linux-kernel - Re: [PATCH v6 3/5] sched: Enforce user requested affinity

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Message-ID: <Yw8mcbgYThZGpMfN@hirez.programming.kicks-ass.net>
Date:   Wed, 31 Aug 2022 11:14:25 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     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>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH v6 3/5] sched: Enforce user requested affinity

On Thu, Aug 25, 2022 at 09:01:17PM -0400, Waiman Long wrote:

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index ac2b103d69dc..1c2f548e5369 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2928,11 +2928,40 @@ static int __set_cpus_allowed_ptr_locked(struct task_struct *p,
>  static int __set_cpus_allowed_ptr(struct task_struct *p,
>  				  const struct cpumask *new_mask, u32 flags)
>  {
> +	struct cpumask *alloc_mask = NULL;
>  	struct rq_flags rf;
>  	struct rq *rq;
> +	int ret;
>  
>  	rq = task_rq_lock(p, &rf);
> -	return __set_cpus_allowed_ptr_locked(p, new_mask, flags, rq, &rf);
> +	if (p->user_cpus_ptr) {
> +
> +		/*
> +		 * A scratch cpumask is allocated on the percpu runqueues
> +		 * to enable additional masking with user_cpus_ptr. This
> +		 * cpumask, once allocated, will not be freed.
> +		 */
> +		if (unlikely(!rq->scratch_mask)) {
> +			alloc_mask = kmalloc(cpumask_size(), GFP_ATOMIC);

This -- absolutely not. You can't have allocations under a
raw_spinlock_t.