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Message-ID: <ZR0vHXDeGi+iVogR@gmail.com>
Date: Wed, 4 Oct 2023 11:23:41 +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 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
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