
> is that releasing the task here from the "exit path" makes the vmap
> stack "earlier" available for reuse.

Sorry, could you clarify your "other" question?

What exactly do you think we could do regardless of PREEMPT_RT?

Oleg.

> 
> > +	if (!IS_ENABLED(CONFIG_PREEMPT_RT)) {
> > +		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
> >  	 * in atomic context because it will indirectly
> > @@ -137,10 +150,6 @@ static inline void put_task_struct(struct task_struct *t)
> >  	 * 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 __put_task_struct_rcu_cb()
> >  	 * to be called in process context.
> >  	 *
> > 
> 
> Sebastian
>