linux-kernel - Re: [PATCH v2] workqueue: set the rescuer worker that belong to freezable wq is freezable

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Message-ID: <CAJhGHyCNXEPEb0t9KLXafx8TPntjWFFH-T9uFUTsuGAQxVcZTw@mail.gmail.com>
Date:   Wed, 23 Jun 2021 11:16:40 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     "Zhang, Qiang" <qiang.zhang@...driver.com>
Cc:     Tejun Heo <tj@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] workqueue: set the rescuer worker that belong to
 freezable wq is freezable

On Tue, Jun 22, 2021 at 8:42 PM <qiang.zhang@...driver.com> wrote:
>
> From: Zqiang <qiang.zhang@...driver.com>
>
> If the rescuer worker belong to freezable wq, when this wq is
> frozen, the rescuer worker also need to be frozen.

Hello,

>From my understanding, the wq subsystem doesn't use set_freezable(),
try_to_freeze() families.  For example, the worker_thread() doesn't
use set_freezable(), neither do in your patch. If kthreads
in the wq subsystem need these functions, please let us know.

The way the wq subsystem to freeze tasks is by freeze_workqueues_begin()
and thaw_workqueues().  When WQs is requested for freezing,
they will stop activating work items.  All the workers including
*rescuers* will be scheduled as TASK_IDLE when all the activated work
items are processed and the whole wq subsystem is frozen unless
there is any semantic change about TASK_IDLE V.S. "frozen".
(rescuers can also only process activated work items.)

Thanks
Lai

>
> Signed-off-by: Zqiang <qiang.zhang@...driver.com>
> ---
>  v1->v2:
>  use kthread_freezable_should_stop() replaces try_to_freeze().
>
>  kernel/workqueue.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 104e3ef04e33..dd7b78bdb7f1 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -2476,6 +2476,9 @@ static int rescuer_thread(void *__rescuer)
>          * doesn't participate in concurrency management.
>          */
>         set_pf_worker(true);
> +
> +       if (wq->flags & WQ_FREEZABLE)
> +               set_freezable();
>  repeat:
>         set_current_state(TASK_IDLE);
>
> @@ -2487,7 +2490,7 @@ static int rescuer_thread(void *__rescuer)
>          * @wq->maydays processing before acting on should_stop so that the
>          * list is always empty on exit.
>          */
> -       should_stop = kthread_should_stop();
> +       should_stop = kthread_freezable_should_stop(NULL);
>
>         /* see whether any pwq is asking for help */
>         raw_spin_lock_irq(&wq_mayday_lock);
> --
> 2.25.1
>