From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756017Ab1CAJUW (ORCPT ); Tue, 1 Mar 2011 04:20:22 -0500 Received: from mail-wy0-f174.google.com ([74.125.82.174]:49680 "EHLO mail-wy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754549Ab1CAJUT (ORCPT ); Tue, 1 Mar 2011 04:20:19 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=subject:from:to:cc:in-reply-to:references:content-type:date :message-id:mime-version:x-mailer:content-transfer-encoding; b=bSxCD3fs+zBp9EqLZNBeo3W7BN5B2UnTDn1Dkb+EUKXBrJsea1P7Q2nFIWhLtlIUyV W9bAj48AtN7uYgp93COy8IIajGqDO8EW1c1s6buWbNZrPoB4E9tujLKvrGosgqkMw8WD Mv49+5BKX0oF2RCKG2P4nMkOzCJkH+pdMr2Eg= Subject: Re: [PATCH 4/4 V2] net,rcu: don't assume the size of struct rcu_head From: Eric Dumazet To: Lai Jiangshan Cc: David Miller , mingo@elte.hu, paulmck@linux.vnet.ibm.com, cl@linux-foundation.org, penberg@kernel.org, mpm@selenic.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org In-Reply-To: <4D6CB414.8050107@cn.fujitsu.com> References: <4D6CA860.3020409@cn.fujitsu.com> <20110301.001638.104075130.davem@davemloft.net> <4D6CB414.8050107@cn.fujitsu.com> Content-Type: text/plain; charset="UTF-8" Date: Tue, 01 Mar 2011 10:20:13 +0100 Message-ID: <1298971213.3284.4.camel@edumazet-laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.30.3 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Le mardi 01 mars 2011 à 16:53 +0800, Lai Jiangshan a écrit : > On 03/01/2011 04:16 PM, David Miller wrote: > > From: Lai Jiangshan > > Date: Tue, 01 Mar 2011 16:03:44 +0800 > > > >> > >> struct dst_entry assumes the size of struct rcu_head as 2 * sizeof(long) > >> and manually adds pads for aligning for "__refcnt". > >> > >> When the size of struct rcu_head is changed, these manual padding > >> is wrong. Use __attribute__((aligned (64))) instead. > >> > >> Signed-off-by: Lai Jiangshan > > > > We don't want to use the align if it's going to waste lots of space. > > > > Instead we want to rearrange the structure so that the alignment comes > > more cheaply. > > Subject: [PATCH 4/4 V2] net,rcu: don't assume the size of struct rcu_head > > struct dst_entry assumes the size of struct rcu_head as 2 * sizeof(long) > and manually adds pads for aligning for "__refcnt". > > When the size of struct rcu_head is changed, these manual padding > are hardly suit for the changes. So we rearrange the structure, > and move the seldom access rcu_head to the end of the structure. > > Signed-off-by: Lai Jiangshan > --- > > diff --git a/include/net/dst.h b/include/net/dst.h > index 93b0310..d8c5296 100644 > --- a/include/net/dst.h > +++ b/include/net/dst.h > @@ -37,7 +37,6 @@ > struct sk_buff; > > struct dst_entry { > - struct rcu_head rcu_head; > struct dst_entry *child; > struct net_device *dev; > short error; > @@ -78,6 +77,13 @@ struct dst_entry { > __u32 __pad2; > #endif > > + unsigned long lastuse; > + union { > + struct dst_entry *next; > + struct rtable __rcu *rt_next; > + struct rt6_info *rt6_next; > + struct dn_route __rcu *dn_next; > + }; > > /* > * Align __refcnt to a 64 bytes alignment > @@ -92,13 +98,7 @@ struct dst_entry { > */ > atomic_t __refcnt; /* client references */ > int __use; > - unsigned long lastuse; > - union { > - struct dst_entry *next; > - struct rtable __rcu *rt_next; > - struct rt6_info *rt6_next; > - struct dn_route __rcu *dn_next; > - }; > + struct rcu_head rcu_head; > }; > > #ifdef __KERNEL__ Nope... "lastuse" and "next" must be in this place, or this introduce false sharing we wanted to avoid in the past. I suggest you leave this code as is, we will address the problem when rcu_head changes (assuming we can test a CONFIG_RCU_HEAD_DEBUG or something) First part of "struct dst_entry" is mostly read, while part beginning after refcnt is often written.