netdev - Re: iptables very slow after commit 784544739a25c30637397ace5489eeb6e15d7d49

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Message-ID: <20090411070854.GC11799@elte.hu>
Date:	Sat, 11 Apr 2009 09:08:54 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Miller <davem@...emloft.net>,
	Lai Jiangshan <laijs@...fujitsu.com>, shemminger@...tta.com,
	jeff.chua.linux@...il.com, dada1@...mosbay.com, jengelh@...ozas.de,
	kaber@...sh.net, r000n@...0n.net,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	netfilter-devel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: iptables very slow after commit
	784544739a25c30637397ace5489eeb6e15d7d49


* Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

> 	I will nevertheless suggest the following egregious hack to
> 	get a consistent sample of one counter for some other CPU:
> 
> 	a.	Disable interrupts
> 	b.	Atomically exchange the bottom 32 bits of the
> 		counter with the value zero.
> 	c.	Atomically exchange the top 32 bits of the counter
> 		with the value zero.
> 	d.	Concatenate the values obtained in (b) and (c), which
> 		is the snapshot value.

Note, i have recently implemented full atomic64_t support on 32-bit 
x86, for the perfcounters code, based on the CMPXCHG8B instruction.

Which, while not the lightest of instructions, is still much better 
than the sequence above.

So i think a better approach would be to also add a dumb generic 
implementation for atomic64_t (using a global lock or so), and then 
generic code could just assume that atomic64_t always exists.

It is far nicer - and faster as well - as the hack above, even on 
32-bit x86.

	Ingo
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