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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <49C36AE5.6040807@cosmosbay.com>
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);

--
To unsubscribe from this list: send the line "unsubscribe netdev" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html