From patchwork Fri Mar 20 10:07:33 2009 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Patchwork-Submitter: Eric Dumazet X-Patchwork-Id: 24738 X-Patchwork-Delegate: davem@davemloft.net Return-Path: X-Original-To: patchwork-incoming@ozlabs.org Delivered-To: patchwork-incoming@ozlabs.org Received: from vger.kernel.org (vger.kernel.org [209.132.176.167]) by ozlabs.org (Postfix) with ESMTP id 9BD3DDDDFF for ; Fri, 20 Mar 2009 21:08:25 +1100 (EST) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1760596AbZCTKHk (ORCPT ); Fri, 20 Mar 2009 06:07:40 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1760589AbZCTKHi (ORCPT ); Fri, 20 Mar 2009 06:07:38 -0400 Received: from gw1.cosmosbay.com ([212.99.114.194]:39462 "EHLO gw1.cosmosbay.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1760579AbZCTKHh convert rfc822-to-8bit (ORCPT ); Fri, 20 Mar 2009 06:07:37 -0400 Received: from [127.0.0.1] (localhost [127.0.0.1]) by gw1.cosmosbay.com (8.13.7/8.13.7) with ESMTP id n2KA7Xp4020604; Fri, 20 Mar 2009 11:07:33 +0100 Message-ID: <49C36AE5.6040807@cosmosbay.com> Date: Fri, 20 Mar 2009 11:07:33 +0100 From: Eric Dumazet User-Agent: Thunderbird 2.0.0.21 (Windows/20090302) MIME-Version: 1.0 To: David Miller CC: netdev@vger.kernel.org Subject: [PATCH] net: remove useless prefetch() call References: <49C354B5.3060404@cosmosbay.com> <20090320.013611.67498837.davem@davemloft.net> <49C36570.4010903@cosmosbay.com> In-Reply-To: <49C36570.4010903@cosmosbay.com> X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-1.6 (gw1.cosmosbay.com [0.0.0.0]); Fri, 20 Mar 2009 11:07:33 +0100 (CET) Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org Eric Dumazet a écrit : > David Miller a écrit : >> From: Eric Dumazet >> 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 >> 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. --- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html 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);