linux-kernel - Re: [PATCH] kthread_worker: re-set CPU affinities if CPU come online

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Message-ID: <20201026135011.GC73258@mtj.duckdns.org>
Date:   Mon, 26 Oct 2020 09:50:11 -0400
From:   Tejun Heo <tj@...nel.org>
To:     qiang.zhang@...driver.com
Cc:     pmladek@...e.com, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] kthread_worker: re-set CPU affinities if CPU come online

On Mon, Oct 26, 2020 at 02:52:13PM +0800, qiang.zhang@...driver.com wrote:
> @@ -737,8 +741,11 @@ __kthread_create_worker(int cpu, unsigned int flags,
>  	if (IS_ERR(task))
>  		goto fail_task;
>  
> -	if (cpu >= 0)
> +	if (cpu >= 0) {
>  		kthread_bind(task, cpu);
> +		worker->bind_cpu = cpu;
> +		cpuhp_state_add_instance_nocalls(kworker_online, &worker->cpuhp_node);
> +	}
>  
>  	worker->flags = flags;
>  	worker->task = task;
...
> +static int kworker_cpu_online(unsigned int cpu, struct hlist_node *node)
> +{
> +	struct kthread_worker *worker = hlist_entry(node, struct kthread_worker, cpuhp_node);
> +	struct task_struct *task = worker->task;
> +
> +	if (cpu == worker->bind_cpu)
> +		WARN_ON_ONCE(set_cpus_allowed_ptr(task, cpumask_of(cpu)) < 0);
> +	return 0;
> +}

I don't think this works. The kthread may have changed its binding while
running using set_cpus_allowed_ptr() as you're doing above. Besides, when a
cpu goes offline, the bound kthread can fall back to other cpus but its cpu
mask isn't cleared, is it?

Thanks.

-- 
tejun