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 18:20:25 +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 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;
	}

?

> +
> +	rq = task_rq_lock(p, &rf);
> +
> +	/*
> +	 * Forcefully restricting the affinity of a deadline task is
> +	 * likely to cause problems, so fail and noisily override the
> +	 * mask entirely.
> +	 */
> +	if (task_has_dl_policy(p) && dl_bandwidth_enabled()) {
> +		err = -EPERM;
> +		goto err_unlock;
> +	}
> +
> +	if (!cpumask_and(new_mask, &p->cpus_mask, subset_mask)) {
> +		err = -EINVAL;
> +		goto err_unlock;
> +	}
> +
> +	/*
> +	 * We're about to butcher the task affinity, so keep track of what
> +	 * the user asked for in case we're able to restore it later on.
> +	 */
> +	if (user_mask) {
> +		cpumask_copy(user_mask, p->cpus_ptr);
> +		p->user_cpus_ptr = user_mask;
> +	}
> +
> +	return __set_cpus_allowed_ptr_locked(p, new_mask, 0, rq, &rf);
> +
> +err_unlock:
> +	task_rq_unlock(rq, p, &rf);
> +	kfree(user_mask);
> +	return err;
> +}