linux-kernel - Re: [PATCH] mm/vmstat: retrieve more accurate vmstat value

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Message-ID: <5655BB0A.90000@suse.cz>
Date:	Wed, 25 Nov 2015 14:43:38 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Michal Hocko <mhocko@...nel.org>, Joonsoo Kim <js1304@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Christoph Lameter <cl@...ux.com>,
	linux-mm@...ck.org, Joonsoo Kim <iamjoonsoo.kim@....com>
Subject: Re: [PATCH] mm/vmstat: retrieve more accurate vmstat value

On 11/25/2015 01:00 PM, Michal Hocko wrote:
> On Tue 24-11-15 15:22:03, Joonsoo Kim wrote:
>> When I tested compaction in low memory condition, I found that
>> my benchmark is stuck in congestion_wait() at shrink_inactive_list().
>> This stuck last for 1 sec and after then it can escape. More investigation
>> shows that it is due to stale vmstat value. vmstat is updated every 1 sec
>> so it is stuck for 1 sec.
> 
> Wouldn't it be sufficient to use zone_page_state_snapshot in
> too_many_isolated?

That sounds better than the ad-hoc half-solution, yeah.
I don't know how performance sensitive the callers are, but maybe it could do a
non-snapshot check first, and only repeat with _snapshot when it's about to wait
(the result is true), just to make sure?

OTOH, how big issue is this? I suspect the system has been genuinely
too_many_isolated(), or very close, in order to hit the condition in the first
place, and the inaccuracy just delays the recovery a bit?

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