linux-kernel - Re: [PATCH v7 13/22] sched: Allow task CPU affinity to be restricted on asymmetric systems

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Date:   Thu, 27 May 2021 09:56:30 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Qais Yousef <qais.yousef@....com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Quentin Perret <qperret@...gle.com>, Tejun Heo <tj@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        kernel-team@...roid.com
Subject: Re: [PATCH v7 13/22] sched: Allow task CPU affinity to be restricted
 on asymmetric systems

On Wed, May 26, 2021 at 06:02:06PM +0100, Will Deacon wrote:
> On Wed, May 26, 2021 at 06:30:08PM +0200, Peter Zijlstra wrote:
> > On Tue, May 25, 2021 at 04:14:23PM +0100, Will Deacon wrote:
> > > @@ -2426,20 +2421,166 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
> > >  
> > >  	__do_set_cpus_allowed(p, new_mask, flags);
> > >  
> > > -	return affine_move_task(rq, p, &rf, dest_cpu, flags);
> > > +	if (flags & SCA_USER)
> > > +		release_user_cpus_ptr(p);
> > > +
> > > +	return affine_move_task(rq, p, rf, dest_cpu, flags);
> > >  
> > >  out:
> > > -	task_rq_unlock(rq, p, &rf);
> > > +	task_rq_unlock(rq, p, rf);
> > >  
> > >  	return ret;
> > >  }
> > 
> > So sys_sched_setaffinity() releases the user_cpus_ptr thingy ?! How does
> > that work?
> 
> Right, I think if the task explicitly changes its affinity then it makes
> sense to forget about what it had before. It then behaves very similar to
> CPU hotplug, which is the analogy I've been trying to follow: if you call
> sched_setaffinity() with a mask containing offline CPUs then those CPUs
> are not added back to the affinity mask when they are onlined.

Oh right, crap semantics all the way down :/ I always forget how
horrible they are.

You're right though; this is consistent with the current mess.