This series replaces the v4 version in mmotm as the changes caused
excessive conflicts. This series is also available at
git://git.kernel.org/pub/scm/linux/kernel/git/mel/linux.git mm-reclaimcongest-v5r4
Changelog since v4
o Costmetic changes (neilb)
o Correct number of writeback throttled tasks (neilb)
o Use wake_up (neilb)
Changelog since v3
o Count writeback completions for NR_THROTTLED_WRITTEN only
o Use IRQ-safe inc_node_page_state
o Remove redundant throttling
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/[email protected]/).
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-v5r4
Amean mmap-4 31.4003 ( 0.00%) 2661.0198 (-8374.52%)
Amean mmap-7 38.1641 ( 0.00%) 149.2891 (-291.18%)
Amean mmap-12 60.0981 ( 0.00%) 187.8105 (-212.51%)
Amean mmap-21 161.2699 ( 0.00%) 213.9107 ( -32.64%)
Amean mmap-30 174.5589 ( 0.00%) 377.7548 (-116.41%)
Amean mmap-48 8106.8160 ( 0.00%) 1070.5616 ( 86.79%)
Stddev mmap-4 41.3455 ( 0.00%) 27573.9676 (-66591.66%)
Stddev mmap-7 53.5556 ( 0.00%) 4608.5860 (-8505.23%)
Stddev mmap-12 171.3897 ( 0.00%) 5559.4542 (-3143.75%)
Stddev mmap-21 1506.6752 ( 0.00%) 5746.2507 (-281.39%)
Stddev mmap-30 557.5806 ( 0.00%) 7678.1624 (-1277.05%)
Stddev mmap-48 61681.5718 ( 0.00%) 14507.2830 ( 76.48%)
Max-90 mmap-4 31.4243 ( 0.00%) 83.1457 (-164.59%)
Max-90 mmap-7 41.0410 ( 0.00%) 41.0720 ( -0.08%)
Max-90 mmap-12 66.5255 ( 0.00%) 53.9073 ( 18.97%)
Max-90 mmap-21 146.7479 ( 0.00%) 105.9540 ( 27.80%)
Max-90 mmap-30 193.9513 ( 0.00%) 64.3067 ( 66.84%)
Max-90 mmap-48 277.9137 ( 0.00%) 591.0594 (-112.68%)
Max mmap-4 1913.8009 ( 0.00%) 299623.9695 (-15555.96%)
Max mmap-7 2423.9665 ( 0.00%) 204453.1708 (-8334.65%)
Max mmap-12 6845.6573 ( 0.00%) 221090.3366 (-3129.64%)
Max mmap-21 56278.6508 ( 0.00%) 213877.3496 (-280.03%)
Max mmap-30 19716.2990 ( 0.00%) 216287.6229 (-997.00%)
Max mmap-48 477923.9400 ( 0.00%) 245414.8238 ( 48.65%)
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 42.2287 ( 0.00%) 49.6838 * -17.65%*
Amean mmap-7 216.4326 ( 0.00%) 47.4451 * 78.08%*
Amean mmap-12 2412.0588 ( 0.00%) 51.7497 ( 97.85%)
Amean mmap-21 5546.2548 ( 0.00%) 51.8862 ( 99.06%)
Amean mmap-30 1085.3121 ( 0.00%) 72.1004 ( 93.36%)
The overall system CPU usage and elapsed time is as follows
5.15.0-rc3 5.15.0-rc3
vanilla mm-reclaimcongest-v5r4
Duration User 6989.03 983.42
Duration System 7308.12 799.68
Duration Elapsed 2277.67 2092.98
The patches reduce system CPU usage by 89% as the vanilla kernel is rarely
stalling.
The high-level /proc/vmstats show
5.15.0-rc1 5.15.0-rc1
vanilla mm-reclaimcongest-v5r2
Ops Direct pages scanned 1056608451.00 503594991.00
Ops Kswapd pages scanned 109795048.00 147289810.00
Ops Kswapd pages reclaimed 63269243.00 31036005.00
Ops Direct pages reclaimed 10803973.00 6328887.00
Ops Kswapd efficiency % 57.62 21.07
Ops Kswapd velocity 48204.98 57572.86
Ops Direct efficiency % 1.02 1.26
Ops Direct velocity 463898.83 196845.97
Kswapd scanned less pages 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 52%
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 77.37
For direct reclaim, vanilla scanned 90.59% of pages where as with the
patches, 77.37% were direct reclaim due to throttling
Ops Page writes by reclaim 2613590.00 1687131.00
Page writes from reclaim context are reduced.
Ops Page writes anon 2932752.00 1917048.00
And there is less swapping.
Ops Page reclaim immediate 996248528.00 107664764.00
The number of pages encountered at the tail of the LRU tagged for immediate
reclaim but still dirty/writeback is reduced by 89%.
Ops Slabs scanned 164284.00 153608.00
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 usec_timeout=100000 usect_delayed=72000 reason=VMSCAN_THROTTLE_WRITEBACK
4 usec_timeout=100000 usect_delayed=20000 reason=VMSCAN_THROTTLE_WRITEBACK
5 usec_timeout=100000 usect_delayed=12000 reason=VMSCAN_THROTTLE_WRITEBACK
6 usec_timeout=100000 usect_delayed=16000 reason=VMSCAN_THROTTLE_WRITEBACK
11 usec_timeout=100000 usect_delayed=100000 reason=VMSCAN_THROTTLE_WRITEBACK
11 usec_timeout=100000 usect_delayed=8000 reason=VMSCAN_THROTTLE_WRITEBACK
94 usec_timeout=100000 usect_delayed=0 reason=VMSCAN_THROTTLE_WRITEBACK
112 usec_timeout=100000 usect_delayed=4000 reason=VMSCAN_THROTTLE_WRITEBACK
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
6624 reason=VMSCAN_THROTTLE_ISOLATED
93246 reason=VMSCAN_THROTTLE_NOPROGRESS
96934 reason=VMSCAN_THROTTLE_WRITEBACK
The most common reason to stall was due to excessive pages tagged for
immediate reclaim at the tail of the LRU followed by a failure to make
forward. 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
9 usec_timeout=20000 usect_delayed=4000 reason=VMSCAN_THROTTLE_ISOLATED
12 usec_timeout=20000 usect_delayed=16000 reason=VMSCAN_THROTTLE_ISOLATED
83 usec_timeout=20000 usect_delayed=20000 reason=VMSCAN_THROTTLE_ISOLATED
6520 usec_timeout=20000 usect_delayed=0 reason=VMSCAN_THROTTLE_ISOLATED
Most did not stall at all. 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=324000 reason=VMSCAN_THROTTLE_NOPROGRESS
1 usec_timeout=500000 usect_delayed=332000 reason=VMSCAN_THROTTLE_NOPROGRESS
1 usec_timeout=500000 usect_delayed=348000 reason=VMSCAN_THROTTLE_NOPROGRESS
1 usec_timeout=500000 usect_delayed=360000 reason=VMSCAN_THROTTLE_NOPROGRESS
2 usec_timeout=500000 usect_delayed=228000 reason=VMSCAN_THROTTLE_NOPROGRESS
2 usec_timeout=500000 usect_delayed=260000 reason=VMSCAN_THROTTLE_NOPROGRESS
2 usec_timeout=500000 usect_delayed=340000 reason=VMSCAN_THROTTLE_NOPROGRESS
2 usec_timeout=500000 usect_delayed=364000 reason=VMSCAN_THROTTLE_NOPROGRESS
2 usec_timeout=500000 usect_delayed=372000 reason=VMSCAN_THROTTLE_NOPROGRESS
2 usec_timeout=500000 usect_delayed=428000 reason=VMSCAN_THROTTLE_NOPROGRESS
2 usec_timeout=500000 usect_delayed=460000 reason=VMSCAN_THROTTLE_NOPROGRESS
2 usec_timeout=500000 usect_delayed=464000 reason=VMSCAN_THROTTLE_NOPROGRESS
3 usec_timeout=500000 usect_delayed=244000 reason=VMSCAN_THROTTLE_NOPROGRESS
3 usec_timeout=500000 usect_delayed=252000 reason=VMSCAN_THROTTLE_NOPROGRESS
3 usec_timeout=500000 usect_delayed=272000 reason=VMSCAN_THROTTLE_NOPROGRESS
4 usec_timeout=500000 usect_delayed=188000 reason=VMSCAN_THROTTLE_NOPROGRESS
4 usec_timeout=500000 usect_delayed=268000 reason=VMSCAN_THROTTLE_NOPROGRESS
4 usec_timeout=500000 usect_delayed=328000 reason=VMSCAN_THROTTLE_NOPROGRESS
4 usec_timeout=500000 usect_delayed=380000 reason=VMSCAN_THROTTLE_NOPROGRESS
4 usec_timeout=500000 usect_delayed=392000 reason=VMSCAN_THROTTLE_NOPROGRESS
4 usec_timeout=500000 usect_delayed=432000 reason=VMSCAN_THROTTLE_NOPROGRESS
5 usec_timeout=500000 usect_delayed=204000 reason=VMSCAN_THROTTLE_NOPROGRESS
5 usec_timeout=500000 usect_delayed=220000 reason=VMSCAN_THROTTLE_NOPROGRESS
5 usec_timeout=500000 usect_delayed=412000 reason=VMSCAN_THROTTLE_NOPROGRESS
5 usec_timeout=500000 usect_delayed=436000 reason=VMSCAN_THROTTLE_NOPROGRESS
6 usec_timeout=500000 usect_delayed=488000 reason=VMSCAN_THROTTLE_NOPROGRESS
7 usec_timeout=500000 usect_delayed=212000 reason=VMSCAN_THROTTLE_NOPROGRESS
7 usec_timeout=500000 usect_delayed=300000 reason=VMSCAN_THROTTLE_NOPROGRESS
7 usec_timeout=500000 usect_delayed=316000 reason=VMSCAN_THROTTLE_NOPROGRESS
7 usec_timeout=500000 usect_delayed=472000 reason=VMSCAN_THROTTLE_NOPROGRESS
8 usec_timeout=500000 usect_delayed=248000 reason=VMSCAN_THROTTLE_NOPROGRESS
8 usec_timeout=500000 usect_delayed=356000 reason=VMSCAN_THROTTLE_NOPROGRESS
8 usec_timeout=500000 usect_delayed=456000 reason=VMSCAN_THROTTLE_NOPROGRESS
9 usec_timeout=500000 usect_delayed=124000 reason=VMSCAN_THROTTLE_NOPROGRESS
9 usec_timeout=500000 usect_delayed=376000 reason=VMSCAN_THROTTLE_NOPROGRESS
9 usec_timeout=500000 usect_delayed=484000 reason=VMSCAN_THROTTLE_NOPROGRESS
10 usec_timeout=500000 usect_delayed=172000 reason=VMSCAN_THROTTLE_NOPROGRESS
10 usec_timeout=500000 usect_delayed=420000 reason=VMSCAN_THROTTLE_NOPROGRESS
10 usec_timeout=500000 usect_delayed=452000 reason=VMSCAN_THROTTLE_NOPROGRESS
11 usec_timeout=500000 usect_delayed=256000 reason=VMSCAN_THROTTLE_NOPROGRESS
12 usec_timeout=500000 usect_delayed=112000 reason=VMSCAN_THROTTLE_NOPROGRESS
12 usec_timeout=500000 usect_delayed=116000 reason=VMSCAN_THROTTLE_NOPROGRESS
12 usec_timeout=500000 usect_delayed=144000 reason=VMSCAN_THROTTLE_NOPROGRESS
12 usec_timeout=500000 usect_delayed=152000 reason=VMSCAN_THROTTLE_NOPROGRESS
12 usec_timeout=500000 usect_delayed=264000 reason=VMSCAN_THROTTLE_NOPROGRESS
12 usec_timeout=500000 usect_delayed=384000 reason=VMSCAN_THROTTLE_NOPROGRESS
12 usec_timeout=500000 usect_delayed=424000 reason=VMSCAN_THROTTLE_NOPROGRESS
12 usec_timeout=500000 usect_delayed=492000 reason=VMSCAN_THROTTLE_NOPROGRESS
13 usec_timeout=500000 usect_delayed=184000 reason=VMSCAN_THROTTLE_NOPROGRESS
13 usec_timeout=500000 usect_delayed=444000 reason=VMSCAN_THROTTLE_NOPROGRESS
14 usec_timeout=500000 usect_delayed=308000 reason=VMSCAN_THROTTLE_NOPROGRESS
14 usec_timeout=500000 usect_delayed=440000 reason=VMSCAN_THROTTLE_NOPROGRESS
14 usec_timeout=500000 usect_delayed=476000 reason=VMSCAN_THROTTLE_NOPROGRESS
16 usec_timeout=500000 usect_delayed=140000 reason=VMSCAN_THROTTLE_NOPROGRESS
17 usec_timeout=500000 usect_delayed=232000 reason=VMSCAN_THROTTLE_NOPROGRESS
17 usec_timeout=500000 usect_delayed=240000 reason=VMSCAN_THROTTLE_NOPROGRESS
17 usec_timeout=500000 usect_delayed=280000 reason=VMSCAN_THROTTLE_NOPROGRESS
18 usec_timeout=500000 usect_delayed=404000 reason=VMSCAN_THROTTLE_NOPROGRESS
20 usec_timeout=500000 usect_delayed=148000 reason=VMSCAN_THROTTLE_NOPROGRESS
20 usec_timeout=500000 usect_delayed=216000 reason=VMSCAN_THROTTLE_NOPROGRESS
20 usec_timeout=500000 usect_delayed=468000 reason=VMSCAN_THROTTLE_NOPROGRESS
21 usec_timeout=500000 usect_delayed=448000 reason=VMSCAN_THROTTLE_NOPROGRESS
23 usec_timeout=500000 usect_delayed=168000 reason=VMSCAN_THROTTLE_NOPROGRESS
23 usec_timeout=500000 usect_delayed=296000 reason=VMSCAN_THROTTLE_NOPROGRESS
25 usec_timeout=500000 usect_delayed=132000 reason=VMSCAN_THROTTLE_NOPROGRESS
25 usec_timeout=500000 usect_delayed=352000 reason=VMSCAN_THROTTLE_NOPROGRESS
26 usec_timeout=500000 usect_delayed=180000 reason=VMSCAN_THROTTLE_NOPROGRESS
27 usec_timeout=500000 usect_delayed=284000 reason=VMSCAN_THROTTLE_NOPROGRESS
28 usec_timeout=500000 usect_delayed=164000 reason=VMSCAN_THROTTLE_NOPROGRESS
29 usec_timeout=500000 usect_delayed=136000 reason=VMSCAN_THROTTLE_NOPROGRESS
30 usec_timeout=500000 usect_delayed=200000 reason=VMSCAN_THROTTLE_NOPROGRESS
30 usec_timeout=500000 usect_delayed=400000 reason=VMSCAN_THROTTLE_NOPROGRESS
31 usec_timeout=500000 usect_delayed=196000 reason=VMSCAN_THROTTLE_NOPROGRESS
32 usec_timeout=500000 usect_delayed=156000 reason=VMSCAN_THROTTLE_NOPROGRESS
33 usec_timeout=500000 usect_delayed=224000 reason=VMSCAN_THROTTLE_NOPROGRESS
35 usec_timeout=500000 usect_delayed=128000 reason=VMSCAN_THROTTLE_NOPROGRESS
35 usec_timeout=500000 usect_delayed=176000 reason=VMSCAN_THROTTLE_NOPROGRESS
36 usec_timeout=500000 usect_delayed=368000 reason=VMSCAN_THROTTLE_NOPROGRESS
36 usec_timeout=500000 usect_delayed=496000 reason=VMSCAN_THROTTLE_NOPROGRESS
37 usec_timeout=500000 usect_delayed=312000 reason=VMSCAN_THROTTLE_NOPROGRESS
38 usec_timeout=500000 usect_delayed=304000 reason=VMSCAN_THROTTLE_NOPROGRESS
40 usec_timeout=500000 usect_delayed=288000 reason=VMSCAN_THROTTLE_NOPROGRESS
43 usec_timeout=500000 usect_delayed=408000 reason=VMSCAN_THROTTLE_NOPROGRESS
55 usec_timeout=500000 usect_delayed=416000 reason=VMSCAN_THROTTLE_NOPROGRESS
56 usec_timeout=500000 usect_delayed=76000 reason=VMSCAN_THROTTLE_NOPROGRESS
58 usec_timeout=500000 usect_delayed=120000 reason=VMSCAN_THROTTLE_NOPROGRESS
59 usec_timeout=500000 usect_delayed=208000 reason=VMSCAN_THROTTLE_NOPROGRESS
61 usec_timeout=500000 usect_delayed=68000 reason=VMSCAN_THROTTLE_NOPROGRESS
71 usec_timeout=500000 usect_delayed=192000 reason=VMSCAN_THROTTLE_NOPROGRESS
71 usec_timeout=500000 usect_delayed=480000 reason=VMSCAN_THROTTLE_NOPROGRESS
79 usec_timeout=500000 usect_delayed=60000 reason=VMSCAN_THROTTLE_NOPROGRESS
82 usec_timeout=500000 usect_delayed=320000 reason=VMSCAN_THROTTLE_NOPROGRESS
82 usec_timeout=500000 usect_delayed=92000 reason=VMSCAN_THROTTLE_NOPROGRESS
85 usec_timeout=500000 usect_delayed=64000 reason=VMSCAN_THROTTLE_NOPROGRESS
85 usec_timeout=500000 usect_delayed=80000 reason=VMSCAN_THROTTLE_NOPROGRESS
88 usec_timeout=500000 usect_delayed=84000 reason=VMSCAN_THROTTLE_NOPROGRESS
90 usec_timeout=500000 usect_delayed=160000 reason=VMSCAN_THROTTLE_NOPROGRESS
90 usec_timeout=500000 usect_delayed=292000 reason=VMSCAN_THROTTLE_NOPROGRESS
94 usec_timeout=500000 usect_delayed=56000 reason=VMSCAN_THROTTLE_NOPROGRESS
118 usec_timeout=500000 usect_delayed=88000 reason=VMSCAN_THROTTLE_NOPROGRESS
119 usec_timeout=500000 usect_delayed=72000 reason=VMSCAN_THROTTLE_NOPROGRESS
126 usec_timeout=500000 usect_delayed=108000 reason=VMSCAN_THROTTLE_NOPROGRESS
146 usec_timeout=500000 usect_delayed=52000 reason=VMSCAN_THROTTLE_NOPROGRESS
148 usec_timeout=500000 usect_delayed=36000 reason=VMSCAN_THROTTLE_NOPROGRESS
148 usec_timeout=500000 usect_delayed=48000 reason=VMSCAN_THROTTLE_NOPROGRESS
159 usec_timeout=500000 usect_delayed=28000 reason=VMSCAN_THROTTLE_NOPROGRESS
178 usec_timeout=500000 usect_delayed=44000 reason=VMSCAN_THROTTLE_NOPROGRESS
183 usec_timeout=500000 usect_delayed=40000 reason=VMSCAN_THROTTLE_NOPROGRESS
237 usec_timeout=500000 usect_delayed=100000 reason=VMSCAN_THROTTLE_NOPROGRESS
266 usec_timeout=500000 usect_delayed=32000 reason=VMSCAN_THROTTLE_NOPROGRESS
313 usec_timeout=500000 usect_delayed=24000 reason=VMSCAN_THROTTLE_NOPROGRESS
347 usec_timeout=500000 usect_delayed=96000 reason=VMSCAN_THROTTLE_NOPROGRESS
470 usec_timeout=500000 usect_delayed=20000 reason=VMSCAN_THROTTLE_NOPROGRESS
559 usec_timeout=500000 usect_delayed=16000 reason=VMSCAN_THROTTLE_NOPROGRESS
964 usec_timeout=500000 usect_delayed=12000 reason=VMSCAN_THROTTLE_NOPROGRESS
2001 usec_timeout=500000 usect_delayed=104000 reason=VMSCAN_THROTTLE_NOPROGRESS
2447 usec_timeout=500000 usect_delayed=8000 reason=VMSCAN_THROTTLE_NOPROGRESS
7888 usec_timeout=500000 usect_delayed=4000 reason=VMSCAN_THROTTLE_NOPROGRESS
22727 usec_timeout=500000 usect_delayed=0 reason=VMSCAN_THROTTLE_NOPROGRESS
51305 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
1 usec_timeout=100000 usect_delayed=44000 reason=VMSCAN_THROTTLE_WRITEBACK
2 usec_timeout=100000 usect_delayed=76000 reason=VMSCAN_THROTTLE_WRITEBACK
3 usec_timeout=100000 usect_delayed=80000 reason=VMSCAN_THROTTLE_WRITEBACK
5 usec_timeout=100000 usect_delayed=48000 reason=VMSCAN_THROTTLE_WRITEBACK
5 usec_timeout=100000 usect_delayed=84000 reason=VMSCAN_THROTTLE_WRITEBACK
6 usec_timeout=100000 usect_delayed=72000 reason=VMSCAN_THROTTLE_WRITEBACK
7 usec_timeout=100000 usect_delayed=88000 reason=VMSCAN_THROTTLE_WRITEBACK
11 usec_timeout=100000 usect_delayed=56000 reason=VMSCAN_THROTTLE_WRITEBACK
12 usec_timeout=100000 usect_delayed=64000 reason=VMSCAN_THROTTLE_WRITEBACK
16 usec_timeout=100000 usect_delayed=92000 reason=VMSCAN_THROTTLE_WRITEBACK
24 usec_timeout=100000 usect_delayed=68000 reason=VMSCAN_THROTTLE_WRITEBACK
28 usec_timeout=100000 usect_delayed=32000 reason=VMSCAN_THROTTLE_WRITEBACK
30 usec_timeout=100000 usect_delayed=60000 reason=VMSCAN_THROTTLE_WRITEBACK
30 usec_timeout=100000 usect_delayed=96000 reason=VMSCAN_THROTTLE_WRITEBACK
32 usec_timeout=100000 usect_delayed=52000 reason=VMSCAN_THROTTLE_WRITEBACK
42 usec_timeout=100000 usect_delayed=40000 reason=VMSCAN_THROTTLE_WRITEBACK
77 usec_timeout=100000 usect_delayed=28000 reason=VMSCAN_THROTTLE_WRITEBACK
99 usec_timeout=100000 usect_delayed=36000 reason=VMSCAN_THROTTLE_WRITEBACK
137 usec_timeout=100000 usect_delayed=24000 reason=VMSCAN_THROTTLE_WRITEBACK
190 usec_timeout=100000 usect_delayed=20000 reason=VMSCAN_THROTTLE_WRITEBACK
339 usec_timeout=100000 usect_delayed=16000 reason=VMSCAN_THROTTLE_WRITEBACK
518 usec_timeout=100000 usect_delayed=12000 reason=VMSCAN_THROTTLE_WRITEBACK
852 usec_timeout=100000 usect_delayed=8000 reason=VMSCAN_THROTTLE_WRITEBACK
3359 usec_timeout=100000 usect_delayed=4000 reason=VMSCAN_THROTTLE_WRITEBACK
7147 usec_timeout=100000 usect_delayed=0 reason=VMSCAN_THROTTLE_WRITEBACK
83962 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.
include/linux/backing-dev.h | 1 -
include/linux/mmzone.h | 15 +++
include/trace/events/vmscan.h | 38 ++++++++
include/trace/events/writeback.h | 7 --
mm/backing-dev.c | 48 ----------
mm/compaction.c | 10 +-
mm/filemap.c | 1 +
mm/internal.h | 21 +++++
mm/memcontrol.c | 10 +-
mm/page-writeback.c | 11 ++-
mm/page_alloc.c | 26 ++----
mm/vmscan.c | 151 ++++++++++++++++++++++++++++---
mm/vmstat.c | 1 +
13 files changed, 237 insertions(+), 103 deletions(-)
--
2.31.1
do_writepages throttles on congestion if the writepages() fails due to a
lack of memory but congestion_wait() is partially broken as the congestion
state is not updated for all BDIs.
This patch stalls waiting for a number of pages to complete writeback
that located on the local node. The main weakness is that there is no
correlation between the location of the inode's pages and locality but
that is still better than congestion_wait.
Signed-off-by: Mel Gorman <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
---
mm/page-writeback.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 4812a17b288c..f34f54fcd5b4 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2366,8 +2366,15 @@ int do_writepages(struct address_space *mapping, struct writeback_control *wbc)
ret = generic_writepages(mapping, wbc);
if ((ret != -ENOMEM) || (wbc->sync_mode != WB_SYNC_ALL))
break;
- cond_resched();
- congestion_wait(BLK_RW_ASYNC, HZ/50);
+
+ /*
+ * Lacking an allocation context or the locality or writeback
+ * state of any of the inode's pages, throttle based on
+ * writeback activity on the local node. It's as good a
+ * guess as any.
+ */
+ reclaim_throttle(NODE_DATA(numa_node_id()),
+ VMSCAN_THROTTLE_WRITEBACK, HZ/50);
}
/*
* Usually few pages are written by now from those we've just submitted
--
2.31.1
The page allocator stalls based on the number of pages that are
waiting for writeback to start but this should now be redundant.
shrink_inactive_list() will wake flusher threads if the LRU tail are
unqueued dirty pages so the flusher should be active. If it fails to make
progress due to pages under writeback not being completed quickly then
it should stall on VMSCAN_THROTTLE_WRITEBACK.
Signed-off-by: Mel Gorman <[email protected]>
---
mm/page_alloc.c | 21 +--------------------
1 file changed, 1 insertion(+), 20 deletions(-)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 78e538067651..8fa0109ff417 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4795,30 +4795,11 @@ should_reclaim_retry(gfp_t gfp_mask, unsigned order,
trace_reclaim_retry_zone(z, order, reclaimable,
available, min_wmark, *no_progress_loops, wmark);
if (wmark) {
- /*
- * If we didn't make any progress and have a lot of
- * dirty + writeback pages then we should wait for
- * an IO to complete to slow down the reclaim and
- * prevent from pre mature OOM
- */
- if (!did_some_progress) {
- unsigned long write_pending;
-
- write_pending = zone_page_state_snapshot(zone,
- NR_ZONE_WRITE_PENDING);
-
- if (2 * write_pending > reclaimable) {
- congestion_wait(BLK_RW_ASYNC, HZ/10);
- return true;
- }
- }
-
ret = true;
- goto out;
+ break;
}
}
-out:
/*
* Memory allocation/reclaim might be called from a WQ context and the
* current implementation of the WQ concurrency control doesn't
--
2.31.1
Tracing of the stutterp workload showed the following delays
1 usect_delayed=124000 reason=VMSCAN_THROTTLE_NOPROGRESS
1 usect_delayed=128000 reason=VMSCAN_THROTTLE_NOPROGRESS
1 usect_delayed=176000 reason=VMSCAN_THROTTLE_NOPROGRESS
1 usect_delayed=536000 reason=VMSCAN_THROTTLE_NOPROGRESS
1 usect_delayed=544000 reason=VMSCAN_THROTTLE_NOPROGRESS
1 usect_delayed=556000 reason=VMSCAN_THROTTLE_NOPROGRESS
1 usect_delayed=624000 reason=VMSCAN_THROTTLE_NOPROGRESS
1 usect_delayed=716000 reason=VMSCAN_THROTTLE_NOPROGRESS
1 usect_delayed=772000 reason=VMSCAN_THROTTLE_NOPROGRESS
2 usect_delayed=512000 reason=VMSCAN_THROTTLE_NOPROGRESS
16 usect_delayed=120000 reason=VMSCAN_THROTTLE_NOPROGRESS
53 usect_delayed=116000 reason=VMSCAN_THROTTLE_NOPROGRESS
116 usect_delayed=112000 reason=VMSCAN_THROTTLE_NOPROGRESS
5907 usect_delayed=108000 reason=VMSCAN_THROTTLE_NOPROGRESS
71741 usect_delayed=104000 reason=VMSCAN_THROTTLE_NOPROGRESS
All the throttling hit the full timeout and then there was wakeup delays
meaning that the wakeups are premature as no other reclaimer such as
kswapd has made progress. This patch increases the maximum timeout.
Signed-off-by: Mel Gorman <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
---
mm/vmscan.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 66da45084af4..35b6ccaa01c3 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1042,7 +1042,7 @@ void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason)
break;
case VMSCAN_THROTTLE_NOPROGRESS:
- timeout = HZ/10;
+ timeout = HZ/2;
break;
case VMSCAN_THROTTLE_ISOLATED:
timeout = HZ/50;
--
2.31.1
Tracing indicates that tasks throttled on NOPROGRESS are woken
prematurely resulting in occasional massive spikes in direct
reclaim activity. This patch wakes tasks throttled on NOPROGRESS
if reclaim efficiency is at least 12%.
Signed-off-by: Mel Gorman <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
---
mm/vmscan.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 35b6ccaa01c3..812d4697d50d 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -3349,8 +3349,11 @@ static inline bool compaction_ready(struct zone *zone, struct scan_control *sc)
static void consider_reclaim_throttle(pg_data_t *pgdat, struct scan_control *sc)
{
- /* If reclaim is making progress, wake any throttled tasks. */
- if (sc->nr_reclaimed) {
+ /*
+ * If reclaim is making progress greater than 12% efficiency then
+ * wake all the NOPROGRESS throttled tasks.
+ */
+ if (sc->nr_reclaimed > (sc->nr_scanned >> 3)) {
wait_queue_head_t *wqh;
wqh = &pgdat->reclaim_wait[VMSCAN_THROTTLE_NOPROGRESS];
--
2.31.1
Neil Brown raised concerns about callers of reclaim_throttle specifying
a timeout value. The original timeout values to congestion_wait() were
probably pulled out of thin air or copy&pasted from somewhere else.
This patch centralises the timeout values and selects a timeout based
on the reason for reclaim throttling. These figures are also pulled
out of the same thin air but better values may be derived
Running a workload that is throttling for inappropriate periods
and tracing mm_vmscan_throttled can be used to pick a more appropriate
value. Excessive throttling would pick a lower timeout where as
excessive CPU usage in reclaim context would select a larger timeout.
Ideally a large value would always be used and the wakeups would
occur before a timeout but that requires careful testing.
Signed-off-by: Mel Gorman <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
---
mm/compaction.c | 2 +-
mm/internal.h | 3 +--
mm/page-writeback.c | 2 +-
mm/vmscan.c | 50 +++++++++++++++++++++++++++++++++------------
4 files changed, 40 insertions(+), 17 deletions(-)
diff --git a/mm/compaction.c b/mm/compaction.c
index 7359093d8ac0..151b04c4dab3 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -828,7 +828,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
if (cc->mode == MIGRATE_ASYNC)
return -EAGAIN;
- reclaim_throttle(pgdat, VMSCAN_THROTTLE_ISOLATED, HZ/10);
+ reclaim_throttle(pgdat, VMSCAN_THROTTLE_ISOLATED);
if (fatal_signal_pending(current))
return -EINTR;
diff --git a/mm/internal.h b/mm/internal.h
index c72d3383ef34..383d9b7e7991 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -129,8 +129,7 @@ extern unsigned long highest_memmap_pfn;
*/
extern int isolate_lru_page(struct page *page);
extern void putback_lru_page(struct page *page);
-extern void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason,
- long timeout);
+extern void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason);
/*
* in mm/rmap.c:
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index f34f54fcd5b4..4b01a6872f9e 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2374,7 +2374,7 @@ int do_writepages(struct address_space *mapping, struct writeback_control *wbc)
* guess as any.
*/
reclaim_throttle(NODE_DATA(numa_node_id()),
- VMSCAN_THROTTLE_WRITEBACK, HZ/50);
+ VMSCAN_THROTTLE_WRITEBACK);
}
/*
* Usually few pages are written by now from those we've just submitted
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 0450f6867d61..66da45084af4 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1006,12 +1006,10 @@ static void handle_write_error(struct address_space *mapping,
unlock_page(page);
}
-void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason,
- long timeout)
+void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason)
{
wait_queue_head_t *wqh = &pgdat->reclaim_wait[reason];
- long ret;
- bool acct_writeback = (reason == VMSCAN_THROTTLE_WRITEBACK);
+ long timeout, ret;
DEFINE_WAIT(wait);
/*
@@ -1023,17 +1021,43 @@ void reclaim_throttle(pg_data_t *pgdat, enum vmscan_throttle_state reason,
current->flags & (PF_IO_WORKER|PF_KTHREAD))
return;
- if (acct_writeback &&
- atomic_inc_return(&pgdat->nr_writeback_throttled) == 1) {
- WRITE_ONCE(pgdat->nr_reclaim_start,
- node_page_state(pgdat, NR_THROTTLED_WRITTEN));
+ /*
+ * These figures are pulled out of thin air.
+ * VMSCAN_THROTTLE_ISOLATED is a transient condition based on too many
+ * parallel reclaimers which is a short-lived event so the timeout is
+ * short. Failing to make progress or waiting on writeback are
+ * potentially long-lived events so use a longer timeout. This is shaky
+ * logic as a failure to make progress could be due to anything from
+ * writeback to a slow device to excessive references pages at the tail
+ * of the inactive LRU.
+ */
+ switch(reason) {
+ case VMSCAN_THROTTLE_WRITEBACK:
+ timeout = HZ/10;
+
+ if (atomic_inc_return(&pgdat->nr_writeback_throttled) == 1) {
+ WRITE_ONCE(pgdat->nr_reclaim_start,
+ node_page_state(pgdat, NR_THROTTLED_WRITTEN));
+ }
+
+ break;
+ case VMSCAN_THROTTLE_NOPROGRESS:
+ timeout = HZ/10;
+ break;
+ case VMSCAN_THROTTLE_ISOLATED:
+ timeout = HZ/50;
+ break;
+ default:
+ WARN_ON_ONCE(1);
+ timeout = HZ;
+ break;
}
prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
ret = schedule_timeout(timeout);
finish_wait(wqh, &wait);
- if (acct_writeback)
+ if (reason == VMSCAN_THROTTLE_WRITEBACK)
atomic_dec(&pgdat->nr_writeback_throttled);
trace_mm_vmscan_throttled(pgdat->node_id, jiffies_to_usecs(timeout),
@@ -2319,7 +2343,7 @@ shrink_inactive_list(unsigned long nr_to_scan, struct lruvec *lruvec,
/* wait a bit for the reclaimer. */
stalled = true;
- reclaim_throttle(pgdat, VMSCAN_THROTTLE_ISOLATED, HZ/10);
+ reclaim_throttle(pgdat, VMSCAN_THROTTLE_ISOLATED);
/* We are about to die and free our memory. Return now. */
if (fatal_signal_pending(current))
@@ -3251,7 +3275,7 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
* until some pages complete writeback.
*/
if (sc->nr.immediate)
- reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK, HZ/10);
+ reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK);
}
/*
@@ -3275,7 +3299,7 @@ static void shrink_node(pg_data_t *pgdat, struct scan_control *sc)
if (!current_is_kswapd() && current_may_throttle() &&
!sc->hibernation_mode &&
test_bit(LRUVEC_CONGESTED, &target_lruvec->flags))
- reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK, HZ/10);
+ reclaim_throttle(pgdat, VMSCAN_THROTTLE_WRITEBACK);
if (should_continue_reclaim(pgdat, sc->nr_reclaimed - nr_reclaimed,
sc))
@@ -3347,7 +3371,7 @@ static void consider_reclaim_throttle(pg_data_t *pgdat, struct scan_control *sc)
/* Throttle if making no progress at high prioities. */
if (sc->priority < DEF_PRIORITY - 2)
- reclaim_throttle(pgdat, VMSCAN_THROTTLE_NOPROGRESS, HZ/10);
+ reclaim_throttle(pgdat, VMSCAN_THROTTLE_NOPROGRESS);
}
/*
--
2.31.1
On 10/22/21 16:46, Mel Gorman wrote:
> The page allocator stalls based on the number of pages that are
> waiting for writeback to start but this should now be redundant.
> shrink_inactive_list() will wake flusher threads if the LRU tail are
> unqueued dirty pages so the flusher should be active. If it fails to make
> progress due to pages under writeback not being completed quickly then
> it should stall on VMSCAN_THROTTLE_WRITEBACK.
>
> Signed-off-by: Mel Gorman <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
> ---
> mm/page_alloc.c | 21 +--------------------
> 1 file changed, 1 insertion(+), 20 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 78e538067651..8fa0109ff417 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4795,30 +4795,11 @@ should_reclaim_retry(gfp_t gfp_mask, unsigned order,
> trace_reclaim_retry_zone(z, order, reclaimable,
> available, min_wmark, *no_progress_loops, wmark);
> if (wmark) {
> - /*
> - * If we didn't make any progress and have a lot of
> - * dirty + writeback pages then we should wait for
> - * an IO to complete to slow down the reclaim and
> - * prevent from pre mature OOM
> - */
> - if (!did_some_progress) {
> - unsigned long write_pending;
> -
> - write_pending = zone_page_state_snapshot(zone,
> - NR_ZONE_WRITE_PENDING);
> -
> - if (2 * write_pending > reclaimable) {
> - congestion_wait(BLK_RW_ASYNC, HZ/10);
> - return true;
> - }
> - }
> -
> ret = true;
> - goto out;
> + break;
> }
> }
>
> -out:
> /*
> * Memory allocation/reclaim might be called from a WQ context and the
> * current implementation of the WQ concurrency control doesn't
>