Date:	Thu, 15 Jul 2010 22:57:21 +0200
From:	Zeno Davatz <zdavatz@...il.com>
To:	Pekka Enberg <penberg@...helsinki.fi>
Cc:	Damien Wyart <damien.wyart@...e.fr>,
	Catalin Marinas <catalin.marinas@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"x86@...nel.org" <x86@...nel.org>, "mingo@...e.hu" <mingo@...e.hu>,
	"yinghai@...nel.org" <yinghai@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: kmemleak, cpu usage jump out of nowhere

Am 15.07.2010 um 22:50 schrieb Pekka Enberg <penberg@...helsinki.fi>:

> On Thu, Jul 15, 2010 at 11:38 PM, Zeno Davatz <zdavatz@...il.com> wrote:
>> Am 15.07.2010 um 22:00 schrieb Damien Wyart <damien.wyart@...e.fr>:
>> 
>>>>> For now, I can't reproduce the problem with CONFIG_NO_BOOTMEM disabled ;
>>>>> with the option and rc5 the problem was happening quite quickly after
>>>>> boot and normal use of the machine. So it seems I can confirme what Zeno
>>>>> has seen and I hope this will give a hint to debug the problem. I guess
>>>>> this has not been reported that much because many testers might not have
>>>>> enabled CONFIG_NO_BOOTMEM... Maybe the scheduler folks could test their
>>>>> benchmark with a kernel having this option enabled?
>>> 
>>> * Pekka Enberg <penberg@...helsinki.fi> [2010-07-15 22:50]:
>>>> To be honest, the bug is bit odd. It's related to boot-time memory
>>>> allocator changes but yet it seems to manifest itself as a scheduling
>>>> problem. So if you have some spare time and want to speed up the
>>>> debugging process, please test v2.6.34 and v2.6.35-rc1 with
>>>> CONFIG_NO_BOOTMEM and if former is good and latter is bad, try to see
>>>> if you can identify the offending commit with "git bisect."
>>> 
>>> Not sure I will have enough time in the coming days (doing that remotely
>>> is fishy since ssh access is almost stuck when the problem occurs); if
>>> Zeno can and would like to do it, maybe this could be done faster.
>>> 
>>> As the scheduler is now very well instrumented (many debugging features
>>> are available), reproducing the bug on a test platform (it happens quite
>>> quickly for me) might also give some hints. So testers, if you have
>>> time, please test 2.6.35-rc5 with CONFIG_NO_BOOTMEM on a Core i7 and see
>>> if you can reproduce the problem!
>> 
>> Will try to do so. Can you point me to the git bisect howto with the versions you want.
> 
> Cool. So like I said, you first want to test 2.6.34 to find a known
> good version. Please remember to make sure you have CONFIG_NO_BOOTMEM
> enabled. You can also try to speed up the process by testing
> 2.6.35-rc1 which is likely to include the offending commit. That's not
> strictly necessary as long as you are sure that you have some
> 2.6.35-rc kernel that's bad.
> 
> After that, bisecting is as simple as:
> 
>  git bisect start
>  git bisect good v2.6.34
>  git bisect bad v2.6.31-rc1 # or some other kernel you know to be bad
>  <compile, boot, and try to trigger the problem>
> 
> then
> 
>  git bisect bad # if you were able to trigger the problem
> 
> or
> 
>  git bisect good # if the problem doesn't exist
> 
> git will then find the next revision to test after which you do
> 
>  <compile, boot, and try to trigger the problem>
> 
> and repeat the "git bisect good/bad" step until git tells you it has
> found the offending commit.
> 
> There's more information on the git bisect man pages:
> 
> http://www.kernel.org/pub/software/scm/git/docs/git-bisect.html
> 
> Let me know if you need more help with this.

Ok, thanks for the guidance, will start some time tomorrow. Hope to make it in the morning.

Best
Zeno--
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