This is a 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 it 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 reclaim conditions with event driven ones 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, let direct reclaim tasks sleep until another task makes progress 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. 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 three components 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 iX 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 anon memory hogs which continually map (100-dirty_ratio)% of memory o Total estimated WSS = (100+dirty_ration) percentage of memory X+Y+1 == NR_WORKERS varying from 4 up to NR_CPUS*4 The intent is to maximise the total WSS to force a lot 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 background readers but it stresses reclaim less. Now the results -- short summary 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. First the results of the "anon latency" latency stutterp 5.15.0-rc1 5.15.0-rc1 vanilla mm-reclaimcongest-v2r5 Amean mmap-4 30.4535 ( 0.00%) 35.8324 ( -17.66%) Amean mmap-7 36.2699 ( 0.00%) 45.2236 ( -24.69%) Amean mmap-12 50.9131 ( 0.00%) 82.3022 ( -61.65%) Amean mmap-21 2237.9029 ( 0.00%) 248.5605 ( 88.89%) Amean mmap-30 13015.0189 ( 0.00%) 285.9522 ( 97.80%) Amean mmap-48 554.7624 ( 0.00%) 573.8247 ( -3.44%) Amean mmap-64 308.4377 ( 0.00%) 349.8743 ( -13.43%) Stddev mmap-4 35.4820 ( 0.00%) 488.2629 (-1276.08%) Stddev mmap-7 78.7472 ( 0.00%) 489.7342 (-521.91%) Stddev mmap-12 133.3145 ( 0.00%) 1468.1633 (-1001.28%) Stddev mmap-21 21210.2154 ( 0.00%) 4416.6959 ( 79.18%) Stddev mmap-30 102917.0729 ( 0.00%) 3816.3041 ( 96.29%) Stddev mmap-48 3839.4931 ( 0.00%) 3830.6352 ( 0.23%) Stddev mmap-64 84.7670 ( 0.00%) 1264.4255 (-1391.65%) Max-99 mmap-4 32.0370 ( 0.00%) 33.3524 ( -4.11%) Max-99 mmap-7 44.9212 ( 0.00%) 43.0797 ( 4.10%) Max-99 mmap-12 65.0430 ( 0.00%) 90.2671 ( -38.78%) Max-99 mmap-21 424.5115 ( 0.00%) 188.7360 ( 55.54%) Max-99 mmap-30 408.4132 ( 0.00%) 286.8658 ( 29.76%) Max-99 mmap-48 521.7660 ( 0.00%) 1183.9219 (-126.91%) Max-99 mmap-64 422.4280 ( 0.00%) 421.6158 ( 0.19%) Max mmap-4 1908.9517 ( 0.00%) 42286.3987 (-2115.16%) Max mmap-7 5514.1997 ( 0.00%) 37353.5532 (-577.41%) Max mmap-12 5385.8229 ( 0.00%) 86358.3761 (-1503.44%) Max mmap-21 226560.1452 ( 0.00%) 152778.0127 ( 32.57%) Max mmap-30 816927.5043 ( 0.00%) 122999.4260 ( 84.94%) Max mmap-48 88771.0063 ( 0.00%) 87701.5264 ( 1.20%) Max mmap-64 2489.0185 ( 0.00%) 37293.3631 (-1398.32% For low thread counts, the time to mmap() is lower and improves for some higher thread counts. The variance is all over the map although the vanilla kernel has much worse variances at low thread counts. In all cases, the differences are not statistically significant due to very large maximum outliers. Max-99 is the maximum latency at the 99% percentile with massive latencies in the remaining 1%. This is partially 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 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. The overall system CPU usage and elapsed time is as follows 5.15.0-rc1 5.15.0-rc1 vanillamm-reclaimcongest-v2r5 Duration User 9775.48 11210.47 Duration System 12434.96 2356.96 Duration Elapsed 3019.60 2432.12 The patches significantly reduce system CPU usage as the vanilla kernel is not stalling and instead hammering the LRU lists uselessly. The high-level /proc/vmstats show 5.15.0-rc1 5.15.0-rc1 vanillamm-reclaimcongest-v2r5 Ops Direct pages scanned 1985928832.00 114592698.00 Ops Kswapd pages scanned 129232402.00 173143704.00 Ops Kswapd pages reclaimed 71154883.00 78204376.00 Ops Direct pages reclaimed 10082328.00 11225695.00 Ops Kswapd efficiency % 55.06 45.17 Ops Kswapd velocity 42797.85 71190.44 Ops Direct efficiency % 0.51 9.80 Ops Direct velocity 657679.44 47116.38 Kswapd scans more pages as a result of the patch and at a higher velocity. This is because direct reclaim is scanning fewer pages at much lower velocity as it gets stalled. >From a detailed graph, what is clear is that direct reclaim is not occuring uniformly during the duration of the test. Instead, there are massive spikes in reclaim activity (both direct and kswapd) but crucially, the number of pages reclaimed is relatively consistent. In other words, with this workload, reclaim rate remains relatively constant but there are massive variations in scan activity representing useless sacnning. Ops Percentage direct scans 93.89 39.83 For the overall scans, vanilla scanned 94.89% of pages where as with the patches, 39.83% were direct reclaim Ops Page writes by reclaim 2228237.00 2310272.00 Page writes from reclaim context remain consistent. Ops Page writes anon 2719172.00 2594781.00 Swap activity remain consistent. Ops Page reclaim immediate 1904282975.00 78707724.00 The number of pages encountered at the tail of the LRU tagged for immediate reclaim but still dirty/writeback is reduced by 95% Ops Slabs scanned 178981.00 155082.00 Slab scan activity is similar Ops Direct inode steals 3111707.00 2909.00 Ops Kswapd inode steals 148176.00 149325.00 But the number of inodes reclaimed from direct reclaim context is massively reduced. ftrace was used to gather stall activity Vanilla ------- 1 writeback_wait_iff_congested: usec_delayed=32000 2 writeback_wait_iff_congested: usec_delayed=24000 2 writeback_wait_iff_congested: usec_delayed=28000 16 writeback_wait_iff_congested: usec_delayed=20000 29 writeback_wait_iff_congested: usec_delayed=16000 82 writeback_wait_iff_congested: usec_delayed=12000 142 writeback_wait_iff_congested: usec_delayed=8000 235 writeback_wait_iff_congested: usec_delayed=4000 128103 writeback_wait_iff_congested: 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_delayed=124000 1 writeback_congestion_wait: usec_delayed=136000 2 writeback_congestion_wait: usec_delayed=112000 2 writeback_congestion_wait: usec_delayed=116000 229 writeback_congestion_wait: usec_delayed=108000 446 writeback_congestion_wait: 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 but still dirty/writeback at the tail of the LRU 1 usect_delayed=84000 reason=VMSCAN_THROTTLE_WRITEBACK 2 usect_delayed=20000 reason=VMSCAN_THROTTLE_WRITEBACK 6 usect_delayed=16000 reason=VMSCAN_THROTTLE_WRITEBACK 12 usect_delayed=12000 reason=VMSCAN_THROTTLE_WRITEBACK 17 usect_delayed=8000 reason=VMSCAN_THROTTLE_WRITEBACK 129 usect_delayed=4000 reason=VMSCAN_THROTTLE_WRITEBACK 205 usect_delayed=0 reason=VMSCAN_THROTTLE_WRITEBACK Some were woken immediately, others stalled for short periods until enough IO was completed but did not hit the timeout. For direct reclaim, the number of times stalled for each reason were 16909 reason=VMSCAN_THROTTLE_ISOLATED 77844 reason=VMSCAN_THROTTLE_NOPROGRESS 113415 reason=VMSCAN_THROTTLE_WRITEBACK Like kswapd, the most common reason was pages tagged for immediate reclaim. A very large number of stalls were due to failing to make progress. 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 1 usect_delayed=36000 reason=VMSCAN_THROTTLE_ISOLATED 3 usect_delayed=24000 reason=VMSCAN_THROTTLE_ISOLATED 7 usect_delayed=20000 reason=VMSCAN_THROTTLE_ISOLATED 49 usect_delayed=16000 reason=VMSCAN_THROTTLE_ISOLATED 94 usect_delayed=12000 reason=VMSCAN_THROTTLE_ISOLATED 105 usect_delayed=8000 reason=VMSCAN_THROTTLE_ISOLATED 417 usect_delayed=4000 reason=VMSCAN_THROTTLE_ISOLATED 16233 usect_delayed=0 reason=VMSCAN_THROTTLE_ISOLATED The vast majority do not stall at all, others stall for short periods by never hit the timeout For VMSCAN_THROTTLE_NOPROGRESS, the breakdown of stalls was 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 This is showing that the timeout was always hit and often overrun by a large margin. If anything, this indicates that stalling on NOPROGRESS should have a timeout larger than HZ/10 For VMSCAN_THROTTLE_WRITEBACK, the delays were all over the map 1 usect_delayed=136000 reason=VMSCAN_THROTTLE_WRITEBACK 1 usect_delayed=500000 reason=VMSCAN_THROTTLE_WRITEBACK 1 usect_delayed=512000 reason=VMSCAN_THROTTLE_WRITEBACK 1 usect_delayed=528000 reason=VMSCAN_THROTTLE_WRITEBACK 2 usect_delayed=128000 reason=VMSCAN_THROTTLE_WRITEBACK 3 usect_delayed=124000 reason=VMSCAN_THROTTLE_WRITEBACK 4 usect_delayed=60000 reason=VMSCAN_THROTTLE_WRITEBACK 5 usect_delayed=56000 reason=VMSCAN_THROTTLE_WRITEBACK 6 usect_delayed=68000 reason=VMSCAN_THROTTLE_WRITEBACK 7 usect_delayed=40000 reason=VMSCAN_THROTTLE_WRITEBACK 7 usect_delayed=88000 reason=VMSCAN_THROTTLE_WRITEBACK 11 usect_delayed=64000 reason=VMSCAN_THROTTLE_WRITEBACK 12 usect_delayed=92000 reason=VMSCAN_THROTTLE_WRITEBACK 14 usect_delayed=44000 reason=VMSCAN_THROTTLE_WRITEBACK 18 usect_delayed=48000 reason=VMSCAN_THROTTLE_WRITEBACK 19 usect_delayed=36000 reason=VMSCAN_THROTTLE_WRITEBACK 19 usect_delayed=72000 reason=VMSCAN_THROTTLE_WRITEBACK 21 usect_delayed=76000 reason=VMSCAN_THROTTLE_WRITEBACK 22 usect_delayed=84000 reason=VMSCAN_THROTTLE_WRITEBACK 22 usect_delayed=96000 reason=VMSCAN_THROTTLE_WRITEBACK 23 usect_delayed=120000 reason=VMSCAN_THROTTLE_WRITEBACK 23 usect_delayed=80000 reason=VMSCAN_THROTTLE_WRITEBACK 33 usect_delayed=52000 reason=VMSCAN_THROTTLE_WRITEBACK 37 usect_delayed=100000 reason=VMSCAN_THROTTLE_WRITEBACK 45 usect_delayed=28000 reason=VMSCAN_THROTTLE_WRITEBACK 50 usect_delayed=116000 reason=VMSCAN_THROTTLE_WRITEBACK 73 usect_delayed=32000 reason=VMSCAN_THROTTLE_WRITEBACK 82 usect_delayed=24000 reason=VMSCAN_THROTTLE_WRITEBACK 95 usect_delayed=20000 reason=VMSCAN_THROTTLE_WRITEBACK 117 usect_delayed=112000 reason=VMSCAN_THROTTLE_WRITEBACK 254 usect_delayed=16000 reason=VMSCAN_THROTTLE_WRITEBACK 367 usect_delayed=12000 reason=VMSCAN_THROTTLE_WRITEBACK 893 usect_delayed=8000 reason=VMSCAN_THROTTLE_WRITEBACK 3914 usect_delayed=108000 reason=VMSCAN_THROTTLE_WRITEBACK 8526 usect_delayed=4000 reason=VMSCAN_THROTTLE_WRITEBACK 32907 usect_delayed=0 reason=VMSCAN_THROTTLE_WRITEBACK 65780 usect_delayed=104000 reason=VMSCAN_THROTTLE_WRITEBACK The majority hit the timeout in direct reclaim context which is not the same as what happened for kswapd. One one hand, this might imply that direct and kswapd should have different timeouts but that would be very clumsy. It would make more sense to increase the timeout for NOPROGRESS and see does that alleviate the pressure. Bottom line, the new throttling mechanism works but the timeouts may need adjutment. -- 2.31.1
