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Subject: Re: [RFC][PATCH 6/8] mm: handle_speculative_fault()
From: Andi Kleen <andi@firstfloor.org>
References: <20100104182429.833180340@chello.nl>
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Date: Tue, 05 Jan 2010 15:15:15 +0100
In-Reply-To: <20100105054536.44bf8002@infradead.org> (Arjan van de Ven's message of "Tue, 5 Jan 2010 05:45:36 -0800")
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To: Arjan van de Ven <arjan@infradead.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, Peter Zijlstra <peterz@infradead.org>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, "minchan.kim@gmail.com" <minchan.kim@gmail.com>, cl@linux-foundation.org, "hugh.dickins" <hugh.dickins@tiscali.co.uk>, Nick Piggin <nickpiggin@yahoo.com.au>, Ingo Molnar <mingo@elte.hu>, Linus Torvalds <torvalds@linux-foundation.org>
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Arjan van de Ven <arjan@infradead.org> writes:

> On Mon, 04 Jan 2010 19:24:35 +0100
> Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
>
>> Generic speculative fault handler, tries to service a pagefault
>> without holding mmap_sem.
>
>
> while I appreciate the goal of reducing contention on this lock...
> wouldn't step one be to remove the page zeroing from under this lock?
> that's by far (easily by 10x I would guess) the most expensive thing
> that's done under the lock, and I would expect a first order of
> contention reduction just by having the zeroing of a page not done
> under the lock...

The cache line bouncing of the shared cache lines hurts too.

I suspect fixing this all properly will need some deeper changes.

-Andi
-- 
ak@linux.intel.com -- Speaking for myself only.

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