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

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Date:	Tue, 5 Jan 2010 10:25:43 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Christoph Lameter <cl@...ux-foundation.org>
cc:	Andi Kleen <andi@...stfloor.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Minchan Kim <minchan.kim@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"hugh.dickins" <hugh.dickins@...cali.co.uk>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC][PATCH 6/8] mm: handle_speculative_fault()

On Tue, 5 Jan 2010, Christoph Lameter wrote:
> 
> If the critical section protected by the spinlock is small then the
> delay will keep the cacheline exclusive until we hit the unlock. This
> is the case here as far as I can tell.

I hope somebody can time it. Because I think the idle reads on all the 
(unsuccessful) spinlocks will kill it.

Think of it this way: under heavy contention, you'll see a lot of people 
waiting for the spinlocks and one of them succeeds at writing it, reading 
the line. So you get an O(n^2) bus traffic access pattern. In contrast, 
with an xadd, you get O(n) behavior - everybody does _one_ acquire-for- 
write bus access.

Remember: the critical section is small, but since you're contending on 
the spinlock, that doesn't much _help_. The readers are all hitting the 
lock (and you can try to solve the O(n*2) issue with back-off, but quite 
frankly, anybody who does that has basically already lost - I'm personally 
convinced you should never do lock backoff, and instead look at what you 
did wrong at a higher level instead).

				Linus
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