linux-kernel - Re: [PATCH] lockup_detector: Make DETECT_HUNT_TASK default depend on LOCKUP

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Date:	Sun, 8 Aug 2010 23:23:11 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Heiko Carstens <heiko.carstens@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Li Zefan <lizf@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	Don Zickus <dzickus@...hat.com>
Subject: Re: [PATCH] lockup_detector: Make DETECT_HUNT_TASK default depend on
 LOCKUP_DETECTOR


* Frederic Weisbecker <fweisbec@...il.com> wrote:

> > The thing is, CONFIG_DETECT_SOFTLOCKUP was default-y before, so many 
> > people had it enabled [and had it forced-enabled if DEBUG_KERNEL was off], 
> > even if they didnt really want or need it.
> 
> Hmm. It was:
> 
> config DETECT_SOFTLOCKUP
> 	bool "Detect Soft Lockups"
> 	depends on DEBUG_KERNEL && !S390
> 	default y
> 
> It means it's default enabled only if DEBUG_KERNEL, right? Then if you don't 
> select CONFIG_DEBUG_KERNEL, it's fine as it won't be selected.

Indeed, you are right.

Anyway, i think the general point remains: i'm not sure we should 
default-enable this feature.

> But I agree with you. There is a bunch of config options for which selection 
> is a duty when you are a kernel developer: PROVE_LOCKING, DETECT_HUNG_TASK, 
> DEBUG_PREEMPT, PROVE_RCU, etc... Because they all show (or prove we can 
> have) bugs that one might miss without these options. Softlockups are rarely 
> part of them because even without the lockup detector enabled, you'll 
> observe something is wrong.

Note that it's now detecting all kinds of lockups: softlockups, hard lockups 
and even unkillable hung tasks.

	Ingo
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