linux-kernel - Re: [BUG] long freezes on thinkpad t60

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Message-ID: <468201A0.5010401@yahoo.com.au>
Date:	Wed, 27 Jun 2007 16:20:16 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Eric Dumazet <dada1@...mosbay.com>,
	Chuck Ebbert <cebbert@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Jarek Poplawski <jarkao2@...pl>,
	Miklos Szeredi <miklos@...redi.hu>, chris@...ee.ca,
	linux-kernel@...r.kernel.org, tglx@...utronix.de,
	akpm@...ux-foundation.org
Subject: Re: [BUG] long freezes on thinkpad t60

Linus Torvalds wrote:
> 
> On Wed, 27 Jun 2007, Nick Piggin wrote:
> 
>>I don't know why my unlock sequence should be that much slower? Unlocked
>>mov vs unlocked add? Definitely in dumb micro-benchmark testing it wasn't
>>twice as slow (IIRC).
> 
> 
> Oh, that releasing "add" can be unlocked, and only the holder of the lock 
> ever touches that field?

Right.


> I must not have looked closely enough. In that case, I withdraw that 
> objection, and the sequence-number-based spinlock sounds like a perfectly 
> fine one.
> 
> Yes, the add will be slightly slower than the plain byte move, and the 
> locked xadd will be slightly slower than a regular locked add, but 
> compared to the serialization cost, that should be small. For some reason 
> I thought you needed a locked instruction for the unlock too.
> 
> So try it with just a byte counter, and test some stupid micro-benchmark 
> on both a P4 and a Core 2 Duo, and if it's in the noise, maybe we can make 
> it the normal spinlock sequence just because it isn't noticeably slower.
> 
> In fact, I think a "incb <mem>" instruction is even a byte shorter than 
> "movb $1,mem", and with "unlock" being inlined, that could actually be a 
> slight _win_.

OK, I'll try running some tests and get back to you on it.

-- 
SUSE Labs, Novell Inc.
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