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Subject[PATCH] mm, vmscan: Do not special-case slab reclaim when watermarks are boosted
Dave Chinner reported a problem pointing a finger at commit
1c30844d2dfe ("mm: reclaim small amounts of memory when an
external fragmentation event occurs"). The report is extensive (see
https://lore.kernel.org/linux-mm/20190807091858.2857-1-david@fromorbit.com/)
and it's worth recording the most relevant parts (colorful language and
typos included).

When running a simple, steady state 4kB file creation test to
simulate extracting tarballs larger than memory full of small
files into the filesystem, I noticed that once memory fills up
the cache balance goes to hell.

The workload is creating one dirty cached inode for every dirty
page, both of which should require a single IO each to clean and
reclaim, and creation of inodes is throttled by the rate at which
dirty writeback runs at (via balance dirty pages). Hence the ingest
rate of new cached inodes and page cache pages is identical and
steady. As a result, memory reclaim should quickly find a steady
balance between page cache and inode caches.

The moment memory fills, the page cache is reclaimed at a much
faster rate than the inode cache, and evidence suggests taht
the inode cache shrinker is not being called when large batches
of pages are being reclaimed. In roughly the same time period
that it takes to fill memory with 50% pages and 50% slab caches,
memory reclaim reduces the page cache down to just dirty pages
and slab caches fill the entirity of memory.

The LRU is largely full of dirty pages, and we're getting spikes
of random writeback from memory reclaim so it's all going to shit.
Behaviour never recovers, the page cache remains pinned at just
dirty pages, and nothing I could tune would make any difference.
vfs_cache_pressure makes no difference - I would it up so high
it should trim the entire inode caches in a singel pass, yet it
didn't do anything. It was clear from tracing and live telemetry
that the shrinkers were pretty much not running except when
there was absolutely no memory free at all, and then they did
the minimum necessary to free memory to make progress.

So I went looking at the code, trying to find places where pages
got reclaimed and the shrinkers weren't called. There's only one
- kswapd doing boosted reclaim as per commit 1c30844d2dfe ("mm:
reclaim small amounts of memory when an external fragmentation
event occurs").

The watermark boosting introduced by the commit is triggered in response
to an allocation "fragmentation event". The boosting was not intended to
target THP specifically and triggers even if THP is disabled. However,
with Dave's perfectly reasonable workload, fragmentation events can be
very common given the ratio of slab to page cache allocations so boosting
remains active for long periods of time.

As high-order allocations might use compaction and compaction cannot move
slab pages the decision was made in the commit to special-case kswapd
when watermarks are boosted -- kswapd avoids reclaiming slab as reclaiming
slab does not directly help compaction.

As Dave notes, this decision means that slab can be artificially protected
for long periods of time and messes up the balance with slab and page
caches.

Removing the special casing can still indirectly help fragmentation by
avoiding fragmentation-causing events due to slab allocation as pages
from a slab pageblock will have some slab objects freed. Furthermore,
with the special casing, reclaim behaviour is unpredictable as kswapd
sometimes examines slab and sometimes does not in a manner that is tricky
to tune or analyse.

This patch removes the special casing. The downside is that this is not a
universal performance win. Some benchmarks that depend on the residency
of data when rereading metadata may see a regression when slab reclaim
is restored to its original behaviour. Similarly, some benchmarks that
only read-once or write-once may perform better when page reclaim is too
aggressive. The primary upside is that slab shrinker is less surprising
(arguably more sane but that's a matter of opinion), behaves consistently
regardless of the fragmentation state of the system and properly obeys
VM sysctls.

A fsmark benchmark configuration was constructed similar to
what Dave reported and is codified by the mmtest configuration
config-io-fsmark-small-file-stream. It was evaluated on a 1-socket machine
to avoid dealing with NUMA-related issues and the timing of reclaim. The
storage was an SSD Samsung Evo and a fresh trimmed XFS filesystem was
used for the test data.

This is not an exact replication of Dave's setup. The configuration
scales its parameters depending on the memory size of the SUT to behave
similarly across machines. The parameters mean the first sample reported
by fs_mark is using 50% of RAM which will barely be throttled and look
like a big outlier. Dave used fake NUMA to have multiple kswapd instances
which I didn't replicate. Finally, the number of iterations differ from
Dave's test as the target disk was not large enough. While not identical,
it should be representative.

fsmark
5.3.0-rc3 5.3.0-rc3
vanilla shrinker-v1r1
Min 1-files/sec 4444.80 ( 0.00%) 4765.60 ( 7.22%)
1st-qrtle 1-files/sec 5005.10 ( 0.00%) 5091.70 ( 1.73%)
2nd-qrtle 1-files/sec 4917.80 ( 0.00%) 4855.60 ( -1.26%)
3rd-qrtle 1-files/sec 4667.40 ( 0.00%) 4831.20 ( 3.51%)
Max-1 1-files/sec 11421.50 ( 0.00%) 9999.30 ( -12.45%)
Max-5 1-files/sec 11421.50 ( 0.00%) 9999.30 ( -12.45%)
Max-10 1-files/sec 11421.50 ( 0.00%) 9999.30 ( -12.45%)
Max-90 1-files/sec 4649.60 ( 0.00%) 4780.70 ( 2.82%)
Max-95 1-files/sec 4491.00 ( 0.00%) 4768.20 ( 6.17%)
Max-99 1-files/sec 4491.00 ( 0.00%) 4768.20 ( 6.17%)
Max 1-files/sec 11421.50 ( 0.00%) 9999.30 ( -12.45%)
Hmean 1-files/sec 5004.75 ( 0.00%) 5075.96 ( 1.42%)
Stddev 1-files/sec 1778.70 ( 0.00%) 1369.66 ( 23.00%)
CoeffVar 1-files/sec 33.70 ( 0.00%) 26.05 ( 22.71%)
BHmean-99 1-files/sec 5053.72 ( 0.00%) 5101.52 ( 0.95%)
BHmean-95 1-files/sec 5053.72 ( 0.00%) 5101.52 ( 0.95%)
BHmean-90 1-files/sec 5107.05 ( 0.00%) 5131.41 ( 0.48%)
BHmean-75 1-files/sec 5208.45 ( 0.00%) 5206.68 ( -0.03%)
BHmean-50 1-files/sec 5405.53 ( 0.00%) 5381.62 ( -0.44%)
BHmean-25 1-files/sec 6179.75 ( 0.00%) 6095.14 ( -1.37%)

5.3.0-rc3 5.3.0-rc3
vanillashrinker-v1r1
Duration User 501.82 497.29
Duration System 4401.44 4424.08
Duration Elapsed 8124.76 8358.05

This is showing a slight skew for the max result representing a
large outlier for the 1st, 2nd and 3rd quartile are similar indicating
that the bulk of the results show little difference. Note that an
earlier version of the fsmark configuration showed a regression but
that included more samples taken while memory was still filling.

Note that the elapsed time is higher. Part of this is that the
configuration included time to delete all the test files when the test
completes -- the test automation handles the possibility of testing fsmark
with multiple thread counts. Without the patch, many of these objects
would be memory resident which is part of what the patch is addressing.

There are other important observations that justify the patch.

1. With the vanilla kernel, the number of dirty pages in the system
is very low for much of the test. With this patch, dirty pages
is generally kept at 10% which matches vm.dirty_background_ratio
which is normal expected historical behaviour.

2. With the vanilla kernel, the ratio of Slab/Pagecache is close to
0.95 for much of the test i.e. Slab is being left alone and dominating
memory consumption. With the patch applied, the ratio varies between
0.35 and 0.45 with the bulk of the measured ratios roughly half way
between those values. This is a different balance to what Dave reported
but it was at least consistent.

3. Slabs are scanned throughout the entire test with the patch applied.
The vanille kernel has periods with no scan activity and then relatively
massive spikes.

4. Without the patch, kswapd scan rates are very variable. With the patch,
the scan rates remain quite stead.

4. Overall vmstats are closer to normal expectations

5.3.0-rc3 5.3.0-rc3
vanilla shrinker-v1r1
Ops Direct pages scanned 99388.00 328410.00
Ops Kswapd pages scanned 45382917.00 33451026.00
Ops Kswapd pages reclaimed 30869570.00 25239655.00
Ops Direct pages reclaimed 74131.00 5830.00
Ops Kswapd efficiency % 68.02 75.45
Ops Kswapd velocity 5585.75 4002.25
Ops Page reclaim immediate 1179721.00 430927.00
Ops Slabs scanned 62367361.00 73581394.00
Ops Direct inode steals 2103.00 1002.00
Ops Kswapd inode steals 570180.00 5183206.00

o Vanilla kernel is hitting direct reclaim more frequently,
not very much in absolute terms but the fact the patch
reduces it is interesting
o "Page reclaim immediate" in the vanilla kernel indicates
dirty pages are being encountered at the tail of the LRU.
This is generally bad and means in this case that the LRU
is not long enough for dirty pages to be cleaned by the
background flush in time. This is much reduced by the
patch.
o With the patch, kswapd is reclaiming 10 times more slab
pages than with the vanilla kernel. This is indicative
of the watermark boosting over-protecting slab

A more complete set of tests were run that were part of the basis
for introducing boosting and while there are some differences, they
are well within tolerances.

Bottom line, the special casing kswapd to avoid slab behaviour is
unpredictable and can lead to abnormal results for normal workloads. This
patch restores the expected behaviour that slab and page cache is
balanced consistently for a workload with a steady allocation ratio of
slab/pagecache pages. It also means that if there are workloads that
favour the preservation of slab over pagecache that it can be tuned via
vm.vfs_cache_pressure where as the vanilla kernel effectively ignores
the parameter when boosting is active.

Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external fragmentation event occurs")
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Cc: stable@vger.kernel.org # v5.0+
---
mm/vmscan.c | 13 ++-----------
1 file changed, 2 insertions(+), 11 deletions(-)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index dbdc46a84f63..c77d1e3761a7 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -88,9 +88,6 @@ struct scan_control {
/* Can pages be swapped as part of reclaim? */
unsigned int may_swap:1;

- /* e.g. boosted watermark reclaim leaves slabs alone */
- unsigned int may_shrinkslab:1;
-
/*
* Cgroups are not reclaimed below their configured memory.low,
* unless we threaten to OOM. If any cgroups are skipped due to
@@ -2714,10 +2711,8 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
shrink_node_memcg(pgdat, memcg, sc, &lru_pages);
node_lru_pages += lru_pages;

- if (sc->may_shrinkslab) {
- shrink_slab(sc->gfp_mask, pgdat->node_id,
- memcg, sc->priority);
- }
+ shrink_slab(sc->gfp_mask, pgdat->node_id, memcg,
+ sc->priority);

/* Record the group's reclaim efficiency */
vmpressure(sc->gfp_mask, memcg, false,
@@ -3194,7 +3189,6 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
.may_writepage = !laptop_mode,
.may_unmap = 1,
.may_swap = 1,
- .may_shrinkslab = 1,
};

/*
@@ -3238,7 +3232,6 @@ unsigned long mem_cgroup_shrink_node(struct mem_cgroup *memcg,
.may_unmap = 1,
.reclaim_idx = MAX_NR_ZONES - 1,
.may_swap = !noswap,
- .may_shrinkslab = 1,
};
unsigned long lru_pages;

@@ -3286,7 +3279,6 @@ unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
.may_writepage = !laptop_mode,
.may_unmap = 1,
.may_swap = may_swap,
- .may_shrinkslab = 1,
};

set_task_reclaim_state(current, &sc.reclaim_state);
@@ -3598,7 +3590,6 @@ static int balance_pgdat(pg_data_t *pgdat, int order, int classzone_idx)
*/
sc.may_writepage = !laptop_mode && !nr_boost_reclaim;
sc.may_swap = !nr_boost_reclaim;
- sc.may_shrinkslab = !nr_boost_reclaim;

/*
* Do some background aging of the anon list, to give
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