linux-kernel - Re: [PATCH 0/4] Convert khugepaged to a task

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Message-ID: <544FB8A8.1090402@redhat.com>
Date:	Tue, 28 Oct 2014 11:39:20 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Andi Kleen <andi@...stfloor.org>, Alex Thorlton <athorlton@....com>
CC:	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Bob Liu <lliubbo@...il.com>,
	David Rientjes <rientjes@...gle.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Hugh Dickins <hughd@...gle.com>,
	Ingo Molnar <mingo@...hat.com>,
	Kees Cook <keescook@...omium.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Mel Gorman <mgorman@...e.de>, Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vladimir Davydov <vdavydov@...allels.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/4] Convert khugepaged to a task_work function

On 10/28/2014 08:58 AM, Rik van Riel wrote:
> On 10/28/2014 08:12 AM, Andi Kleen wrote:
>> Alex Thorlton <athorlton@....com> writes:
>>
>>> Last week, while discussing possible fixes for some unexpected/unwanted behavior
>>> from khugepaged (see: https://lkml.org/lkml/2014/10/8/515) several people
>>> mentioned possibly changing changing khugepaged to work as a task_work function
>>> instead of a kernel thread.  This will give us finer grained control over the
>>> page collapse scans, eliminate some unnecessary scans since tasks that are
>>> relatively inactive will not be scanned often, and eliminate the unwanted
>>> behavior described in the email thread I mentioned.
>>
>> With your change, what would happen in a single threaded case?
>>
>> Previously one core would scan and another would run the workload.
>> With your change both scanning and running would be on the same
>> core.
>>
>> Would seem like a step backwards to me.
>
> It's not just scanning, either.
>
> Memory compaction can spend a lot of time waiting on
> locks. Not consuming CPU or anything, but just waiting.
>
> I am not convinced that moving all that waiting to task
> context is a good idea.

It may be worth investigating how the hugepage code calls
the memory allocation & compaction code.

Doing only async compaction from task_work context should
probably be ok.

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