linux-kernel - Re: [PATCH v4] sched/core: Use zero length to reset cpumasks in sched

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Date:   Wed, 4 Oct 2023 12:06:29 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Waiman Long <longman@...hat.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>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        linux-kernel@...r.kernel.org, Phil Auld <pauld@...hat.com>,
        Brent Rowsell <browsell@...hat.com>,
        Peter Hunt <pehunt@...hat.com>,
        Florian Weimer <fweimer@...hat.com>
Subject: Re: [PATCH v4] sched/core: Use zero length to reset cpumasks in
 sched_setaffinity()


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Wed, Oct 04, 2023 at 11:23:41AM +0200, Ingo Molnar wrote:
> 
> > >  	if (user_mask) {
> > > -		cpumask_copy(user_mask, in_mask);
> > > +		/*
> > > +		 * All-set user cpumask resets affinity and drops the explicit
> > > +		 * user mask.
> > > +		 */
> > > +		cpumask_and(user_mask, in_mask, cpu_possible_mask);
> > > +		if (cpumask_equal(user_mask, cpu_possible_mask)) {
> > > +			kfree(user_mask);
> > > +			user_mask = NULL;
> > > +		}
> > 
> > Question: is there any observable behavioral difference between current 
> > (old) all-set cpumask calls and the patched (new) one?
> 
> Very little I think -- the main difference is that we no longer carry
> the ->user_cpus_ptr mask around, and that saves a little masking.

So calling with a full mask would actually work fine on 'old' kernels too,
as it's a 'reset' event in essence. (With a bit of allocation & masking
overhead.)

This pretty unambiguously marks the full-mask solution as the superior ABI ...

Thanks,

	Ingo