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

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Message-ID: <imrfubmkw3a6qdznnpounrnen5ituzchwtbjmouocuk77upn67@ljrz32ppyqyr>
Date: Wed, 16 Jul 2025 09:08:36 -0700
From: Breno Leitao <leitao@...ian.org>
To: Andrea Righi <arighi@...dia.com>
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 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?

Thanks
--breno