linux-kernel - Re: [GIT PULL] adaptive spinning mutexes

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Date:	Wed, 14 Jan 2009 21:14:35 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	torvalds@...ux-foundation.org, a.p.zijlstra@...llo.nl,
	paulmck@...ux.vnet.ibm.com, ghaskins@...ell.com, matthew@....cx,
	andi@...stfloor.org, chris.mason@...cle.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-btrfs@...r.kernel.org, tglx@...utronix.de, npiggin@...e.de,
	pmorreale@...ell.com, SDietrich@...ell.com,
	dmitry.adamushko@...il.com, hannes@...xchg.org
Subject: Re: [GIT PULL] adaptive spinning mutexes


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Wed, 14 Jan 2009 20:00:08 +0100
> Ingo Molnar <mingo@...e.hu> wrote:
> 
> > 
> > * Andrew Morton <akpm@...ux-foundation.org> wrote:
> > 
> > > On Wed, 14 Jan 2009 19:33:19 +0100 Ingo Molnar <mingo@...e.hu> wrote:
> > > 
> > > > Please pull the adaptive-mutexes-for-linus git tree
> > > 
> > > <fear>
> > > 
> > > - It seems a major shortcoming that the feature is disabled if
> > >   CONFIG_DEBUG_MUTEXES=y.  It means that lots of people won't test it.
> 
> ^^^?
>  
> > > - When people hit performance/latency oddities, it would be nice if
> > >   they had a /proc knob with which they could disable this feature at
> > >   runtime.
> > > 
> > >   This would also be useful for comparative performance testing.
> > 
> > Yeah. From my other mail:
> > 
> > > > We still have the /sys/debug/sched_features tunable under 
> > > > CONFIG_SCHED_DEBUG=y, so should this cause any performance regressions 
> > > > somewhere, it can be pinned down and blamed back on this change 
> > > > easily, without bisection and without rebooting the box.
> > 
> > This kind of easy knob was included early on - this is how all those spin 
> > versus no-spin numbers were done.
> 
> Do people enable CONFIG_SCHED_DEBUG?

If they suspect performance problems and want to analyze them?

Note that CONFIG_SCHED_DEBUG=y is also the default.

> CONFIG_DEBUG_MUTEXES=n, CONFIG_SCHED_DEBUG=y is getting to be a pretty 
> small subset?

Those two are the default config settings actually, so i'd expect it to be 
the most commonly occuring combinations.

	Ingo
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