linux-kernel - Re: [PATCH v11 08/16] sched: Allow task CPU affinity to be restricted on asymmetric systems

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Date:   Wed, 18 Aug 2021 12:56:41 +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>,
        Valentin Schneider <valentin.schneider@....com>,
        Mark Rutland <mark.rutland@....com>, kernel-team@...roid.com
Subject: Re: [PATCH v11 08/16] sched: Allow task CPU affinity to be
 restricted on asymmetric systems

On Wed, Aug 18, 2021 at 11:42:28AM +0100, Will Deacon wrote:
> As for your other suggestion:
> 
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -2733,6 +2733,7 @@ static int __set_cpus_allowed_ptr_locked
> >  	const struct cpumask *cpu_allowed_mask = task_cpu_possible_mask(p);
> >  	const struct cpumask *cpu_valid_mask = cpu_active_mask;
> >  	bool kthread = p->flags & PF_KTHREAD;
> > +	struct cpumask *user_mask = NULL;
> >  	unsigned int dest_cpu;
> >  	int ret = 0;
> >  
> > @@ -2792,9 +2793,13 @@ static int __set_cpus_allowed_ptr_locked
> >  	__do_set_cpus_allowed(p, new_mask, flags);
> >  
> >  	if (flags & SCA_USER)
> > -		release_user_cpus_ptr(p);
> > +		swap(user_mask, p->user_cpus_ptr);
> > +
> > +	ret = affine_move_task(rq, p, rf, dest_cpu, flags);
> > +
> > +	kfree(user_mask);
> >  
> > -	return affine_move_task(rq, p, rf, dest_cpu, flags);
> > +	return ret;
> >  
> >  out:
> >  	task_rq_unlock(rq, p, rf);
> > @@ -2954,8 +2959,10 @@ void relax_compatible_cpus_allowed_ptr(s
> >  		return;
> >  
> >  	raw_spin_lock_irqsave(&p->pi_lock, flags);
> > -	release_user_cpus_ptr(p);
> > +	p->user_cpus_ptr = NULL;
> >  	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
> > +
> > +	kfree(mask);
> 
> I think the idea looks good, but perhaps we could wrap things up a bit:
> 
> /* Comment about why this is useful with RT */
> static cpumask_t *clear_user_cpus_ptr(struct task_struct *p)
> {
> 	struct cpumask *user_mask = NULL;
> 
> 	swap(user_mask, p->user_cpus_ptr);
> 	return user_mask;
> }
> 
> void release_user_cpus_ptr(struct task_struct *p)
> {
> 	kfree(clear_user_cpus_ptr(p));
> }
> 
> Then just use clear_user_cpus_ptr() in sched/core.c where we know what
> we're doing (well, at least one of us does!).

OK, I'll go make it like that.