linux-kernel - Re: [PATCH 06/34] mm, vmscan: have kswapd only scan based on the highest requested zone

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Message-ID: <20160712140504.GC5881@cmpxchg.org>
Date:	Tue, 12 Jul 2016 10:05:04 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Mel Gorman <mgorman@...hsingularity.net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>, Rik van Riel <riel@...riel.com>,
	Vlastimil Babka <vbabka@...e.cz>,
	Minchan Kim <minchan@...nel.org>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 06/34] mm, vmscan: have kswapd only scan based on the
 highest requested zone

On Fri, Jul 08, 2016 at 10:34:42AM +0100, Mel Gorman wrote:
> kswapd checks all eligible zones to see if they need balancing even if it
> was woken for a lower zone.  This made sense when we reclaimed on a
> per-zone basis because we wanted to shrink zones fairly so avoid
> age-inversion problems.  Ideally this is completely unnecessary when
> reclaiming on a per-node basis.  In theory, there may still be anomalies
> when all requests are for lower zones and very old pages are preserved in
> higher zones but this should be the exceptional case.
> 
> Signed-off-by: Mel Gorman <mgorman@...hsingularity.net>
> Acked-by: Vlastimil Babka <vbabka@...e.cz>

Acked-by: Johannes Weiner <hannes@...xchg.org>

I wasn't quite sure at first what the rationale is for this patch,
since it probably won't make much difference in pratice. But I do
agree that the code is cleaner to have kswapd check exactly what it
was asked to check, rather than some do-the-"right"-thing magic.

A hypothetical onslaught of low-zone allocations will wreak havoc to
the page age in higher zones anyway, right? So I don't think that case
matters all that much.