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Date: Thu, 13 Oct 2016 08:19:53 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Michal Hocko <mhocko@...nel.org>,
Andrew Morton <akpm@...ux-foundation.org>
Cc: Mel Gorman <mgorman@...e.de>, Joonsoo Kim <js1304@...il.com>,
Dave Chinner <david@...morbit.com>, linux-mm@...ck.org,
LKML <linux-kernel@...r.kernel.org>,
Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH] mm, compaction: allow compaction for GFP_NOFS requests
On 10/12/2016 01:47 PM, Michal Hocko wrote:
> From: Michal Hocko <mhocko@...e.com>
>
> compaction has been disabled for GFP_NOFS and GFP_NOIO requests since
> the direct compaction was introduced by 56de7263fcf3 ("mm: compaction:
> direct compact when a high-order allocation fails"). The main reason
> is that the migration of page cache pages might recurse back to fs/io
> layer and we could potentially deadlock. This is overly conservative
> because all the anonymous memory is migrateable in the GFP_NOFS context
> just fine. This might be a large portion of the memory in many/most
> workkloads.
>
> Remove the GFP_NOFS restriction and make sure that we skip all fs pages
> (those with a mapping) while isolating pages to be migrated. We cannot
> consider clean fs pages because they might need a metadata update so
> only isolate pages without any mapping for nofs requests.
>
> The effect of this patch will be probably very limited in many/most
> workloads because higher order GFP_NOFS requests are quite rare,
> although different configurations might lead to very different results.
> David Chinner has mentioned a heavy metadata workload with 64kB block
> which to quote him:
> "
> Unfortunately, there was an era of cargo cult configuration tweaks
> in the Ceph community that has resulted in a large number of
> production machines with XFS filesystems configured this way. And a
> lot of them store large numbers of small files and run under
> significant sustained memory pressure.
>
> I slowly working towards getting rid of these high order allocations
> and replacing them with the equivalent number of single page
> allocations, but I haven't got that (complex) change working yet.
> "
>
> We can do the following to simulate that workload:
> $ mkfs.xfs -f -n size=64k <dev>
> $ mount <dev> /mnt/scratch
> $ time ./fs_mark -D 10000 -S0 -n 100000 -s 0 -L 32 \
> -d /mnt/scratch/0 -d /mnt/scratch/1 \
> -d /mnt/scratch/2 -d /mnt/scratch/3 \
> -d /mnt/scratch/4 -d /mnt/scratch/5 \
> -d /mnt/scratch/6 -d /mnt/scratch/7 \
> -d /mnt/scratch/8 -d /mnt/scratch/9 \
> -d /mnt/scratch/10 -d /mnt/scratch/11 \
> -d /mnt/scratch/12 -d /mnt/scratch/13 \
> -d /mnt/scratch/14 -d /mnt/scratch/15
>
> and indeed is hammers the system with many high order GFP_NOFS requests as
> per a simle tracepoint during the load:
> $ echo '!(gfp_flags & 0x80) && (gfp_flags &0x400000)' > $TRACE_MNT/events/kmem/mm_page_alloc/filter
> I am getting
> 5287609 order=0
> 37 order=1
> 1594905 order=2
> 3048439 order=3
> 6699207 order=4
> 66645 order=5
>
> My testing was done in a kvm guest so performance numbers should be
> taken with a grain of salt but there seems to be a difference when the
> patch is applied:
>
> * Original kernel
> FSUse% Count Size Files/sec App Overhead
> 1 1600000 0 4300.1 20745838
> 3 3200000 0 4239.9 23849857
> 5 4800000 0 4243.4 25939543
> 6 6400000 0 4248.4 19514050
> 8 8000000 0 4262.1 20796169
> 9 9600000 0 4257.6 21288675
> 11 11200000 0 4259.7 19375120
> 13 12800000 0 4220.7 22734141
> 14 14400000 0 4238.5 31936458
> 16 16000000 0 4231.5 23409901
> 18 17600000 0 4045.3 23577700
> 19 19200000 0 2783.4 58299526
> 21 20800000 0 2678.2 40616302
> 23 22400000 0 2693.5 83973996
>
> and xfs complaining about memory allocation not making progress
> [ 2304.372647] XFS: fs_mark(3289) possible memory allocation deadlock size 65624 in kmem_alloc (mode:0x2408240)
> [ 2304.443323] XFS: fs_mark(3285) possible memory allocation deadlock size 65728 in kmem_alloc (mode:0x2408240)
> [ 4796.772477] XFS: fs_mark(3424) possible memory allocation deadlock size 46936 in kmem_alloc (mode:0x2408240)
> [ 4796.775329] XFS: fs_mark(3423) possible memory allocation deadlock size 51416 in kmem_alloc (mode:0x2408240)
> [ 4797.388808] XFS: fs_mark(3424) possible memory allocation deadlock size 65728 in kmem_alloc (mode:0x2408240)
>
> * Patched kernel
> FSUse% Count Size Files/sec App Overhead
> 1 1600000 0 4289.1 19243934
> 3 3200000 0 4241.6 32828865
> 5 4800000 0 4248.7 32884693
> 6 6400000 0 4314.4 19608921
> 8 8000000 0 4269.9 24953292
> 9 9600000 0 4270.7 33235572
> 11 11200000 0 4346.4 40817101
> 13 12800000 0 4285.3 29972397
> 14 14400000 0 4297.2 20539765
> 16 16000000 0 4219.6 18596767
> 18 17600000 0 4273.8 49611187
> 19 19200000 0 4300.4 27944451
> 21 20800000 0 4270.6 22324585
> 22 22400000 0 4317.6 22650382
> 24 24000000 0 4065.2 22297964
>
> So the dropdown at Count 19200000 didn't happen and there was only a
> single warning about allocation not making progress
> [ 3063.815003] XFS: fs_mark(3272) possible memory allocation deadlock size 65624 in kmem_alloc (mode:0x2408240)
>
> This suggests that the patch has helped even though there is not all
> that much of anonymous memory as the workload mostly generates fs
> metadata. I assume the success rate would be higher with more anonymous
> memory which should be the case in many workloads.
>
> Changes since RFC
> - testing results from the test case suggested by David
> - fix kcompactd and proc triggered compaction by giving them GFP_KERNEL
> gfp_mask as per Vlastimil
>
> Signed-off-by: Michal Hocko <mhocko@...e.com>
Acked-by: Vlastimil Babka <vbabka@...e.cz>
Small nitpick below.
> @@ -1696,14 +1703,16 @@ enum compact_result try_to_compact_pages(gfp_t gfp_mask, unsigned int order,
> unsigned int alloc_flags, const struct alloc_context *ac,
> enum compact_priority prio)
> {
> - int may_enter_fs = gfp_mask & __GFP_FS;
> int may_perform_io = gfp_mask & __GFP_IO;
> struct zoneref *z;
> struct zone *zone;
> enum compact_result rc = COMPACT_SKIPPED;
>
> - /* Check if the GFP flags allow compaction */
> - if (!may_enter_fs || !may_perform_io)
> + /*
> + * Check if the GFP flags allow compaction - GFP_NOIO is really
> + * tricky context because the migration might require IO and
"and" ?
> + */
> + if (!may_perform_io)
> return COMPACT_SKIPPED;
>
> trace_mm_compaction_try_to_compact_pages(order, gfp_mask, prio);
> @@ -1770,6 +1779,7 @@ static void compact_node(int nid)
> .mode = MIGRATE_SYNC,
> .ignore_skip_hint = true,
> .whole_zone = true,
> + .gfp_mask = GFP_KERNEL,
> };
>
>
> @@ -1895,6 +1905,7 @@ static void kcompactd_do_work(pg_data_t *pgdat)
> .classzone_idx = pgdat->kcompactd_classzone_idx,
> .mode = MIGRATE_SYNC_LIGHT,
> .ignore_skip_hint = true,
> + .gfp_mask = GFP_KERNEL,
>
> };
> trace_mm_compaction_kcompactd_wake(pgdat->node_id, cc.order,
>
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