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Date:	Tue, 8 Feb 2011 00:12:28 +0100
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: khugepaged eating 100%CPU

Hello Michal,

On Mon, Feb 07, 2011 at 10:16:01PM +0100, Michal Hocko wrote:
> On Mon 07-02-11 22:06:54, Michal Hocko wrote:
> > Hi Andrea,
> > 
> > I am currently running into an issue when khugepaged is running 100% on
> > one of my CPUs for a long time (at least one hour as I am writing the
> > email). The kernel is the clean 2.6.38-rc3 (i386) vanilla kernel.
> > 
> > I have tried to disable defrag but it didn't help (I haven't rebooted
> > after setting the value). I am not sure what information is helpful and
> > also not sure whether I am able to reproduce it after restart (it is the
> > first time I can see this problem) so sorry for the poor report.
> > 
> > Here is some basic info which might be useful (config and sysrq+t are
> > attached):
> > =========
> 
> And I have just realized that I forgot about the daemon stack:
> # cat /proc/573/stack 
> [<c019c981>] shrink_zone+0x1b9/0x455
> [<c019d462>] do_try_to_free_pages+0x9d/0x301
> [<c019d803>] try_to_free_pages+0xb3/0x104
> [<c01966d7>] __alloc_pages_nodemask+0x358/0x589
> [<c01bf314>] khugepaged+0x13f/0xc60
> [<c014c301>] kthread+0x67/0x6c
> [<c0102db6>] kernel_thread_helper+0x6/0x10
> [<ffffffff>] 0xffffffff

It would be great to know if __alloc_pages_nodemask returned or if it
was calling it in a loop.

When __alloc_pages_nodemask fails in collapse_huge_page, hpage is set
to ERR_PTR(-ENOMEM), then khugepaged_scan_pmd returns 1, then
khugepaged_scan_mm_slot goto breakouterloop_mmap_sem and return
progress, then the khugepaged_do_scan main loop should notice that
IS_ERR(*hpage) is set and break out of the loop and return void, then
khugepaged_loop should notice that IS_ERR(hpage) is set and it should
throttle for alloc_sleep_millisecs inside khugepaged_alloc_sleep
before setting hpage to NULL and trying again to allocate. I wonder
what could be going wrong in khugepaged.. I wonder if it's a bug inside
__alloc_pages_nodemask and not a khugepaged issue. Best would be if
you run SYSRQ+l several times (/proc/*/stack don't seem to be the best
for running tasks even if it should be accurate enough already, but if
you run it often and with sysrq+l it'll be more clear what is
running).

I hope you can reproduce, if it's an allocator issue you should notice
it again by keeping the same workload on that same system. I doubt I
can reproduce at the moment as I don't know what's going on to
simulate your load.

Thanks a lot,
Andrea
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