The patch titled
Subject: mm/vmscan: throttle reclaim until some writeback completes if congested
has been added to the -mm tree. Its filename is
mm-vmscan-throttle-reclaim-until-some-writeback-completes-if-congested.patch
This patch should soon appear at
https://ozlabs.org/~akpm/mmots/broken-out/mm-vmscan-throttle-reclaim-until-some-writeback-completes-if-congested.patch
and later at
https://ozlabs.org/~akpm/mmotm/broken-out/mm-vmscan-throttle-reclaim-until-some-writeback-completes-if-congested.patch
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The -mm tree is included into linux-next and is updated
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------------------------------------------------------
From: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Subject: mm/vmscan: throttle reclaim until some writeback completes if congested
Patch series "Remove dependency on congestion_wait in mm/", v4.
This series that removes all calls to congestion_wait in mm/ and deletes
wait_iff_congested. It's not a clever implementation but congestion_wait
has been broken for a long time
(https://lore.kernel.org/linux-mm/45d8b7a6-8548-65f5-cccf-9f451d4ae3d4@xxxxxxxxx/).
Even if congestion throttling worked, it was never a great idea. While
excessive dirty/writeback pages at the tail of the LRU is one possibility
that reclaim may be slow, there is also the problem of too many pages
being isolated and reclaim failing for other reasons (elevated references,
too many pages isolated, excessive LRU contention etc).
This series replaces the "congestion" throttling with 3 different types.
o If there are too many dirty/writeback pages, sleep until a timeout
or enough pages get cleaned
o If too many pages are isolated, sleep until enough isolated pages
are either reclaimed or put back on the LRU
o If no progress is being made, direct reclaim tasks sleep until
another task makes progress with acceptable efficiency.
This was initially tested with a mix of workloads that used to trigger
corner cases that no longer work. A new test case was created called
"stutterp" (pagereclaim-stutterp-noreaders in mmtests) using a freshly
created XFS filesystem. Note that it may be necessary to increase the
timeout of ssh if executing remotely as ssh itself can get throttled and
the connection may timeout.
stutterp varies the number of "worker" processes from 4 up to NR_CPUS*4 to
check the impact as the number of direct reclaimers increase. It has four
types of worker.
o One "anon latency" worker creates small mappings with mmap() and times
how long it takes to fault the mapping reading it 4K at a time
o X file writers which is fio randomly writing X files where the total
size of the files add up to the allowed dirty_ratio. fio is allowed
to run for a warmup period to allow some file-backed pages to
accumulate. The duration of the warmup is based on the best-case
linear write speed of the storage.
o Y file readers which is fio randomly reading small files
o Z anon memory hogs which continually map (100-dirty_ratio)% of
memory
o Total estimated WSS = (100+dirty_ration) percentage of memory
X+Y+Z+1 == NR_WORKERS varying from 4 up to NR_CPUS*4
The intent is to maximise the total WSS with a mix of file and anon memory
where some anonymous memory must be swapped and there is a high likelihood
of dirty/writeback pages reaching the end of the LRU.
The test can be configured to have no background readers to stress
dirty/writeback pages. The results below are based on having zero readers.
The short summary of the results is that the series works and stalls
until some event occurs but the timeouts may need adjustment.
The test results are not broken down by patch as the series should be
treated as one block that replaces a broken throttling mechanism with a
working one.
Finally, three machines were tested but I'm reporting the worst set of
results. The other two machines had much better latencies for example.
First the results of the "anon latency" latency
stutterp
5.15.0-rc1 5.15.0-rc1
vanilla mm-reclaimcongest-v4r2
Amean mmap-4 31.4003 ( 0.00%) 176.8729 (-463.28%)
Amean mmap-7 38.1641 ( 0.00%) 605.8866 (-1487.58%)
Amean mmap-12 60.0981 ( 0.00%) 2866.8561 (-4670.29%)
Amean mmap-21 161.2699 ( 0.00%) 118.6587 ( 26.42%)
Amean mmap-30 174.5589 ( 0.00%) 3729.2263 (-2036.37%)
Amean mmap-48 8106.8160 ( 0.00%) 1463.7815 ( 81.94%)
Stddev mmap-4 41.3455 ( 0.00%) 5847.5425 (-14043.13%)
Stddev mmap-7 53.5556 ( 0.00%) 12091.9011 (-22478.20%)
Stddev mmap-12 171.3897 ( 0.00%) 28785.9881 (-16695.63%)
Stddev mmap-21 1506.6752 ( 0.00%) 1609.0361 ( -6.79%)
Stddev mmap-30 557.5806 ( 0.00%) 32712.2440 (-5766.82%)
Stddev mmap-48 61681.5718 ( 0.00%) 15971.4654 ( 74.11%)
Max-90 mmap-4 31.4243 ( 0.00%) 30.1957 ( 3.91%)
Max-90 mmap-7 41.0410 ( 0.00%) 36.7782 ( 10.39%)
Max-90 mmap-12 66.5255 ( 0.00%) 121.8574 ( -83.17%)
Max-90 mmap-21 146.7479 ( 0.00%) 132.2327 ( 9.89%)
Max-90 mmap-30 193.9513 ( 0.00%) 61.6135 ( 68.23%)
Max-90 mmap-48 277.9137 ( 0.00%) 593.7413 (-113.64%)
Max mmap-4 1913.8009 ( 0.00%) 239690.5578 (-12424.32%)
Max mmap-7 2423.9665 ( 0.00%) 270122.1751 (-11043.81%)
Max mmap-12 6845.6573 ( 0.00%) 308761.7416 (-4410.33%)
Max mmap-21 56278.6508 ( 0.00%) 79286.8553 ( -40.88%)
Max mmap-30 19716.2990 ( 0.00%) 306793.2333 (-1456.04%)
Max mmap-48 477923.9400 ( 0.00%) 229791.8793 ( 51.92%)
For most thread counts, the time to mmap() is unfortunately increased. In
earlier versions of the series, this was lower but a large number of
throttling events were reaching their timeout increasing the amount of
inefficient scanning of the LRU. There is no prioritisation of reclaim
tasks making progress based on each tasks rate of page allocation versus
progress of reclaim. The variance is also impacted for high worker counts
but in all cases, the differences in latency are not statistically
significant due to very large maximum outliers. Max-90 shows that 90% of
the stalls are comparable but the Max results show the massive outliers
which are increased to to stalling.
It is expected that this will be very machine dependant. Due to the test
design, reclaim is difficult so allocations stall and there are variances
depending on whether THPs can be allocated or not. The amount of memory
will affect exactly how bad the corner cases are and how often they
trigger. The warmup period calculation is not ideal as it's based on
linear writes where as fio is randomly writing multiple files from
multiple tasks so the start state of the test is variable. For example,
these are the latencies on a single-socket machine that had more memory
Amean mmap-4 20.5437 ( 0.00%) 19.1772 * 6.65%*
Amean mmap-6 39.2860 ( 0.00%) 69.4987 ( -76.90%)
Amean mmap-8 2476.1950 ( 0.00%) 151.7673 ( 93.87%)
Amean mmap-12 178.0936 ( 0.00%) 209.1427 ( -17.43%)
Amean mmap-18 3238.9125 ( 0.00%) 262.5806 ( 91.89%)
Amean mmap-24 7922.7016 ( 0.00%) 322.9738 ( 95.92%)
Amean mmap-30 1766.8392 ( 0.00%) 405.8898 ( 77.03%)
Amean mmap-32 7542.2844 ( 0.00%) 555.6236 ( 92.63%)
Amean mmap-32 7542.2844 ( 0.00%) 512.1812 ( 93.21%)
The overall system CPU usage and elapsed time is as follows
5.15.0-rc3 5.15.0-rc3
vanilla mm-reclaimcongest-v4r2
Duration User 6989.03 2368.70
Duration System 7308.12 843.35
Duration Elapsed 2277.67 2131.77
The patches reduce system CPU usage by 88% as the vanilla kernel is rarely
stalling.
The high-level /proc/vmstats show
5.15.0-rc1 5.15.0-rc1
vanilla mm-reclaimcongest-v4r2
Ops Direct pages scanned 1056608451.00 76886196.00
Ops Kswapd pages scanned 109795048.00 82179688.00
Ops Kswapd pages reclaimed 63269243.00 27410157.00
Ops Direct pages reclaimed 10803973.00 8016444.00
Ops Kswapd efficiency % 57.62 33.35
Ops Kswapd velocity 48204.98 38549.98
Ops Direct efficiency % 1.02 10.43
Ops Direct velocity 463898.83 36066.83
Kswapd scanned lesspages but the detailed pattern is different. The
vanilla kernel scans slowly over time where as the patches exhibits burst
patterns of scan activity. Direct reclaim scanning is reduced by 92% due
to stalling.
The pattern for stealing pages is also slightly different. Both kernels
exhibit spikes but the vanilla kernel when reclaiming shows pages being
reclaimed over a period of time where as the patches tend to reclaim in
spikes. The difference is that vanilla is not throttling and instead
scanning constantly finding some pages over time where as the patched
kernel throttles and reclaims in spikes.
Ops Percentage direct scans 90.59 48.34
For direct reclaim, vanilla scanned 90.59% of pages where as with the
patches, 48.34% were direct reclaim due to throttling
Ops Page writes by reclaim 2613590.00 1882533.00
Page writes from reclaim context are reduced.
Ops Page writes anon 2932752.00 2266749.00
And there is slightly less swapping.
Ops Page reclaim immediate 996248528.00 29230920.00
The number of pages encountered at the tail of the LRU tagged for immediate
reclaim but still dirty/writeback is reduced by 97%.
Ops Slabs scanned 164284.00 166646.00
Slab scan activity is similar.
ftrace was used to gather stall activity
Vanilla
-------
1 writeback_wait_iff_congested: usec_timeout=100000 usec_delayed=16000
2 writeback_wait_iff_congested: usec_timeout=100000 usec_delayed=12000
8 writeback_wait_iff_congested: usec_timeout=100000 usec_delayed=8000
29 writeback_wait_iff_congested: usec_timeout=100000 usec_delayed=4000
82394 writeback_wait_iff_congested: usec_timeout=100000 usec_delayed=0
The fast majority of wait_iff_congested calls do not stall at all.
What is likely happening is that cond_resched() reschedules the task for
a short period when the BDI is not registering congestion (which it never
will in this test setup).
1 writeback_congestion_wait: usec_timeout=100000 usec_delayed=120000
2 writeback_congestion_wait: usec_timeout=100000 usec_delayed=132000
4 writeback_congestion_wait: usec_timeout=100000 usec_delayed=112000
380 writeback_congestion_wait: usec_timeout=100000 usec_delayed=108000
778 writeback_congestion_wait: usec_timeout=100000 usec_delayed=104000
congestion_wait if called always exceeds the timeout as there is no
trigger to wake it up.
Bottom line: Vanilla will throttle but it's not effective.
Patch series
------------
Kswapd throttle activity was always due to scanning pages tagged for
immediate reclaim at the tail of the LRU
1 usect_delayed=12000 reason=VMSCAN_THROTTLE_WRITEBACK
1 usect_delayed=16000 reason=VMSCAN_THROTTLE_WRITEBACK
1 usect_delayed=28000 reason=VMSCAN_THROTTLE_WRITEBACK
1 usect_delayed=68000 reason=VMSCAN_THROTTLE_WRITEBACK
1 usect_delayed=96000 reason=VMSCAN_THROTTLE_WRITEBACK
2 usect_delayed=24000 reason=VMSCAN_THROTTLE_WRITEBACK
8 usect_delayed=8000 reason=VMSCAN_THROTTLE_WRITEBACK
23 usect_delayed=100000 reason=VMSCAN_THROTTLE_WRITEBACK
52 usect_delayed=4000 reason=VMSCAN_THROTTLE_WRITEBACK
61 usect_delayed=0 reason=VMSCAN_THROTTLE_WRITEBAC
The majority of events did not stall or stalled for a short period.
Roughly 16% of stalls reached the timeout before expiry. For direct
reclaim, the number of times stalled for each reason were
13594 reason=VMSCAN_THROTTLE_ISOLATED
72247 reason=VMSCAN_THROTTLE_WRITEBACK
77203 reason=VMSCAN_THROTTLE_NOPROGRESS
The most common reason to stall was due to a failure to make forward
progress followed closely by excessive pages tagged for immediate reclaim
at the tail of the LRU. A relatively small number were due to too many
pages isolated from the LRU by parallel threads
For VMSCAN_THROTTLE_ISOLATED, the breakdown of delays was
3 usec_timeout=20000 usect_delayed=16000 reason=VMSCAN_THROTTLE_ISOLATED
8 usec_timeout=20000 usect_delayed=8000 reason=VMSCAN_THROTTLE_ISOLATED
9 usec_timeout=20000 usect_delayed=12000 reason=VMSCAN_THROTTLE_ISOLATED
18 usec_timeout=20000 usect_delayed=4000 reason=VMSCAN_THROTTLE_ISOLATED
69 usec_timeout=20000 usect_delayed=20000 reason=VMSCAN_THROTTLE_ISOLATED
1946 usec_timeout=20000 usect_delayed=0 reason=VMSCAN_THROTTLE_ISOLATED
Most did not stall at all or for a short period. A small number reached
the timeout.
For VMSCAN_THROTTLE_NOPROGRESS, the breakdown of stalls were all over the
map
1 usec_timeout=500000 usect_delayed=188000 reason=VMSCAN_THROTTLE_NOPROGRESS
1 usec_timeout=500000 usect_delayed=204000 reason=VMSCAN_THROTTLE_NOPROGRESS
1 usec_timeout=500000 usect_delayed=264000 reason=VMSCAN_THROTTLE_NOPROGRESS
1 usec_timeout=500000 usect_delayed=268000 reason=VMSCAN_THROTTLE_NOPROGRESS
1 usec_timeout=500000 usect_delayed=276000 reason=VMSCAN_THROTTLE_NOPROGRESS
1 usec_timeout=500000 usect_delayed=360000 reason=VMSCAN_THROTTLE_NOPROGRESS
1 usec_timeout=500000 usect_delayed=364000 reason=VMSCAN_THROTTLE_NOPROGRESS
1 usec_timeout=500000 usect_delayed=380000 reason=VMSCAN_THROTTLE_NOPROGRESS
1 usec_timeout=500000 usect_delayed=388000 reason=VMSCAN_THROTTLE_NOPROGRESS
1 usec_timeout=500000 usect_delayed=400000 reason=VMSCAN_THROTTLE_NOPROGRESS
1 usec_timeout=500000 usect_delayed=432000 reason=VMSCAN_THROTTLE_NOPROGRESS
1 usec_timeout=500000 usect_delayed=468000 reason=VMSCAN_THROTTLE_NOPROGRESS
2 usec_timeout=500000 usect_delayed=180000 reason=VMSCAN_THROTTLE_NOPROGRESS
2 usec_timeout=500000 usect_delayed=236000 reason=VMSCAN_THROTTLE_NOPROGRESS
2 usec_timeout=500000 usect_delayed=284000 reason=VMSCAN_THROTTLE_NOPROGRESS
2 usec_timeout=500000 usect_delayed=396000 reason=VMSCAN_THROTTLE_NOPROGRESS
2 usec_timeout=500000 usect_delayed=404000 reason=VMSCAN_THROTTLE_NOPROGRESS
2 usec_timeout=500000 usect_delayed=420000 reason=VMSCAN_THROTTLE_NOPROGRESS
2 usec_timeout=500000 usect_delayed=436000 reason=VMSCAN_THROTTLE_NOPROGRESS
2 usec_timeout=500000 usect_delayed=456000 reason=VMSCAN_THROTTLE_NOPROGRESS
2 usec_timeout=500000 usect_delayed=464000 reason=VMSCAN_THROTTLE_NOPROGRESS
2 usec_timeout=500000 usect_delayed=472000 reason=VMSCAN_THROTTLE_NOPROGRESS
3 usec_timeout=500000 usect_delayed=156000 reason=VMSCAN_THROTTLE_NOPROGRESS
3 usec_timeout=500000 usect_delayed=328000 reason=VMSCAN_THROTTLE_NOPROGRESS
3 usec_timeout=500000 usect_delayed=336000 reason=VMSCAN_THROTTLE_NOPROGRESS
3 usec_timeout=500000 usect_delayed=476000 reason=VMSCAN_THROTTLE_NOPROGRESS
4 usec_timeout=500000 usect_delayed=168000 reason=VMSCAN_THROTTLE_NOPROGRESS
4 usec_timeout=500000 usect_delayed=348000 reason=VMSCAN_THROTTLE_NOPROGRESS
4 usec_timeout=500000 usect_delayed=412000 reason=VMSCAN_THROTTLE_NOPROGRESS
4 usec_timeout=500000 usect_delayed=452000 reason=VMSCAN_THROTTLE_NOPROGRESS
4 usec_timeout=500000 usect_delayed=484000 reason=VMSCAN_THROTTLE_NOPROGRESS
5 usec_timeout=500000 usect_delayed=164000 reason=VMSCAN_THROTTLE_NOPROGRESS
5 usec_timeout=500000 usect_delayed=240000 reason=VMSCAN_THROTTLE_NOPROGRESS
5 usec_timeout=500000 usect_delayed=256000 reason=VMSCAN_THROTTLE_NOPROGRESS
5 usec_timeout=500000 usect_delayed=316000 reason=VMSCAN_THROTTLE_NOPROGRESS
5 usec_timeout=500000 usect_delayed=352000 reason=VMSCAN_THROTTLE_NOPROGRESS
5 usec_timeout=500000 usect_delayed=408000 reason=VMSCAN_THROTTLE_NOPROGRESS
5 usec_timeout=500000 usect_delayed=444000 reason=VMSCAN_THROTTLE_NOPROGRESS
5 usec_timeout=500000 usect_delayed=492000 reason=VMSCAN_THROTTLE_NOPROGRESS
6 usec_timeout=500000 usect_delayed=280000 reason=VMSCAN_THROTTLE_NOPROGRESS
6 usec_timeout=500000 usect_delayed=332000 reason=VMSCAN_THROTTLE_NOPROGRESS
6 usec_timeout=500000 usect_delayed=368000 reason=VMSCAN_THROTTLE_NOPROGRESS
7 usec_timeout=500000 usect_delayed=116000 reason=VMSCAN_THROTTLE_NOPROGRESS
7 usec_timeout=500000 usect_delayed=132000 reason=VMSCAN_THROTTLE_NOPROGRESS
7 usec_timeout=500000 usect_delayed=136000 reason=VMSCAN_THROTTLE_NOPROGRESS
7 usec_timeout=500000 usect_delayed=272000 reason=VMSCAN_THROTTLE_NOPROGRESS
7 usec_timeout=500000 usect_delayed=308000 reason=VMSCAN_THROTTLE_NOPROGRESS
7 usec_timeout=500000 usect_delayed=440000 reason=VMSCAN_THROTTLE_NOPROGRESS
8 usec_timeout=500000 usect_delayed=292000 reason=VMSCAN_THROTTLE_NOPROGRESS
8 usec_timeout=500000 usect_delayed=324000 reason=VMSCAN_THROTTLE_NOPROGRESS
8 usec_timeout=500000 usect_delayed=448000 reason=VMSCAN_THROTTLE_NOPROGRESS
9 usec_timeout=500000 usect_delayed=144000 reason=VMSCAN_THROTTLE_NOPROGRESS
9 usec_timeout=500000 usect_delayed=152000 reason=VMSCAN_THROTTLE_NOPROGRESS
9 usec_timeout=500000 usect_delayed=184000 reason=VMSCAN_THROTTLE_NOPROGRESS
10 usec_timeout=500000 usect_delayed=392000 reason=VMSCAN_THROTTLE_NOPROGRESS
10 usec_timeout=500000 usect_delayed=424000 reason=VMSCAN_THROTTLE_NOPROGRESS
11 usec_timeout=500000 usect_delayed=220000 reason=VMSCAN_THROTTLE_NOPROGRESS
11 usec_timeout=500000 usect_delayed=228000 reason=VMSCAN_THROTTLE_NOPROGRESS
11 usec_timeout=500000 usect_delayed=252000 reason=VMSCAN_THROTTLE_NOPROGRESS
12 usec_timeout=500000 usect_delayed=140000 reason=VMSCAN_THROTTLE_NOPROGRESS
12 usec_timeout=500000 usect_delayed=148000 reason=VMSCAN_THROTTLE_NOPROGRESS
12 usec_timeout=500000 usect_delayed=384000 reason=VMSCAN_THROTTLE_NOPROGRESS
13 usec_timeout=500000 usect_delayed=212000 reason=VMSCAN_THROTTLE_NOPROGRESS
14 usec_timeout=500000 usect_delayed=176000 reason=VMSCAN_THROTTLE_NOPROGRESS
14 usec_timeout=500000 usect_delayed=488000 reason=VMSCAN_THROTTLE_NOPROGRESS
15 usec_timeout=500000 usect_delayed=196000 reason=VMSCAN_THROTTLE_NOPROGRESS
17 usec_timeout=500000 usect_delayed=216000 reason=VMSCAN_THROTTLE_NOPROGRESS
18 usec_timeout=500000 usect_delayed=112000 reason=VMSCAN_THROTTLE_NOPROGRESS
20 usec_timeout=500000 usect_delayed=124000 reason=VMSCAN_THROTTLE_NOPROGRESS
20 usec_timeout=500000 usect_delayed=300000 reason=VMSCAN_THROTTLE_NOPROGRESS
21 usec_timeout=500000 usect_delayed=304000 reason=VMSCAN_THROTTLE_NOPROGRESS
24 usec_timeout=500000 usect_delayed=120000 reason=VMSCAN_THROTTLE_NOPROGRESS
24 usec_timeout=500000 usect_delayed=312000 reason=VMSCAN_THROTTLE_NOPROGRESS
27 usec_timeout=500000 usect_delayed=224000 reason=VMSCAN_THROTTLE_NOPROGRESS
27 usec_timeout=500000 usect_delayed=68000 reason=VMSCAN_THROTTLE_NOPROGRESS
28 usec_timeout=500000 usect_delayed=416000 reason=VMSCAN_THROTTLE_NOPROGRESS
29 usec_timeout=500000 usect_delayed=200000 reason=VMSCAN_THROTTLE_NOPROGRESS
30 usec_timeout=500000 usect_delayed=160000 reason=VMSCAN_THROTTLE_NOPROGRESS
30 usec_timeout=500000 usect_delayed=60000 reason=VMSCAN_THROTTLE_NOPROGRESS
30 usec_timeout=500000 usect_delayed=76000 reason=VMSCAN_THROTTLE_NOPROGRESS
31 usec_timeout=500000 usect_delayed=496000 reason=VMSCAN_THROTTLE_NOPROGRESS
32 usec_timeout=500000 usect_delayed=192000 reason=VMSCAN_THROTTLE_NOPROGRESS
32 usec_timeout=500000 usect_delayed=296000 reason=VMSCAN_THROTTLE_NOPROGRESS
38 usec_timeout=500000 usect_delayed=232000 reason=VMSCAN_THROTTLE_NOPROGRESS
38 usec_timeout=500000 usect_delayed=320000 reason=VMSCAN_THROTTLE_NOPROGRESS
39 usec_timeout=500000 usect_delayed=208000 reason=VMSCAN_THROTTLE_NOPROGRESS
47 usec_timeout=500000 usect_delayed=108000 reason=VMSCAN_THROTTLE_NOPROGRESS
47 usec_timeout=500000 usect_delayed=52000 reason=VMSCAN_THROTTLE_NOPROGRESS
52 usec_timeout=500000 usect_delayed=128000 reason=VMSCAN_THROTTLE_NOPROGRESS
54 usec_timeout=500000 usect_delayed=80000 reason=VMSCAN_THROTTLE_NOPROGRESS
55 usec_timeout=500000 usect_delayed=288000 reason=VMSCAN_THROTTLE_NOPROGRESS
61 usec_timeout=500000 usect_delayed=72000 reason=VMSCAN_THROTTLE_NOPROGRESS
63 usec_timeout=500000 usect_delayed=84000 reason=VMSCAN_THROTTLE_NOPROGRESS
68 usec_timeout=500000 usect_delayed=64000 reason=VMSCAN_THROTTLE_NOPROGRESS
75 usec_timeout=500000 usect_delayed=44000 reason=VMSCAN_THROTTLE_NOPROGRESS
80 usec_timeout=500000 usect_delayed=48000 reason=VMSCAN_THROTTLE_NOPROGRESS
83 usec_timeout=500000 usect_delayed=88000 reason=VMSCAN_THROTTLE_NOPROGRESS
88 usec_timeout=500000 usect_delayed=56000 reason=VMSCAN_THROTTLE_NOPROGRESS
97 usec_timeout=500000 usect_delayed=100000 reason=VMSCAN_THROTTLE_NOPROGRESS
99 usec_timeout=500000 usect_delayed=36000 reason=VMSCAN_THROTTLE_NOPROGRESS
99 usec_timeout=500000 usect_delayed=92000 reason=VMSCAN_THROTTLE_NOPROGRESS
102 usec_timeout=500000 usect_delayed=480000 reason=VMSCAN_THROTTLE_NOPROGRESS
149 usec_timeout=500000 usect_delayed=40000 reason=VMSCAN_THROTTLE_NOPROGRESS
187 usec_timeout=500000 usect_delayed=32000 reason=VMSCAN_THROTTLE_NOPROGRESS
196 usec_timeout=500000 usect_delayed=28000 reason=VMSCAN_THROTTLE_NOPROGRESS
245 usec_timeout=500000 usect_delayed=96000 reason=VMSCAN_THROTTLE_NOPROGRESS
322 usec_timeout=500000 usect_delayed=24000 reason=VMSCAN_THROTTLE_NOPROGRESS
406 usec_timeout=500000 usect_delayed=20000 reason=VMSCAN_THROTTLE_NOPROGRESS
588 usec_timeout=500000 usect_delayed=16000 reason=VMSCAN_THROTTLE_NOPROGRESS
843 usec_timeout=500000 usect_delayed=12000 reason=VMSCAN_THROTTLE_NOPROGRESS
1299 usec_timeout=500000 usect_delayed=104000 reason=VMSCAN_THROTTLE_NOPROGRESS
2839 usec_timeout=500000 usect_delayed=8000 reason=VMSCAN_THROTTLE_NOPROGRESS
10111 usec_timeout=500000 usect_delayed=4000 reason=VMSCAN_THROTTLE_NOPROGRESS
21492 usec_timeout=500000 usect_delayed=0 reason=VMSCAN_THROTTLE_NOPROGRESS
36441 usec_timeout=500000 usect_delayed=500000 reason=VMSCAN_THROTTLE_NOPROGRESS
The full timeout is often hit but a large number also do not stall at all.
The remainder slept a little allowing other reclaim tasks to make progress.
While this timeout could be further increased, it could also negatively
impact worst-case behaviour when there is no prioritisation of what
task should make progress.
For VMSCAN_THROTTLE_WRITEBACK, the breakdown was
5 usec_timeout=100000 usect_delayed=48000 reason=VMSCAN_THROTTLE_WRITEBACK
7 usec_timeout=100000 usect_delayed=80000 reason=VMSCAN_THROTTLE_WRITEBACK
8 usec_timeout=100000 usect_delayed=60000 reason=VMSCAN_THROTTLE_WRITEBACK
9 usec_timeout=100000 usect_delayed=28000 reason=VMSCAN_THROTTLE_WRITEBACK
12 usec_timeout=100000 usect_delayed=72000 reason=VMSCAN_THROTTLE_WRITEBACK
12 usec_timeout=100000 usect_delayed=84000 reason=VMSCAN_THROTTLE_WRITEBACK
13 usec_timeout=100000 usect_delayed=40000 reason=VMSCAN_THROTTLE_WRITEBACK
14 usec_timeout=100000 usect_delayed=44000 reason=VMSCAN_THROTTLE_WRITEBACK
14 usec_timeout=100000 usect_delayed=76000 reason=VMSCAN_THROTTLE_WRITEBACK
16 usec_timeout=100000 usect_delayed=88000 reason=VMSCAN_THROTTLE_WRITEBACK
21 usec_timeout=100000 usect_delayed=68000 reason=VMSCAN_THROTTLE_WRITEBACK
24 usec_timeout=100000 usect_delayed=36000 reason=VMSCAN_THROTTLE_WRITEBACK
25 usec_timeout=100000 usect_delayed=56000 reason=VMSCAN_THROTTLE_WRITEBACK
26 usec_timeout=100000 usect_delayed=32000 reason=VMSCAN_THROTTLE_WRITEBACK
32 usec_timeout=100000 usect_delayed=52000 reason=VMSCAN_THROTTLE_WRITEBACK
45 usec_timeout=100000 usect_delayed=96000 reason=VMSCAN_THROTTLE_WRITEBACK
50 usec_timeout=100000 usect_delayed=92000 reason=VMSCAN_THROTTLE_WRITEBACK
60 usec_timeout=100000 usect_delayed=64000 reason=VMSCAN_THROTTLE_WRITEBACK
74 usec_timeout=100000 usect_delayed=24000 reason=VMSCAN_THROTTLE_WRITEBACK
122 usec_timeout=100000 usect_delayed=16000 reason=VMSCAN_THROTTLE_WRITEBACK
134 usec_timeout=100000 usect_delayed=20000 reason=VMSCAN_THROTTLE_WRITEBACK
310 usec_timeout=100000 usect_delayed=12000 reason=VMSCAN_THROTTLE_WRITEBACK
568 usec_timeout=100000 usect_delayed=8000 reason=VMSCAN_THROTTLE_WRITEBACK
2038 usec_timeout=100000 usect_delayed=4000 reason=VMSCAN_THROTTLE_WRITEBACK
7061 usec_timeout=100000 usect_delayed=0 reason=VMSCAN_THROTTLE_WRITEBACK
61547 usec_timeout=100000 usect_delayed=100000 reason=VMSCAN_THROTTLE_WRITEBACK
The majority hit the timeout in direct reclaim context although a sizable
number did not stall at all. This is very different to kswapd where only
a tiny percentage of stalls due to writeback reached the timeout.
Bottom line, the throttling appears to work and the wakeup events may
limit worst case stalls. There might be some grounds for adjusting
timeouts but it's likely futile as the worst-case scenarios depend on the
workload, memory size and the speed of the storage. A better approach to
improve the series further would be to prioritise tasks based on their
rate of allocation with the caveat that it may be very expensive to track.
This patch (of 8):
Page reclaim throttles on wait_iff_congested under the following conditions
o kswapd is encountering pages under writeback and marked for immediate
reclaim implying that pages are cycling through the LRU faster than
pages can be cleaned.
o Direct reclaim will stall if all dirty pages are backed by congested
inodes.
wait_iff_congested is almost completely broken with few exceptions. This
patch adds a new node-based workqueue and tracks the number of throttled
tasks and pages written back since throttling started. If enough pages
belonging to the node are written back then the throttled tasks will wake
early. If not, the throttled tasks sleeps until the timeout expires.
[neilb@xxxxxxx: Uninterruptible sleep and simpler wakeups]
[hdanton@xxxxxxxx: Avoid race when reclaim starts]
[vbabka@xxxxxxx: vmstat irq-safe api, clarifications]
Link: https://lkml.kernel.org/r/20211019090108.25501-1-mgorman@xxxxxxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20211019090108.25501-2-mgorman@xxxxxxxxxxxxxxxxxxx
Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
Cc: NeilBrown <neilb@xxxxxxx>
Cc: "Theodore Ts'o" <tytso@xxxxxxx>
Cc: Andreas Dilger <adilger.kernel@xxxxxxxxx>
Cc: "Darrick J . Wong" <djwong@xxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Dave Chinner <david@xxxxxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Hillf Danton <hdanton@xxxxxxxx>
Cc: Yang Shi <shy828301@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/backing-dev.h | 1
include/linux/mmzone.h | 9 +++
include/trace/events/vmscan.h | 34 ++++++++++++
include/trace/events/writeback.h | 7 --
mm/backing-dev.c | 48 ----------------
mm/filemap.c | 1
mm/internal.h | 11 +++
mm/page_alloc.c | 1
mm/vmscan.c | 82 ++++++++++++++++++++++++-----
mm/vmstat.c | 1
10 files changed, 127 insertions(+), 68 deletions(-)
--- a/include/linux/backing-dev.h~mm-vmscan-throttle-reclaim-until-some-writeback-completes-if-congested
+++ a/include/linux/backing-dev.h
@@ -154,7 +154,6 @@ static inline int wb_congested(struct bd
}
long congestion_wait(int sync, long timeout);
-long wait_iff_congested(int sync, long timeout);
static inline bool mapping_can_writeback(struct address_space *mapping)
{
--- a/include/linux/mmzone.h~mm-vmscan-throttle-reclaim-until-some-writeback-completes-if-congested
+++ a/include/linux/mmzone.h
@@ -199,6 +199,7 @@ enum node_stat_item {
NR_VMSCAN_IMMEDIATE, /* Prioritise for reclaim when writeback ends */
NR_DIRTIED, /* page dirtyings since bootup */
NR_WRITTEN, /* page writings since bootup */
+ NR_THROTTLED_WRITTEN, /* NR_WRITTEN while reclaim throttled */
NR_KERNEL_MISC_RECLAIMABLE, /* reclaimable non-slab kernel pages */
NR_FOLL_PIN_ACQUIRED, /* via: pin_user_page(), gup flag: FOLL_PIN */
NR_FOLL_PIN_RELEASED, /* pages returned via unpin_user_page() */
@@ -272,6 +273,10 @@ enum lru_list {
NR_LRU_LISTS
};
+enum vmscan_throttle_state {
+ VMSCAN_THROTTLE_WRITEBACK,
+};
+
#define for_each_lru(lru) for (lru = 0; lru < NR_LRU_LISTS; lru++)
#define for_each_evictable_lru(lru) for (lru = 0; lru <= LRU_ACTIVE_FILE; lru++)
@@ -841,6 +846,10 @@ typedef struct pglist_data {
int node_id;
wait_queue_head_t kswapd_wait;
wait_queue_head_t pfmemalloc_wait;
+ wait_queue_head_t reclaim_wait; /* wq for throttling reclaim */
+ atomic_t nr_reclaim_throttled; /* nr of throtted tasks */
+ unsigned long nr_reclaim_start; /* nr pages written while throttled
+ * when throttling started. */
struct task_struct *kswapd; /* Protected by
mem_hotplug_begin/end() */
int kswapd_order;
--- a/include/trace/events/vmscan.h~mm-vmscan-throttle-reclaim-until-some-writeback-completes-if-congested
+++ a/include/trace/events/vmscan.h
@@ -27,6 +27,14 @@
{RECLAIM_WB_ASYNC, "RECLAIM_WB_ASYNC"} \
) : "RECLAIM_WB_NONE"
+#define _VMSCAN_THROTTLE_WRITEBACK (1 << VMSCAN_THROTTLE_WRITEBACK)
+
+#define show_throttle_flags(flags) \
+ (flags) ? __print_flags(flags, "|", \
+ {_VMSCAN_THROTTLE_WRITEBACK, "VMSCAN_THROTTLE_WRITEBACK"} \
+ ) : "VMSCAN_THROTTLE_NONE"
+
+
#define trace_reclaim_flags(file) ( \
(file ? RECLAIM_WB_FILE : RECLAIM_WB_ANON) | \
(RECLAIM_WB_ASYNC) \
@@ -454,6 +462,32 @@ DEFINE_EVENT(mm_vmscan_direct_reclaim_en
TP_ARGS(nr_reclaimed)
);
+TRACE_EVENT(mm_vmscan_throttled,
+
+ TP_PROTO(int nid, int usec_timeout, int usec_delayed, int reason),
+
+ TP_ARGS(nid, usec_timeout, usec_delayed, reason),
+
+ TP_STRUCT__entry(
+ __field(int, nid)
+ __field(int, usec_timeout)
+ __field(int, usec_delayed)
+ __field(int, reason)
+ ),
+
+ TP_fast_assign(
+ __entry->nid = nid;
+ __entry->usec_timeout = usec_timeout;
+ __entry->usec_delayed = usec_delayed;
+ __entry->reason = 1U << reason;
+ ),
+
+ TP_printk("nid=%d usec_timeout=%d usect_delayed=%d reason=%s",
+ __entry->nid,
+ __entry->usec_timeout,
+ __entry->usec_delayed,
+ show_throttle_flags(__entry->reason))
+);
#endif /* _TRACE_VMSCAN_H */
/* This part must be outside protection */
--- a/include/trace/events/writeback.h~mm-vmscan-throttle-reclaim-until-some-writeback-completes-if-congested
+++ a/include/trace/events/writeback.h
@@ -763,13 +763,6 @@ DEFINE_EVENT(writeback_congest_waited_te
TP_ARGS(usec_timeout, usec_delayed)
);
-DEFINE_EVENT(writeback_congest_waited_template, writeback_wait_iff_congested,
-
- TP_PROTO(unsigned int usec_timeout, unsigned int usec_delayed),
-
- TP_ARGS(usec_timeout, usec_delayed)
-);
-
DECLARE_EVENT_CLASS(writeback_single_inode_template,
TP_PROTO(struct inode *inode,
--- a/mm/backing-dev.c~mm-vmscan-throttle-reclaim-until-some-writeback-completes-if-congested
+++ a/mm/backing-dev.c
@@ -1041,51 +1041,3 @@ long congestion_wait(int sync, long time
return ret;
}
EXPORT_SYMBOL(congestion_wait);
-
-/**
- * wait_iff_congested - Conditionally wait for a backing_dev to become uncongested or a pgdat to complete writes
- * @sync: SYNC or ASYNC IO
- * @timeout: timeout in jiffies
- *
- * In the event of a congested backing_dev (any backing_dev) this waits
- * for up to @timeout jiffies for either a BDI to exit congestion of the
- * given @sync queue or a write to complete.
- *
- * The return value is 0 if the sleep is for the full timeout. Otherwise,
- * it is the number of jiffies that were still remaining when the function
- * returned. return_value == timeout implies the function did not sleep.
- */
-long wait_iff_congested(int sync, long timeout)
-{
- long ret;
- unsigned long start = jiffies;
- DEFINE_WAIT(wait);
- wait_queue_head_t *wqh = &congestion_wqh[sync];
-
- /*
- * If there is no congestion, yield if necessary instead
- * of sleeping on the congestion queue
- */
- if (atomic_read(&nr_wb_congested[sync]) == 0) {
- cond_resched();
-
- /* In case we scheduled, work out time remaining */
- ret = timeout - (jiffies - start);
- if (ret < 0)
- ret = 0;
-
- goto out;
- }
-
- /* Sleep until uncongested or a write happens */
- prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
- ret = io_schedule_timeout(timeout);
- finish_wait(wqh, &wait);
-
-out:
- trace_writeback_wait_iff_congested(jiffies_to_usecs(timeout),
- jiffies_to_usecs(jiffies - start));
-
- return ret;
-}
-EXPORT_SYMBOL(wait_iff_congested);
--- a/mm/filemap.c~mm-vmscan-throttle-reclaim-until-some-writeback-completes-if-congested
+++ a/mm/filemap.c
@@ -1613,6 +1613,7 @@ void end_page_writeback(struct page *pag
smp_mb__after_atomic();
wake_up_page(page, PG_writeback);
+ acct_reclaim_writeback(page);
put_page(page);
}
EXPORT_SYMBOL(end_page_writeback);
--- a/mm/internal.h~mm-vmscan-throttle-reclaim-until-some-writeback-completes-if-congested
+++ a/mm/internal.h
@@ -34,6 +34,17 @@
void page_writeback_init(void);
+void __acct_reclaim_writeback(pg_data_t *pgdat, struct page *page,
+ int nr_throttled);
+static inline void acct_reclaim_writeback(struct page *page)
+{
+ pg_data_t *pgdat = page_pgdat(page);
+ int nr_throttled = atomic_read(&pgdat->nr_reclaim_throttled);
+
+ if (nr_throttled)
+ __acct_reclaim_writeback(pgdat, page, nr_throttled);
+}
+
vm_fault_t do_swap_page(struct vm_fault *vmf);
void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
--- a/mm/page_alloc.c~mm-vmscan-throttle-reclaim-until-some-writeback-completes-if-congested
+++ a/mm/page_alloc.c
@@ -7415,6 +7415,7 @@ static void __meminit pgdat_init_interna
init_waitqueue_head(&pgdat->kswapd_wait);
init_waitqueue_head(&pgdat->pfmemalloc_wait);
+ init_waitqueue_head(&pgdat->reclaim_wait);
pgdat_page_ext_init(pgdat);
lruvec_init(&pgdat->__lruvec);
--- a/mm/vmscan.c~mm-vmscan-throttle-reclaim-until-some-writeback-completes-if-congested
+++ a/mm/vmscan.c
@@ -1006,6 +1006,64 @@ static void handle_write_error(struct ad
unlock_page(page);
}
+static void
+reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason,
+ long timeout)
+{
+ wait_queue_head_t *wqh = &pgdat->reclaim_wait;
+ long ret;
+ DEFINE_WAIT(wait);
+
+ /*
+ * Do not throttle IO workers, kthreads other than kswapd or
+ * workqueues. They may be required for reclaim to make
+ * forward progress (e.g. journalling workqueues or kthreads).
+ */
+ if (!current_is_kswapd() &&
+ current->flags & (PF_IO_WORKER|PF_KTHREAD))
+ return;
+
+ if (atomic_inc_return(&pgdat->nr_reclaim_throttled) == 1) {
+ WRITE_ONCE(pgdat->nr_reclaim_start,
+ node_page_state(pgdat, NR_THROTTLED_WRITTEN));
+ }
+
+ prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
+ ret = schedule_timeout(timeout);
+ finish_wait(wqh, &wait);
+ atomic_dec(&pgdat->nr_reclaim_throttled);
+
+ trace_mm_vmscan_throttled(pgdat->node_id, jiffies_to_usecs(timeout),
+ jiffies_to_usecs(timeout - ret),
+ reason);
+}
+
+/*
+ * Account for pages written if tasks are throttled waiting on dirty
+ * pages to clean. If enough pages have been cleaned since throttling
+ * started then wakeup the throttled tasks.
+ */
+void __acct_reclaim_writeback(pg_data_t *pgdat, struct page *page,
+ int nr_throttled)
+{
+ unsigned long nr_written;
+
+ inc_node_page_state(page, NR_THROTTLED_WRITTEN);
+
+ /*
+ * This is an inaccurate read as the per-cpu deltas may not
+ * be synchronised. However, given that the system is
+ * writeback throttled, it is not worth taking the penalty
+ * of getting an accurate count. At worst, the throttle
+ * timeout guarantees forward progress.
+ */
+ nr_written = node_page_state(pgdat, NR_THROTTLED_WRITTEN) -
+ READ_ONCE(pgdat->nr_reclaim_start);
+
+ if (nr_written > SWAP_CLUSTER_MAX * nr_throttled)
+ wake_up_all(&pgdat->reclaim_wait);
+}
+
/* possible outcome of pageout() */
typedef enum {
/* failed to write page out, page is locked */
@@ -1418,9 +1476,8 @@ retry:
/*
* The number of dirty pages determines if a node is marked
- * reclaim_congested which affects wait_iff_congested. kswapd
- * will stall and start writing pages if the tail of the LRU
- * is all dirty unqueued pages.
+ * reclaim_congested. kswapd will stall and start writing
+ * pages if the tail of the LRU is all dirty unqueued pages.
*/
page_check_dirty_writeback(page, &dirty, &writeback);
if (dirty || writeback)
@@ -3186,19 +3243,19 @@ again:
* If kswapd scans pages marked for immediate
* reclaim and under writeback (nr_immediate), it
* implies that pages are cycling through the LRU
- * faster than they are written so also forcibly stall.
+ * faster than they are written so forcibly stall
+ * until some pages complete writeback.
*/
if (sc->nr.immediate)
- congestion_wait(BLK_RW_ASYNC, HZ/10);
+ reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK, HZ/10);
}
/*
- * Tag a node/memcg as congested if all the dirty pages
- * scanned were backed by a congested BDI and
- * wait_iff_congested will stall.
+ * Tag a node/memcg as congested if all the dirty pages were marked
+ * for writeback and immediate reclaim (counted in nr.congested).
*
* Legacy memcg will stall in page writeback so avoid forcibly
- * stalling in wait_iff_congested().
+ * stalling in reclaim_throttle().
*/
if ((current_is_kswapd() ||
(cgroup_reclaim(sc) && writeback_throttling_sane(sc))) &&
@@ -3206,15 +3263,15 @@ again:
set_bit(LRUVEC_CONGESTED, &target_lruvec->flags);
/*
- * Stall direct reclaim for IO completions if underlying BDIs
- * and node is congested. Allow kswapd to continue until it
+ * Stall direct reclaim for IO completions if the lruvec is
+ * node is congested. Allow kswapd to continue until it
* starts encountering unqueued dirty pages or cycling through
* the LRU too quickly.
*/
if (!current_is_kswapd() && current_may_throttle() &&
!sc->hibernation_mode &&
test_bit(LRUVEC_CONGESTED, &target_lruvec->flags))
- wait_iff_congested(BLK_RW_ASYNC, HZ/10);
+ reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK, HZ/10);
if (should_continue_reclaim(pgdat, sc->nr_reclaimed - nr_reclaimed,
sc))
@@ -4292,6 +4349,7 @@ static int kswapd(void *p)
WRITE_ONCE(pgdat->kswapd_order, 0);
WRITE_ONCE(pgdat->kswapd_highest_zoneidx, MAX_NR_ZONES);
+ atomic_set(&pgdat->nr_reclaim_throttled, 0);
for ( ; ; ) {
bool ret;
--- a/mm/vmstat.c~mm-vmscan-throttle-reclaim-until-some-writeback-completes-if-congested
+++ a/mm/vmstat.c
@@ -1225,6 +1225,7 @@ const char * const vmstat_text[] = {
"nr_vmscan_immediate_reclaim",
"nr_dirtied",
"nr_written",
+ "nr_throttled_written",
"nr_kernel_misc_reclaimable",
"nr_foll_pin_acquired",
"nr_foll_pin_released",
_
Patches currently in -mm which might be from mgorman@xxxxxxxxxxxxxxxxxxx are
mm-vmscan-throttle-reclaim-until-some-writeback-completes-if-congested.patch
mm-vmscan-throttle-reclaim-and-compaction-when-too-may-pages-are-isolated.patch
mm-vmscan-throttle-reclaim-when-no-progress-is-being-made.patch
mm-writeback-throttle-based-on-page-writeback-instead-of-congestion.patch
mm-page_alloc-remove-the-throttling-logic-from-the-page-allocator.patch
mm-vmscan-centralise-timeout-values-for-reclaim_throttle.patch
mm-vmscan-increase-the-timeout-if-page-reclaim-is-not-making-progress.patch
mm-vmscan-delay-waking-of-tasks-throttled-on-noprogress.patch
