linux-kernel - Re: [RFC][PATCH 6/8] mm: handle_speculative

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Message-ID: <87637gd4ek.fsf@basil.nowhere.org>
Date:	Tue, 05 Jan 2010 15:15:15 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"minchan.kim@...il.com" <minchan.kim@...il.com>,
	cl@...ux-foundation.org,
	"hugh.dickins" <hugh.dickins@...cali.co.uk>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC][PATCH 6/8] mm: handle_speculative_fault()

Arjan van de Ven <arjan@...radead.org> writes:

> On Mon, 04 Jan 2010 19:24:35 +0100
> Peter Zijlstra <a.p.zijlstra@...llo.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@...ux.intel.com -- Speaking for myself only.
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