linux-kernel - Re: AIM7 40% regression with 2.6.26-rc1

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Date:	Tue, 6 May 2008 19:49:54 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"J. Bruce Fields" <bfields@...i.umich.edu>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@....linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: AIM7 40% regression with 2.6.26-rc1

* Matthew Wilcox <matthew@....cx> wrote:

> > down(), down_interruptible() and down_try() should use 
> > spin_lock_irq(), not irqsave.
> 
> We talked about this ... the BKL actually requires that you be able to 
> acquire it with interrupts disabled. [...]

hm, where does it require it, besides the early bootup code? (which 
should just be fixed)

down_trylock() is OK as irqsave/irqrestore for legacy reasons, but that 
is fundamentally atomic anyway.

> > up() seems to be doing wake-one, FIFO which is nice.  Did the 
> > implementation which we just removed also do that?  Was it perhaps 
> > accidentally doing LIFO or something like that?
> 
> That's a question for someone who knows x86 assembler, I think.

the assembly is mostly just for the fastpath - and a 40% regression 
cannot be about fastpath differences. In the old code the scheduling 
happens in lib/semaphore-sleeper.c, and from the looks of it it appears 
to be a proper FIFO as well. (plus this small wakeup weirdness it has)

i reviewed the new code in kernel/semaphore.c as well and can see 
nothing bad in it - it does proper wake-up, FIFO queueing, like the 
mutex code.

	Ingo
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