linux-kernel - Re: [PATCH 7/9] mm, page_alloc: remove stop_machine from build_all

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Date:   Thu, 20 Jul 2017 11:21:25 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...e.de>,
        Johannes Weiner <hannes@...xchg.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 7/9] mm, page_alloc: remove stop_machine from
 build_all_zonelists

On Thu 20-07-17 09:24:58, Vlastimil Babka wrote:
> On 07/14/2017 10:00 AM, Michal Hocko wrote:
> > From: Michal Hocko <mhocko@...e.com>
> > 
> > build_all_zonelists has been (ab)using stop_machine to make sure that
> > zonelists do not change while somebody is looking at them. This is
> > is just a gross hack because a) it complicates the context from which
> > we can call build_all_zonelists (see 3f906ba23689 ("mm/memory-hotplug:
> > switch locking to a percpu rwsem")) and b) is is not really necessary
> > especially after "mm, page_alloc: simplify zonelist initialization".
> > 
> > Updates of the zonelists happen very seldom, basically only when a zone
> > becomes populated during memory online or when it loses all the memory
> > during offline. A racing iteration over zonelists could either miss a
> > zone or try to work on one zone twice. Both of these are something we
> > can live with occasionally because there will always be at least one
> > zone visible so we are not likely to fail allocation too easily for
> > example.
> > 
> > Signed-off-by: Michal Hocko <mhocko@...e.com>
> 
> Some stress testing of this would still be worth, IMHO.

I have run the pathological online/offline of the single memblock in the
movable zone while stressing the same small node with some memory pressure.
Node 1, zone      DMA
  pages free     0
        min      0
        low      0
        high     0
        spanned  0
        present  0
        managed  0
        protection: (0, 943, 943, 943)
Node 1, zone    DMA32
  pages free     227310
        min      8294
        low      10367
        high     12440
        spanned  262112
        present  262112
        managed  241436
        protection: (0, 0, 0, 0)
Node 1, zone   Normal
  pages free     0
        min      0
        low      0
        high     0
        spanned  0
        present  0
        managed  0
        protection: (0, 0, 0, 1024)
Node 1, zone  Movable
  pages free     32722
        min      85
        low      117
        high     149
        spanned  32768
        present  32768
        managed  32768
        protection: (0, 0, 0, 0)

root@...t1:/sys/devices/system/node/node1# while true
do 
echo offline > memory34/state
echo online_movable > memory34/state
done

root@...t1:/mnt/data/test/linux-3.7-rc5# numactl --preferred=1 make -j4

and it survived without any unexpected behavior. While this is not
really a great testing coverage it should exercise the allocation path
quite a lot.

I can add this to the changelog if you think it is worth it.
 
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
-- 
Michal Hocko
SUSE Labs