linux-kernel - Re: [PATCH] reiserfs: kill-the-BKL

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Date:	Thu, 9 Apr 2009 08:35:22 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andi Kleen <andi@...stfloor.org>
cc:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Jeff Mahoney <jeffm@...e.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	ReiserFS Development List <reiserfs-devel@...r.kernel.org>,
	Bron Gondwana <brong@...tmail.fm>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] reiserfs: kill-the-BKL

On Thu, 9 Apr 2009, Andi Kleen wrote:
>
> You have to be very careful with this. Mutexes can be slower than
> spinlocks (and the new BKL is a spinlock) in some situations, they
> typically schedule much more etc., which can be costly.

Actually, with the new adaptive spinning, that basically shouldn't be true 
any more. Or rather, you should need some really bad/unlucky situation for 
it to scheduler more than necessary, and if the locker _acts_ like a 
spinlock (ie it doesn't block while holding the lock), performance should 
approach a spinlock.

That said, there are definitely reasons why a mutex can be slower than the 
BKL, and the whole "BKL gets implicitly dropped at sleep time" is very 
high on that list of reasons. The sleeping patterns can be _very_ 
different with a mutex than with a BKL.

> Better would be to use spinlocks if possible. I guess you just would
> need to find all sleep points and wrap them with lock dropping?

I do agree that a filesystem should try to avoid sleeping locks if at all 
possible, especially on the paths that the VM uses for writeback. But on 
the other hand, I think the issue with reiserfs is just the bad latencies 
that the BKL can cause, and then it doesn't matter.

			Linus
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