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

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Message-ID: <m2x412e6f7f1005070301h627d5c76z35b3277f3a369c69@mail.gmail.com>
Date:	Fri, 7 May 2010 18:01:09 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Eric Dumazet <eric.dumazet@...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

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)
}

-- 
Regards，
Changli Gao(xiaosuo@...il.com)
--
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