linux-kernel - Re: [patch 1/2] mm, zone: track number of pages in free area by migratetype

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Date:   Thu, 17 Nov 2016 18:04:08 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     David Rientjes <rientjes@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...e.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [patch 1/2] mm, zone: track number of pages in free area by
 migratetype

On 11/17/2016 02:32 AM, David Rientjes wrote:
> Each zone's free_area tracks the number of free pages for all free lists.
> This does not allow the number of free pages for a specific migratetype
> to be determined without iterating its free list.
> 
> An upcoming change will use this information to preclude doing async
> memory compaction when the number of MIGRATE_UNMOVABLE pageblocks is
> below a certain threshold.
> 
> The total number of free pages is still tracked, however, to not make
> zone_watermark_ok() more expensive.  Reading /proc/pagetypeinfo, however,
> is faster.

Yeah I've already seen a case with /proc/pagetypeinfo causing soft
lockups due to high number of iterations...

> This patch introduces no functional change and increases the amount of
> per-zone metadata at worst by 48 bytes per memory zone (when CONFIG_CMA
> and CONFIG_MEMORY_ISOLATION are enabled).

Isn't it 48 bytes per zone and order?

> Signed-off-by: David Rientjes <rientjes@...gle.com>

I'd be for this if there are no performance regressions. It affects hot
paths and increases cache footprint. I think at least some allocator
intensive microbenchmark should be used.

Vlastimil