netdev - Re: [PATCH net-next v2] net: stop napi kthreads when THREADED napi is disabled

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Message-ID: <CAEA6p_BxSA16cMXr5NaJCLZ+KWD2YVVwEdvVX_QG=_gyvNCP=w@mail.gmail.com>
Date: Wed, 21 May 2025 09:41:02 -0700
From: Wei Wang <weiwan@...gle.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: Samiullah Khawaja <skhawaja@...gle.com>, "David S . Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, almasrymina@...gle.com, 
	willemb@...gle.com, jdamato@...tly.com, mkarsten@...terloo.ca, 
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next v2] net: stop napi kthreads when THREADED napi is disabled

On Tue, May 20, 2025 at 7:09 PM Jakub Kicinski <kuba@...nel.org> wrote:
>
> On Mon, 19 May 2025 22:43:25 +0000 Samiullah Khawaja wrote:
> > -/* Called with irq disabled */
> > -static inline void ____napi_schedule(struct softnet_data *sd,
> > -                                  struct napi_struct *napi)
> > +static inline bool ____try_napi_schedule_threaded(struct softnet_data *sd,
> > +                                               struct napi_struct *napi)
> >  {
> >       struct task_struct *thread;
> > +     unsigned long new, val;
> >
> > -     lockdep_assert_irqs_disabled();
> > +     do {
> > +             val = READ_ONCE(napi->state);
> > +
> > +             if (!(val & NAPIF_STATE_THREADED))
> > +                     return false;
>
> Do we really need to complicate the fastpath to make the slowpath easy?
>
> Plus I'm not sure it works.
>
>           CPU 0 (IRQ)             CPU 1 (NAPI thr)          CPU 2 (config)
>                          if (test_bit(NAPI_STATE_SCHED_THREADED))
>                                ...
>
>   ____napi_schedule()
>   cmpxchg(...)
>   wake_up_process(thread);
>                                                        clear_bit(NAPI_STATE_THREADED)
>                                                        kthread_stop(thread)
>
>                          if (kthread_should_stop())
>                                 exit
>
> Right?
>
Hmm right... I think the main issue is that while dev_set_threaded()
clears STATE_THREADED, SCHED_THREADED could already be set meaning the
napi is already scheduled. And napi_thread_wait() does not really
check STATE_THREADED...

> I think the shutting down thread should do this:
>
>         while (true) {
>                 state = READ_ONCE()
>
>                 // safe to clear if thread owns the NAPI,
>                 // or NAPI is completely idle
>                 if (state & SCHED_THREADED || !(state & SCHED)) {

I think we should make sure SCHED_THREADED is cleared as well, or
otherwise the thread is in the middle of calling napi_threaded_poll()
and we can't just disable the thread?

>                         state &= ~THREADED;

STATE_THREADED to be exact. Right?

>                 } else {
>                         msleep(1);
>                         continue;
>                 }
>
>                 if (try_cmpxchg())
>                         break;
>         }
>
> But that's just an idea, it could also be wrong... :S