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

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Date:   Wed, 26 May 2021 17:35:25 +0100
From:   Will Deacon <will@...nel.org>
To:     Peter Zijlstra <peterz@...radead.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 06:20:25PM +0200, Peter Zijlstra wrote:
> On Tue, May 25, 2021 at 04:14:23PM +0100, Will Deacon wrote:
> > +static int restrict_cpus_allowed_ptr(struct task_struct *p,
> > +				     struct cpumask *new_mask,
> > +				     const struct cpumask *subset_mask)
> > +{
> > +	struct rq_flags rf;
> > +	struct rq *rq;
> > +	int err;
> > +	struct cpumask *user_mask = NULL;
> > +
> > +	if (!p->user_cpus_ptr) {
> > +		user_mask = kmalloc(cpumask_size(), GFP_KERNEL);
> 
> 		if (!user_mask)
> 			return -ENOMEM;
> 	}
> 
> ?

We won't blow up if we continue without user_mask here, but I agree that
it's more straightforward to return an error and have
force_compatible_cpus_allowed_ptr() noisily override the mask.

We're in pretty deep trouble if we're failing this allocation anyway.

Will