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

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Date:	Fri, 07 May 2010 11:51:21 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org, therbert@...gle.com
Subject: [PATCH net-next-2.6] rps: avoid one atomic in enqueue_to_backlog

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);
 		}


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