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

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Message-Id: <20090114130642.cf2b18b2.akpm@linux-foundation.org>
Date:	Wed, 14 Jan 2009 13:06:42 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
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

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 :(

> > > Note that CONFIG_SCHED_DEBUG=y is also the default.
> > 
> > akpm:/usr/src/25> echo $ARCH
> > x86_64
> > akpm:/usr/src/25> make defconfig
> > *** Default configuration is based on 'x86_64_defconfig'
> 
> x86 defconfig is used too, but it's a pretty rare usage.
> 
> Under default i mean the customary meaning of default config: it's the 
> default if you come via 'make oldconfig' or if you derive your config from 
> a distro config:
> 
> | config SCHED_DEBUG
> |        bool "Collect scheduler debugging info"
> |        depends on DEBUG_KERNEL && PROC_FS
> |        default y
> 

This simply isn't reliable.
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