netdev - Re: [PATCH net-next-2.6] rps: avoid one atomic in enqueue_to

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Date:	Fri, 07 May 2010 12:05:58 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Changli Gao <xiaosuo@...il.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	therbert@...gle.com
Subject: Re: [PATCH net-next-2.6] rps: avoid one atomic in
 enqueue_to_backlog

Le vendredi 07 mai 2010 à 18:01 +0800, Changli Gao a écrit :
> On Fri, May 7, 2010 at 5:51 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > Le vendredi 07 mai 2010 à 07:16 +0200, Eric Dumazet a écrit :
> >> Le jeudi 06 mai 2010 à 22:07 -0700, David Miller a écrit :
> >>
> >> > Looks great, applied, thanks Eric.
> >>
> >> Thanks, I have a followup to avoid one atomic in enqueue phase too ;)
> >>
> >
> > [PATCH net-next-2.6] rps: avoid one atomic in enqueue_to_backlog
> >
> > If CONFIG_SMP=y, then we own a queue spinlock, we can avoid the atomic
> > test_and_set_bit() from napi_schedule_prep().
> >
> > We now have same number of atomic ops per netif_rx() calls than with
> > pre-RPS kernel.
> >
> > Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
> > ---
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index 32611c8..49fa5a6 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -2426,8 +2426,10 @@ enqueue:
> >                        return NET_RX_SUCCESS;
> >                }
> >
> > -               /* Schedule NAPI for backlog device */
> > -               if (napi_schedule_prep(&sd->backlog)) {
> > +               /* Schedule NAPI for backlog device
> > +                * We can use non atomic operation since we own the queue lock
> > +                */
> > +               if (!__test_and_set_bit(NAPI_STATE_SCHED, &sd->backlog.state)) {
> >                        if (!rps_ipi_queued(sd))
> >                                ____napi_schedule(sd, &sd->backlog);
> >                }
> >
> 
> Why not use a wrapper function?
> 
> sth. like:
> 
> static inline int __napi_schedule_prep(struct napi_struct *n)
> {
>    return (!__test_and_set_bit(NAPI_STATE_SCHED, &n->state)
> }
> 


For one user ? 
Not sure it helps code readability.

Right now we all have our minds knowing every bit of this code, but next
year ?



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