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Date: Wed, 12 Oct 2016 13:47:21 +0200
From: Michal Hocko <mhocko@...nel.org>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Mel Gorman <mgorman@...e.de>, Vlastimil Babka <vbabka@...e.cz>,
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: [PATCH] mm, compaction: allow compaction for GFP_NOFS requests
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>
---
Hi,
I have previously posted this as an RFC [1] but I feel this is worth
pursuing after knowing that there really might be some workloads
which trigger heavy GFP_NOFS workloads. My testing suggests that
the patch makes difference even in fs metadata heavy workloads which
do not involve a lot of anonymous memory.
[1] http://lkml.kernel.org/r/20161004081215.5563-1-mhocko@kernel.org
mm/compaction.c | 17 ++++++++++++++---
1 file changed, 14 insertions(+), 3 deletions(-)
diff --git a/mm/compaction.c b/mm/compaction.c
index 0409a4ad6ea1..d1d90e96ef4b 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -834,6 +834,13 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
page_count(page) > page_mapcount(page))
goto isolate_fail;
+ /*
+ * Only allow to migrate anonymous pages in GFP_NOFS context
+ * because those do not depend on fs locks.
+ */
+ if (!(cc->gfp_mask & __GFP_FS) && page_mapping(page))
+ goto isolate_fail;
+
/* If we already hold the lock, we can skip some rechecking */
if (!locked) {
locked = compact_trylock_irqsave(zone_lru_lock(zone),
@@ -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
+ */
+ 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,
--
2.9.3
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