linux-kernel - Re: kmemleak, cpu usage jump out of nowhere

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Date:	Thu, 15 Jul 2010 23:52:25 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Damien Wyart <damien.wyart@...e.fr>
Cc:	Zeno Davatz <zdavatz@...il.com>,
	Catalin Marinas <catalin.marinas@....com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
	mingo@...e.hu, yinghai@...nel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: kmemleak, cpu usage jump out of nowhere

On Thu, Jul 15, 2010 at 11:00 PM, Damien Wyart <damien.wyart@...e.fr> wrote:
>> > 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!

Yeah, there's "perf sched" tool available for that:

  http://lwn.net/Articles/353295/

The only problem is that we'd need a scheduler hacker to decipher the
report and all of them seem to be missing at the moment (probably at
OLS). Anyway, like I said, git bisect will probably speed up the
debugging process, that's all.

                          Pekka
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