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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date:   Wed, 4 Oct 2023 11:23:41 +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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 <20231004083648.GI27267@noisy.programming.kicks-ass.net>
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* Peter Zijlstra <peterz@infradead.org> wrote:

> On Tue, Oct 03, 2023 at 04:57:35PM -0400, Waiman Long wrote:
> > Since commit 8f9ea86fdf99 ("sched: Always preserve the user requested
> > cpumask"), user provided CPU affinity via sched_setaffinity(2) is
> > perserved even if the task is being moved to a different cpuset. However,
> > that affinity is also being inherited by any subsequently created child
> > processes which may not want or be aware of that affinity.
> > 
> > One way to solve this problem is to provide a way to back off from that
> > user provided CPU affinity.  This patch implements such a scheme by
> > using an input cpumask length of 0 to signal a reset of the cpumasks
> > to the default as allowed by the current cpuset.  A non-NULL cpumask
> > should still be provided to avoid problem with older kernel.
> > 
> > If sched_setaffinity(2) has been called previously to set a user
> > supplied cpumask, a value of 0 will be returned to indicate success.
> > Otherwise, an error value of -EINVAL will be returned.
> > 
> > We may have to update the sched_setaffinity(2) manpage to document
> > this new side effect of passing in an input length of 0.
> 
> Bah.. so while this is less horrible than some of the previous hacks,
> but I still think an all set mask is the sanest option.
> 
> Adding FreeBSD's CPU_FILL() to glibc() isn't the hardest thing ever, but
> even without that, it's a single memset() away.
> 
> 
> Would not the below two patches, one kernel, one glibc, be all it takes?

I'd much prefer this ABI variant, it's a pretty natural extension of the 
existing ABI and principles:

>  	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?

Thanks,

	Ingo