netdev - [PATCH] net: remove useless prefetch() call

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Date:	Fri, 20 Mar 2009 11:07:33 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	David Miller <davem@...emloft.net>
CC:	netdev@...r.kernel.org
Subject: [PATCH] net: remove useless prefetch() call

Eric Dumazet a écrit :
> David Miller a écrit :
>> From: Eric Dumazet <dada1@...mosbay.com>
>> Date: Fri, 20 Mar 2009 09:32:53 +0100
>>
>>> There is no point to use prefetch() call here.
>>> start_xmit() is a function like others...
>>>
>>> Signed-off-by: Eric Dumazet <dada1@...mosbay.com>
>> Yes but the operation pointer might not be in the CPU
>> cache at this time?
>>
>> And if it's not we can get it into the cpu whilst we do
>> other processing, such as the dev_queue_xmit_nit() stuff.
> 
> This slow down fast path, but we can find a compromise.
> 

Hmm.. it seems that ndo_select_queue is accessed right before
ndo_start_xmit - by dev_pick_tx()) - and they share same cache line,
so operation pointer is in CPU cache.

So first patch is OK, what about this updated Changelog ?

Thank you

[PATCH] net: remove useless prefetch() call

There is no gain using prefetch() in dev_hard_start_xmit(), since
we already had to read ops->ndo_select_queue pointer in dev_pick_tx(),
and both pointers are probably located in the same cache line.

This prefetch call slows down fast path because of a stall in address
computation.

diff --git a/net/core/dev.c b/net/core/dev.c
index c013031..c97e27d 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1670,7 +1670,6 @@ int dev_hard_start_xmit(struct sk_buff *skb, struct net_device *dev,
 	const struct net_device_ops *ops = dev->netdev_ops;
 	int rc;
 
-	prefetch(&dev->netdev_ops->ndo_start_xmit);
 	if (likely(!skb->next)) {
 		if (!list_empty(&ptype_all))
 			dev_queue_xmit_nit(skb, dev);

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