netdev - Re: [PATCH] netfilter: use per-cpu spinlock rather than RCU

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Message-ID: <Pine.LNX.4.62.0904132102030.10428@wlug.westbo.se>
Date:	Mon, 13 Apr 2009 21:06:10 +0200 (CEST)
From:	Martin Josefsson <gandalf@...fs.se>
To:	Stephen Hemminger <shemminger@...tta.com>
cc:	paulmck@...ux.vnet.ibm.com, David Miller <davem@...emloft.net>,
	paulus@...ba.org, mingo@...e.hu, torvalds@...ux-foundation.org,
	laijs@...fujitsu.com, jeff.chua.linux@...il.com,
	dada1@...mosbay.com, jengelh@...ozas.de, kaber@...sh.net,
	r000n@...0n.net, linux-kernel@...r.kernel.org,
	netfilter-devel@...r.kernel.org, netdev@...r.kernel.org,
	benh@...nel.crashing.org
Subject: Re: [PATCH] netfilter: use per-cpu spinlock rather than RCU

On Mon, 13 Apr 2009, Stephen Hemminger wrote:

> This is an alternative version of ip/ip6/arp tables locking using
> per-cpu locks.  This avoids the overhead of synchronize_net() during
> update but still removes the expensive rwlock in earlier versions.
>
> The idea for this came from an earlier version done by Eric Duzamet.
> Locking is done per-cpu, the fast path locks on the current cpu
> and updates counters.  The slow case involves acquiring the locks on
> all cpu's.

Doesn't spin_lock() result in a pipeline flush on x86?

iirc there was a benchmark in an RCU paper that tested using per cpu 
spin_locks and the result was that it didn't scale well at all.

/Martin
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