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

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Date:	Wed, 7 May 2008 08:00:04 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
cc:	Andi Kleen <andi@...stfloor.org>, Matthew Wilcox <matthew@....cx>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@....linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: AIM7 40% regression with 2.6.26-rc1



On Wed, 7 May 2008, Alan Cox wrote:
>
> > But my preferred option would indeed be just turning it back into a 
> > spinlock - and screw latency and BKL preemption - and having the RT people 
> > who care deeply just work on removing the BKL in the long run.
> 
> It isn't as if the RT build can't use a different lock type to the
> default build.

Well, considering just *how* bad the new BKL apparently is, I think that's 
a separate issue. The semaphore implementation is simply not worth it. At 
a minimum, it should be a mutex.

> > Is BKL preemption worth it? Sounds very dubious. Sounds even more dubious 
> > when we now apparently have even more reason to aim for removing the BKL 
> > rather than trying to mess around with it.
> 
> We have some horrible long lasting BKL users left unfortunately.

Quite frankly, maybe we _need_ to have a bad BKL for those to ever get 
fixed. As it was, people worked on trying to make the BKL behave better, 
and it was a failure. Rather than spend the effort on trying to make it 
work better (at a horrible cost), why not just say "Hell no - if you have 
issues with it, you need to work with people to get rid of the BKL 
rather than cluge around it".

			Linus
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