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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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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