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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date:   Wed, 4 Oct 2023 12:06:29 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Waiman Long <longman@redhat.com>, Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        linux-kernel@vger.kernel.org, Phil Auld <pauld@redhat.com>,
        Brent Rowsell <browsell@redhat.com>,
        Peter Hunt <pehunt@redhat.com>,
        Florian Weimer <fweimer@redhat.com>
Subject: Re: [PATCH v4] sched/core: Use zero length to reset cpumasks in
 sched_setaffinity()
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* Peter Zijlstra <peterz@infradead.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