From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail202.messagelabs.com (mail202.messagelabs.com [216.82.254.227]) by kanga.kvack.org (Postfix) with SMTP id D57DC8D0039 for ; Tue, 1 Mar 2011 21:46:44 -0500 (EST) Message-ID: <4D6DAF86.2000407@cn.fujitsu.com> Date: Wed, 02 Mar 2011 10:46:30 +0800 From: Lai Jiangshan MIME-Version: 1.0 Subject: Re: [PATCH 4/4 V2] net,rcu: don't assume the size of struct rcu_head References: <4D6CA860.3020409@cn.fujitsu.com> <20110301.001638.104075130.davem@davemloft.net> <4D6CB414.8050107@cn.fujitsu.com> <1298971213.3284.4.camel@edumazet-laptop> In-Reply-To: <1298971213.3284.4.camel@edumazet-laptop> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Sender: owner-linux-mm@kvack.org List-ID: To: Eric Dumazet 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 On 03/01/2011 05:20 PM, Eric Dumazet wrote: > Le mardi 01 mars 2011 =C3=A0 16:53 +0800, Lai Jiangshan a =C3=A9crit : >> On 03/01/2011 04:16 PM, David Miller wrote: >>> From: Lai Jiangshan >>> Date: Tue, 01 Mar 2011 16:03:44 +0800 >>> >>>> >>>> struct dst=5Fentry assumes the size of struct rcu=5Fhead as 2 * sizeof= (long) >>>> and manually adds pads for aligning for "=5F=5Frefcnt". >>>> >>>> When the size of struct rcu=5Fhead is changed, these manual padding >>>> is wrong. Use =5F=5Fattribute=5F=5F((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=5Fh= ead >> >> struct dst=5Fentry assumes the size of struct rcu=5Fhead as 2 * sizeof(l= ong) >> and manually adds pads for aligning for "=5F=5Frefcnt". >> >> When the size of struct rcu=5Fhead is changed, these manual padding >> are hardly suit for the changes. So we rearrange the structure, >> and move the seldom access rcu=5Fhead 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=5Fbuff; >> =20 >> struct dst=5Fentry { >> - struct rcu=5Fhead rcu=5Fhead; >> struct dst=5Fentry *child; >> struct net=5Fdevice *dev; >> short error; >> @@ -78,6 +77,13 @@ struct dst=5Fentry { >> =5F=5Fu32 =5F=5Fpad2; >> #endif >> =20 >> + unsigned long lastuse; >> + union { >> + struct dst=5Fentry *next; >> + struct rtable =5F=5Frcu *rt=5Fnext; >> + struct rt6=5Finfo *rt6=5Fnext; >> + struct dn=5Froute =5F=5Frcu *dn=5Fnext; >> + }; >> =20 >> /* >> * Align =5F=5Frefcnt to a 64 bytes alignment >> @@ -92,13 +98,7 @@ struct dst=5Fentry { >> */ >> atomic=5Ft =5F=5Frefcnt; /* client references */ >> int =5F=5Fuse; >> - unsigned long lastuse; >> - union { >> - struct dst=5Fentry *next; >> - struct rtable =5F=5Frcu *rt=5Fnext; >> - struct rt6=5Finfo *rt6=5Fnext; >> - struct dn=5Froute =5F=5Frcu *dn=5Fnext; >> - }; >> + struct rcu=5Fhead rcu=5Fhead; >> }; >> =20 >> #ifdef =5F=5FKERNEL=5F=5F >=20 > Nope... >=20 > "lastuse" and "next" must be in this place, or this introduce false > sharing we wanted to avoid in the past. >=20 > I suggest you leave this code as is, we will address the problem when > rcu=5Fhead changes (assuming we can test a CONFIG=5FRCU=5FHEAD=5FDEBUG or > something) >=20 > First part of "struct dst=5Fentry" is mostly read, while part beginning > after refcnt is often written. >=20 Is it the cause of false sharing? I thought that all are rare write(except = =5F=5Frefcnt) since it is protected by RCU. Do you allow me just move the seldom access rcu=5Fhead to the end of the st= ructure and add pads before =5F=5Frefcnt? I guess it increases about 3% the size of= dst=5Fentry. I accept that I leave this code as is, when I change rcu=5Fhead I will noti= fy you. Thanks, Lai -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org