linux-kernel - Re: [PATCH -tip V3 0/8] workqueue: break affinity initiatively

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Date:   Tue, 12 Jan 2021 12:33:03 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, Qian Cai <cai@...hat.com>,
        Vincent Donnefort <vincent.donnefort@....com>,
        Dexuan Cui <decui@...rosoft.com>,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Paul McKenney <paulmck@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH -tip V3 0/8] workqueue: break affinity initiatively

> Well yes, but afaict the workqueue stuff hasn't been settled yet, and
> the rcutorture patch Paul did was just plain racy and who knows what
> other daft kthread users are out there. That and we're at -rc3.

I just send the V4 patchset for the workqueue.  Please take a look.

> @@ -1861,6 +1861,8 @@ static void worker_attach_to_pool(struct worker *worker,
>         */
>         if (pool->flags & POOL_DISASSOCIATED)
>                 worker->flags |= WORKER_UNBOUND;
> +       else
> +               kthread_set_per_cpu(worker->task, true);

I think kthread_set_per_cpu(worker->task, false) is also needed for
worker_detach_from_pool() or at least rescuer_thread() who doesn't
go to die after detached from the pool.

> I thought only pcpu pools would get the POOL_DISASSOCIATED flag on
> offline, but it seems unbound pools also get it at init time. Did I get
> that right?

You are right.

The POOL_DISASSOCIATED flag indicates whether the pool is concurrency
management or not (negative way, POOL_DISASSOCIATED means "not concurrency
management"). So it should be applied for all unbound pools.

When !POOL_DISASSOCIATED means it is a percpu pool, and the pool->cpu
is online and the offline callback has not been called yet even the pool->cpu
is going to be offline.  So !POOL_DISASSOCIATED is used a lot in the code.