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

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Message-ID: <Y75mGsoe5XUVtqqa@linutronix.de>
Date:   Wed, 11 Jan 2023 08:32:42 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Wei Wang <weiwan@...gle.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        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 2023-01-10 16:42:56 [-0800], Wei Wang wrote:
> I was not able to see the entire changelog, but I don't think
> > -               set_current_state(TASK_INTERRUPTIBLE);
> is redundant.
> 
> It makes sure that if the previous if statement:
>     if (test_bit(NAPI_STATE_SCHED_THREADED, &napi->state) || woken)
> is false, this napi thread yields the CPU to other threads waiting to
> be run by calling schedule().

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.

Sebastian