netdev - Re: [PATCH 4/4 V2] net,rcu: don't assume the size of struct rcu

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Message-ID: <1299034925.2930.52.camel@edumazet-laptop>
Date:	Wed, 02 Mar 2011 04:02:05 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	David Miller <davem@...emloft.net>, mingo@...e.hu,
	paulmck@...ux.vnet.ibm.com, cl@...ux-foundation.org,
	penberg@...nel.org, mpm@...enic.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 4/4 V2] net,rcu: don't assume the size of struct
 rcu_head

Le mercredi 02 mars 2011 à 10:46 +0800, Lai Jiangshan a écrit :

> Is it the cause of false sharing? I thought that all are rare write(except __refcnt)
> since it is protected by RCU.
> 
> Do you allow me just move the seldom access rcu_head to the end of the structure
> and add pads before __refcnt? I guess it increases about 3% the size of dst_entry.


dst_entry is a base class.

Its included at the beginning of other structs.

Moving rcu_head "at the end" just move it right in the middle of upper
objects as a matter of fact. This might add one cache line miss on
critical network object. A complete audit is needed.

David is doing some changes in this area, so things move fast anyway.

> I accept that I leave this code as is, when I change rcu_head I will
notify you.
> 

Thanks


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