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

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Message-ID: <CAAywjhTjdgjz=oD0NUtp-k7Lccek-4e9wCJfMG-p0AGpDHwJiQ@mail.gmail.com>
Date: Wed, 21 May 2025 12:51:33 -0700
From: Samiullah Khawaja <skhawaja@...gle.com>
To: Wei Wang <weiwan@...gle.com>
Cc: Jakub Kicinski <kuba@...nel.org>, "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 Wed, May 21, 2025 at 10:28 AM Samiullah Khawaja <skhawaja@...gle.com> wrote:
>
> On Wed, May 21, 2025 at 9:41 AM Wei Wang <weiwan@...gle.com> wrote:
> >
> > 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?
> +1
>
> If the kthread checks whether it was not SCHED before it died then
> this should not occur.
> > >
> > 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)) {
> This might suffer from the problem you highlighted earlier,
> CPU 0 (IRQ)             CPU 1 (NAPI thr)          CPU 2 (config)
>
>   ____napi_schedule()
>     if (test_bit(NAPI_STATE_THREADED))
>     if (thread) {
>
>  kthread_stop()
>                               if (state & SCHED_THREADED || !(state & SCHED)) {
>                                    state &= ~THREADED;
>                               if (try_cmp_xchg())
>                                    break
>
>        set_bit(NAPI_STATE_SCHED_THREADED)
>        wake_up_process(thread);
>
> This would happen without the try_cmp_xchg logic that I added in my
> patch in the __napi_schedule (in the fast path). __napi_schedule would
> have to make sure that the kthread is not stopping while it is trying
> to do SCHED. This is similar to the logic we have in
> napi_schedule_prep that handles the STATE_DISABLE, STATE_SCHED and
> STATE_MISSED scenarios. Also if it falls back to normal softirq, it
> needs to make sure that the kthread is not polling at the same time.
Discard this as the SCHED would be set in napi_schedule_prepare before
__napi_schedule is called in IRQ, so try_cmp_xchg would return false.
I think if the thread stops if the napi is idle(SCHED is not) set then
it should do. This should make sure any pending SCHED_THREADED are
also done. The existing logic in napi_schedulle_prep should handle all
the cases.
>
> > 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?
> +1
>
> We need to make sure that any scheduled polling should be completed.
>
> >
> > >                         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