linux-kernel - Re: [MM] Make mm counters per cpu instead of atomic

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Date:	Mon, 23 Nov 2009 08:31:40 -0600 (CST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"hugh.dickins@...cali.co.uk" <hugh.dickins@...cali.co.uk>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, Tejun Heo <tj@...nel.org>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [MM] Make mm counters per cpu instead of atomic

On Mon, 23 Nov 2009, Zhang, Yanmin wrote:

> Another theoretic issue is below scenario:
> Process A get the read lock on cpu 0 and is scheduled to cpu 2 to unlock. Then
> it's scheduled back to cpu 0 to repeat the step. eventually, the reader counter
> will overflow. Considering multiple thread cases, it might be faster to
> overflow than what we imagine. When it overflows, processes will hang there.

True.... We need to find some alternative to per cpu data to scale mmap
sem then.
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