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

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Message-ID: <20090114211458.GD6519@elte.hu>
Date:	Wed, 14 Jan 2009 22:14:58 +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 21:51:22 +0100
> Ingo Molnar <mingo@...e.hu> wrote:
> 
> > 
> > * Andrew Morton <akpm@...ux-foundation.org> wrote:
> > 
> > > > > Do people enable CONFIG_SCHED_DEBUG?
> > > > 
> > > > If they suspect performance problems and want to analyze them?
> > > 
> > > The vast majority of users do not and usually cannot compile their own 
> > > kernels.
> > 
> > ... which they derive from distro kernels or some old .config they always 
> > used, via 'make oldconfig'. You are arguing against well-established facts 
> > here.
> > 
> > If you dont believe my word for it, here's an analysis of all kernel 
> > configs posted to lkml in the past 8 months:
> > 
> >    $ grep ^CONFIG_SCHED_DEBUG linux-kernel | wc -l
> >    424
> > 
> >    $ grep 'CONFIG_SCHED_DEBUG is not' linux-kernel | wc -l
> >    109
> > 
> > i.e. CONFIG_SCHED_DEBUG=y is set in 80% of the configs. A large majority 
> > of testers has it enabled and /sys/debug/sched_features was always a good 
> > mechanism that we used for runtime toggles.
> 
> You just disproved your own case :(

how so? 80% is not enough? I also checked Fedora and it has SCHED_DEBUG=y 
in its kernel rpms.

note that there's also a performance issue here: we generally _dont want_ 
a debug sysctl overhead in the mutex code or in any fastpath for that 
matter. So making it depend on SCHED_DEBUG is useful.

sched_feat() features get optimized out at build time when SCHED_DEBUG is 
disabled. So it gives us the best of two worlds: the utility of sysctls in 
the SCHED_DEBUG=y, and they get compiled out in the !SCHED_DEBUG case.

	Ingo
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