Re: Remove __napi_schedule_irqoff?

Jakub Kicinski
2020-10-19T17:55:21Z

On Mon, 19 Oct 2020 12:33:12 +0200 Thomas Gleixner wrote:
> On Sun, Oct 18 2020 at 10:19, Jakub Kicinski wrote:
> > On Sun, 18 Oct 2020 10:20:41 +0200 Heiner Kallweit wrote:
> >> >> Otherwise a non-solution could be to make IRQ_FORCED_THREADING
> >> >> configurable.
> >> >
> >> > I have to say I do not understand why we want to defer to a thread the
> >> > hard IRQ that we use in NAPI model.
> >> >
> >> Seems like the current forced threading comes with the big hammer and
> >> thread-ifies all hard irq's. To avoid this all NAPI network drivers
> >> would have to request the interrupt with IRQF_NO_THREAD.
>
> In a !RT kernel, forced threading (via commandline option) is mostly a
> debug aid. It's pretty useful when something crashes in hard interrupt
> context which usually takes the whole machine down. It's rather unlikely
> to be used on production systems, and if so then the admin surely should
> know what he's doing.
>
> > Right, it'd work for some drivers. Other drivers try to take spin locks
> > in their IRQ handlers.
>
> I checked a few which do and some of these spinlocks just protect
> register access and are not used for more complex serialization. So
> these could be converted to raw spinlocks because their scope is short
> and limited. But yes, you are right that this might be an issue in
> general.
>
> > What gave me a pause was that we have a busy loop in napi_schedule_prep:
> >
> > bool napi_schedule_prep(struct napi_struct *n)
> > {
> > unsigned long val, new;
> >
> > do {
> > val = READ_ONCE(n->state);
> > if (unlikely(val & NAPIF_STATE_DISABLE))
> > return false;
> > new = val | NAPIF_STATE_SCHED;
> >
> > /* Sets STATE_MISSED bit if STATE_SCHED was already set
> > * This was suggested by Alexander Duyck, as compiler
> > * emits better code than :
> > * if (val & NAPIF_STATE_SCHED)
> > * new |= NAPIF_STATE_MISSED;
> > */
> > new |= (val & NAPIF_STATE_SCHED) / NAPIF_STATE_SCHED *
> > NAPIF_STATE_MISSED;
> > } while (cmpxchg(&n->state, val, new) != val);
> >
> > return !(val & NAPIF_STATE_SCHED);
> > }
> >
> >
> > Dunno how acceptable this is to run in an IRQ handler on RT..
>
> In theory it's bad, but I don't think it's a big deal in reality.

Awesome, thanks for advice and clearing things up!
Let me apply Heiner's IRQF_NO_THREAD patch, then.