linux-kernel - Re: [git pull] scheduler fixes

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Message-ID: <20080511144203.GB3220@elte.hu>
Date:	Sun, 11 May 2008 16:42:03 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Sven Wegener <sven.wegener@...aler.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Andi Kleen <andi@...stfloor.org>,
	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>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [git pull] scheduler fixes


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

> > > Current XFS CVS has a fix from myself and Christoph that gets rid 
> > > of the l_flushsema and replaces it with a staggered wakeup of each 
> > > task that's waiting as the previously woken task clears the 
> > > critical section.
> > 
> > the solution is to reduce semaphore usage by converting them to 
> > mutexes. Is anyone working on removing legacy semaphore use from 
> > XFS?
> 
> This race is completely irrelevant to converting semaphores to 
> mutexes. [...]

i was not talking about the race. I was just reacting on your comments 
about thundering herds and staggered wakeups - which is a performance 
detail. Semaphores should not regress AIM7 by 50% but otherwise they are 
legacy code and their use should be reduced monotonically, so i was 
asking why anyone still cares about tuning semaphore details in XFS 
instead of just working on removing semaphore use from them.

> [...] It can only occur for semaphores which /can't/ be converted to 
> mutexes.

exactly what usecase is that? Perhaps it could be converted to an atomic 
counter + the wait_event() APIs.

	Ingo
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