linux-kernel - Re: [PATCH -v3 5/9] workqueue: Tag bound workers with KTHREAD_IS_PER

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Message-ID: <jhja6t2mkef.mognet@arm.com>
Date:   Thu, 21 Jan 2021 14:31:20 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
        tglx@...utronix.de
Cc:     linux-kernel@...r.kernel.org, jiangshanlai@...il.com,
        cai@...hat.com, vincent.donnefort@....com, decui@...rosoft.com,
        paulmck@...nel.org, vincent.guittot@...aro.org,
        rostedt@...dmis.org, tj@...nel.org, peterz@...radead.org
Subject: Re: [PATCH -v3 5/9] workqueue: Tag bound workers with KTHREAD_IS_PER_CPU

On 21/01/21 11:17, Peter Zijlstra wrote:
> @@ -4972,9 +4977,11 @@ static void rebind_workers(struct worker
>  	 * of all workers first and then clear UNBOUND.  As we're called
>  	 * from CPU_ONLINE, the following shouldn't fail.
>  	 */
> -	for_each_pool_worker(worker, pool)
> +	for_each_pool_worker(worker, pool) {
> +		kthread_set_per_cpu(worker->task, pool->cpu);
>  		WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task,
>  						  pool->attrs->cpumask) < 0);

At the end of this series, is_cpu_allowed() allows kthreads with
KTHREAD_IS_PER_CPU on any online CPU, even if it isn't the designated
kthread->cpu.

I thought there might be a race here, given this gives us a window where a
pcpu kworker has the flag but is still affined to cpus_possible_mask. Now,
given cpus_write_lock(), we can't have a CPU going up while another goes
down. So I think it's actually fine, and I've been chasing ghosts yet again.

> +	}
>  
>  	raw_spin_lock_irq(&pool->lock);
>