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From: Andi Kleen <ak@linux.intel.com>
Subject: Re: [PATCH 1/2] mm: NUMA stats code cleanup and enhancement
References: <1511848824-18709-1-git-send-email-kemi.wang@intel.com>
	<9b4d5612-24eb-4bea-7164-49e42dc76f30@suse.cz>
Date: Tue, 28 Nov 2017 10:40:52 -0800
In-Reply-To: <9b4d5612-24eb-4bea-7164-49e42dc76f30@suse.cz> (Vlastimil Babka's
	message of "Tue, 28 Nov 2017 09:09:11 +0100")
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To: Vlastimil Babka <vbabka@suse.cz>
Cc: Kemi Wang <kemi.wang@intel.com>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Andrew Morton <akpm@linux-foundation.org>, Michal Hocko <mhocko@suse.com>, Mel Gorman <mgorman@techsingularity.net>, Johannes Weiner <hannes@cmpxchg.org>, Christopher Lameter <cl@linux.com>, YASUAKI ISHIMATSU <yasu.isimatu@gmail.com>, Andrey Ryabinin <aryabinin@virtuozzo.com>, Nikolay Borisov <nborisov@suse.com>, Pavel Tatashin <pasha.tatashin@oracle.com>, David Rientjes <rientjes@google.com>, Sebastian Andrzej Siewior <bigeasy@linutronix.de>, Dave <dave.hansen@linux.intel.com>, Andi Kleen <andi.kleen@intel.com>, Tim Chen <tim.c.chen@intel.com>, Jesper Dangaard Brouer <brouer@redhat.com>, Ying Huang <ying.huang@intel.com>, Aaron Lu <aaron.lu@intel.com>, Aubrey Li <aubrey.li@intel.com>, Linux MM <linux-mm@kvack.org>, Linux Kernel <linux-kernel@vger.kernel.org>

Vlastimil Babka <vbabka@suse.cz> writes:
>
> I'm worried about the "for_each_possible..." approach here and elsewhere
> in the patch as it can be rather excessive compared to the online number
> of cpus (we've seen BIOSes report large numbers of possible CPU's). IIRC

Even if they report a few hundred extra reading some more shared cache lines
is very cheap. The prefetcher usually quickly figures out such a pattern
and reads it all in parallel.

I doubt it will be noticeable, especially not in a slow path
like reading something from proc/sys.

> the general approach with vmstat is to query just online cpu's / nodes,
> and if they go offline, transfer their accumulated stats to some other
> "victim"?

That's very complicated, and unlikely to be worth it.

-Andi

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