Re: Remove __napi_schedule_irqoff?

Jakub Kicinski
2020-10-18T17:19:51Z

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.

Right, it'd work for some drivers. Other drivers try to take spin locks
in their IRQ handlers.

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..