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Date:   Wed, 4 Oct 2023 11:43:30 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ingo Molnar <mingo@kernel.org>
Cc:     Waiman Long <longman@redhat.com>, Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
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        Brent Rowsell <browsell@redhat.com>,
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Subject: Re: [PATCH v4] sched/core: Use zero length to reset cpumasks in
 sched_setaffinity()
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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.