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

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Message-ID: <Z_af4PF18ocF0bBW@uudg.org>
Date: Wed, 9 Apr 2025 13:27:12 -0300
From: "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Clark Williams <clrkwllms@...nel.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>
Subject: Re: [PATCH] sched: do not call __put_task_struct() on rt if
 pi_blocked_on is set

On Wed, Apr 09, 2025 at 11:43:30AM -0400, Steven Rostedt wrote:
> On Tue, 8 Apr 2025 21:27:37 -0300
> "Luis Claudio R. Goncalves" <lgoncalv@...hat.com> wrote:
> 
> >  {
> > +	bool defer = false;
> > +
> >  	if (!refcount_dec_and_test(&t->usage))
> >  		return;
> >  
> >  	/*
> >  	 * In !RT, it is always safe to call __put_task_struct().
> > -	 * Under RT, we can only call it in preemptible context.
> > +	 * Under RT, we can only call it in preemptible context,
> > +	 * when not blocked on a PI chain.
> >  	 */
> > -	if (!IS_ENABLED(CONFIG_PREEMPT_RT) || preemptible()) {
> > +#ifdef CONFIG_PREEMPT_RT
> > +	if (!preemptible() || current->pi_blocked_on)
> > +		defer = true;
> > +#endif
> 
> Why add the ugly #ifdef back?
> 
> 	if (!IS_ENABLED(CONFIG_PREEMPT_RT) ||
> 	    (preemptible() && !current->pi_blocked_on)) {

I had to add the #ifdef to avoid the build failing if CONFIG_RT_MUTEXES is
not set. I do know SMP, FUTEX, I2C, PSTORE and a few more things depend on
CONFIG_RT_MUTEXES being enabled, but I opted for being thorough.

I would be more than glad getting rid of the #ifdef and simplifying the patch
if that possible build failure is not a case to worry about, if RT_MUTEXES are
always enabled nowadays.

Luis

> -- Steve
> 
> > +
> > +	if (!defer) {
> >  		static DEFINE_WAIT_OVERRIDE_MAP(put_task_map, LD_WAIT_SLEEP);
> >  
> >  
> 
---end quoted text---