linux-kernel - Re: [PATCH v1] net/dev.c : Remove redundant state settings after waking up

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Date:   Wed, 11 Jan 2023 10:20:58 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Wei Wang <weiwan@...gle.com>, Eric Dumazet <edumazet@...gle.com>,
        李哲 <sensor1010@....com>, davem@...emloft.net,
        pabeni@...hat.com, imagedong@...cent.com, kuniyu@...zon.com,
        petrm@...dia.com, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] net/dev.c : Remove redundant state settings after
 waking up

On Wed, 11 Jan 2023 08:32:42 +0100 Sebastian Andrzej Siewior wrote:
> It made sense in the beginning but now the suggested patch is a clean
> up. First the `woken' parameter was added in commit
>    cb038357937ee ("net: fix race between napi kthread mode and busy poll")
> 
> and then the `napi_disable_pending' check was removed in commit
>    27f0ad71699de ("net: fix hangup on napi_disable for threaded napi")
> 
> which renders the code to:
> |         while (!kthread_should_stop()) {
> |                 if (test_bit(NAPI_STATE_SCHED_THREADED, &napi->state) || woken) {
> |                         WARN_ON(!list_empty(&napi->poll_list));
> |                         __set_current_state(TASK_RUNNING);
> |                         return 0;
> |                 }
> | 
> |                 schedule();
> |                 /* woken being true indicates this thread owns this napi. */
> |                 woken = true;
> |                 set_current_state(TASK_INTERRUPTIBLE);
> |         }
> |         __set_current_state(TASK_RUNNING);
> 
> so when you get out of schedule() woken is set and even if
> NAPI_STATE_SCHED_THREADED is not set, the while() loop is left due to
> `woken = true'. So changing state to TASK_INTERRUPTIBLE makes no sense
> since it will be set back to TASK_RUNNING cycles later.

Ah, fair point, forgot about the woken optimization.