linux-kernel - Re: [PATCH rcu 13/14] workqueue: Make queue_rcu_work() use call_rcu

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Date:   Sun, 23 Oct 2022 20:36:00 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, rostedt@...dmis.org,
        Uladzislau Rezki <urezki@...il.com>
Subject: Re: [PATCH rcu 13/14] workqueue: Make queue_rcu_work() use call_rcu_flush()

Hello,

On Wed, Oct 19, 2022 at 6:51 PM Paul E. McKenney <paulmck@...nel.org> wrote:
>
> From: Uladzislau Rezki <urezki@...il.com>
>
> call_rcu() changes to save power will slow down RCU workqueue items
> queued via queue_rcu_work(). This may not be an issue, however we cannot
> assume that workqueue users are OK with long delays. Use
> call_rcu_flush() API instead which reverts to the old behavio

On ChromeOS, I can see that queue_rcu_work() is pretty noisy and the
batching is much better if we can just keep it as call_rcu() instead
of call_rcu_flush().

Is there really any reason to keep it as call_rcu_flush() ?  If I
recall, the real reason Vlad's system was slowing down was because of
scsi and the queue_rcu_work() conversion was really a red herring.

Vlad, any thoughts?

thanks,

 - Joel

.
>
> Signed-off-by: Uladzislau Rezki <urezki@...il.com>
> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> ---
>  kernel/workqueue.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 7cd5f5e7e0a1b..b4b0e828b529e 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -1771,7 +1771,7 @@ bool queue_rcu_work(struct workqueue_struct *wq, struct rcu_work *rwork)
>
>         if (!test_and_set_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(work))) {
>                 rwork->wq = wq;
> -               call_rcu(&rwork->rcu, rcu_work_rcufn);
> +               call_rcu_flush(&rwork->rcu, rcu_work_rcufn);
>                 return true;
>         }
>
> --
> 2.31.1.189.g2e36527f23
>