On Tue, 2017-02-28 at 09:20 -0800, Alexander Duyck wrote:
> On Mon, Feb 27, 2017 at 6:21 AM, Eric Dumazet <eric.duma...@gmail.com> wrote:
> > +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;
> > + if (unlikely(val & NAPIF_STATE_SCHED))
> > + new |= NAPIF_STATE_MISSED;
>
> You might want to consider just using a combination AND, divide,
> multiply, and OR to avoid having to have any conditional branches
> being added due to this code path. Basically the logic would look
> like:
> new = val | NAPIF_STATE_SCHED;
> new |= (val & NAPIF_STATE_SCHED) / NAPIF_STATE_SCHED * NAPIF_STATE_MISSED;
>
> In assembler that all ends up getting translated out to AND, SHL, OR.
> You avoid the branching, or MOV/OR/TEST/CMOV type code you would end
> up with otherwise.
Sure, I can try to optimize this a bit ;)
> > + } while (cmpxchg(&n->state, val, new) != val);
> > +
> > + if (unlikely(val & NAPIF_STATE_MISSED))
> > + __napi_schedule(n);
> > +
> > return true;
> > }
>
> If you rescheduled napi should you really be returning true? Seems
> like you should be returning "!(val & NAPIF_STATE_MISSED)" to try to
> avoid letting this occur again.
Good idea.
Hmm... you mean that many drivers test napi_complete_done() return
value ?
;)
Thanks !
