linux-kernel - Re: [patch] speed up / fix the new generic semaphore code (fix AIM7 40% regression with 2.6.26-rc1)

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Message-ID: <alpine.LFD.1.10.0805080859140.3024@woody.linux-foundation.org>
Date:	Thu, 8 May 2008 09:02:22 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Andi Kleen <andi@...stfloor.org>,
	Matthew Wilcox <matthew@....cx>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@....linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [patch] speed up / fix the new generic semaphore code (fix AIM7
 40% regression with 2.6.26-rc1)



On Thu, 8 May 2008, Ingo Molnar wrote:
> 
> Peter pointed it out that because sem->count is u32, the <= 0 is in fact 
> a "== 0" condition - the patch below does that. As expected gcc figured 
> out the same thing too so the resulting code output did not change. (so 
> this is just a cleanup)

Why don't we just make it do the same thing that the x86 semaphores used 
to do: make it signed, and decrement unconditionally. And callt eh 
slow-path if it became negative.

IOW, make the fast-path be

	spin_lock_irqsave(&sem->lock, flags);
	if (--sem->count < 0)
		__down();
	spin_unlock_irqrestore(&sem->lock, flags);

and now we have an existing known-good implementation to look at?

Rather than making up a totally new and untested thing.

		Linus
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