linux-kernel - Re: [PATCH] workqueue: fix selection of wake_cpu in kick

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Date: Tue, 16 Apr 2024 13:06:39 -1000
From: Tejun Heo <tj@...nel.org>
To: Sven Schnelle <svens@...ux.ibm.com>
Cc: Lai Jiangshan <jiangshanlai@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] workqueue: fix selection of wake_cpu in kick_pool()

Hello,

On Mon, Apr 15, 2024 at 07:35:49AM +0200, Sven Schnelle wrote:
> @@ -1277,7 +1277,8 @@ static bool kick_pool(struct worker_pool *pool)
>  	    !cpumask_test_cpu(p->wake_cpu, pool->attrs->__pod_cpumask)) {
>  		struct work_struct *work = list_first_entry(&pool->worklist,
>  						struct work_struct, entry);
> -		p->wake_cpu = cpumask_any_distribute(pool->attrs->__pod_cpumask);
> +		p->wake_cpu = cpumask_any_and_distribute(pool->attrs->__pod_cpumask,
> +							 cpu_online_mask);

I think this can still race with the last CPU in the pod going down and
return nr_cpu_ids. Maybe something like the following would be better?

	int wake_cpu;

	wake_cpu = cpumask_any_distribute_and(...);
	if (wake_cpu < nr_cpus_ids) {
		p->wake_cpu = wake_cpu;
		// update stat;
	}

This generally seems like a good idea but isn't this still racy? The CPU may
go down between setting p->wake_cpu and wake_up_process().

Thanks.

-- 
tejun