linux-kernel - Re: [PATCH] sched/ext: Suppress warning in __this_cpu

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Message-ID: <aHfPlm2wXPJQQiQn@gpd4>
Date: Wed, 16 Jul 2025 18:13:10 +0200
From: Andrea Righi <arighi@...dia.com>
To: Breno Leitao <leitao@...ian.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Tejun Heo <tj@...nel.org>,
	David Vernet <void@...ifault.com>,
	Changwoo Min <changwoo@...lia.com>, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>, sched-ext@...ts.linux.dev,
	linux-kernel@...r.kernel.org, kernel-team@...a.com,
	jake@...lion.co.uk
Subject: Re: [PATCH] sched/ext: Suppress warning in __this_cpu_write() by
 disabling preemption

On Wed, Jul 16, 2025 at 09:08:36AM -0700, Breno Leitao wrote:
> On Wed, Jul 16, 2025 at 04:26:23PM +0200, Andrea Righi wrote:
> > On Wed, Jul 16, 2025 at 03:36:31PM +0200, Peter Zijlstra wrote:
> > > On Wed, Jul 16, 2025 at 03:15:12PM +0200, Andrea Righi wrote:
> > > 
> > > > The idea is to track the scx callbacks that are invoked with a rq lock held
> > > > and, in those cases, store the locked rq. However, some callbacks may also
> > > > be invoked from an unlocked context, where no rq is locked and in this case
> > > > rq should be NULL.
> > > > 
> > > > In the latter case, it's acceptable for preemption to remain enabled, but
> > > > we still want to explicitly set locked_rq = NULL. If during the execution
> > > > of the callback we jump on another CPU, it'd still be in an unlocked state,
> > > > so it's locked_rq is still NULL.
> > > 
> > > Right; but doing superfluous NULL stores seems pointless. So better to
> > > avoid the store entirely, rather than making it more expensive and no
> > > less pointless, right?
> > 
> > Right, we can definitely avoid rewriting NULL.
> > The following should do the trick.
> > 
> > Breno, can you give it a try?
> 
> Sure thing. I've tested it and I don't see the warning on my side.
> 
> Would you like to me post the patch, probably removing the WARN_ONCE()
> as raised by peterz?

Sure, go ahead. Yes, let's remove the WARN_ON_ONCE().
And thanks Peter for all the suggestions!

-Andrea