linux-kernel - Re: [PATCH 28/40] autonuma: make khugepaged pte

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Date:	Thu, 12 Jul 2012 20:50:31 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Hillf Danton <dhillf@...il.com>, Dan Smith <danms@...ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Paul Turner <pjt@...gle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Mike Galbraith <efault@....de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Bharata B Rao <bharata.rao@...il.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Johannes Weiner <hannes@...xchg.org>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	Alex Shi <alex.shi@...el.com>,
	Mauricio Faria de Oliveira <mauricfo@...ux.vnet.ibm.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Don Morris <don.morris@...com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [PATCH 28/40] autonuma: make khugepaged pte_numa aware

On Mon, Jul 02, 2012 at 12:24:36AM -0400, Rik van Riel wrote:
> On 06/28/2012 08:56 AM, Andrea Arcangeli wrote:
> > If any of the ptes that khugepaged is collapsing was a pte_numa, the
> > resulting trans huge pmd will be a pmd_numa too.
> 
> Why?
> 
> If some of the ptes already got faulted in and made really
> resident again, why do you want to incur a new NUMA fault
> on the newly collapsed hugepage?

If we don't set pmd_numa on the collapsed hugepage, the result is that
we'll understimate the thread NUMA affinity to the node where the
hugepage is located (mm affinity is recorded independently by the NUMA
hinting page faults).

If it's better or worse I guess depends on luck, we just lose
information.

I guess overstimating the node affinity with a node with hugepages
just collapsed is better than understimating it, more often than not.

I doubt it matters much if just 1 pte_numa or all pte_numa creates a
pmd_numa.

With the pmd scan mode (default enabled) we fault in at
pmd-granularity regardless of THP or not, so either ways it's the
same, this only an issue when you set knuma_scand/pmd = 0 at runtime.

> Is there something on we should know about?
> 
> If so, could you document it?

I'll add a note.
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