linux-kernel - Re: [BUG] mm, thp: khugepaged can't allocate on requested node when confined to a cpuset

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Date:	Tue, 14 Oct 2014 11:31:05 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>
CC:	Alex Thorlton <athorlton@....com>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Ingo Molnar <mingo@...nel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Hugh Dickins <hughd@...gle.com>, Bob Liu <lliubbo@...il.com>,
	Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org
Subject: Re: [BUG] mm, thp: khugepaged can't allocate on requested node when
 confined to a cpuset

On 10/14/2014 10:54 AM, Peter Zijlstra wrote:
> On Tue, Oct 14, 2014 at 02:48:28PM +0300, Kirill A. Shutemov wrote:
>
>> Why whould you want to pin khugpeaged? Is there a valid use-case?
>> Looks like userspace shoots to its leg.
>
> Its just bad design to put so much work in another context. But the
> use-case is isolating other cpus.
>
>> Is there a reason why we should respect cpuset limitation for kernel
>> threads?
>
> Yes, because we want to allow isolating CPUs from 'random' activity.
>
>> Should we bypass cpuset for PF_KTHREAD completely?
>
> No. That'll break stuff.

Agreed on the above.

I suspect the sane thing to do is allow a cpuset
limited task to still allocate pages elsewhere,
if it explicitly asks for it.

Ask for something potentially stupid? Get it :)

In many of the cases where something asks for memory
in a specific location, it has a good reason to do
so, and there's no reason to deny it.
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