memcg: flush lruvec stats in the refault

Linux Kernel / Linux Kernel - Shakeel Butt [google.com] - 23 September 2021 17:09 UTC

Prior to the commit 7e1c0d6f5820 ("memcg: switch lruvec stats to rstat") and the commit aa48e47e3906 ("memcg: infrastructure to flush memcg stats"), each lruvec memcg stats can be off by (nr_cgroups * nr_cpus * 32) at worst and for unbounded amount of time. The commit aa48e47e3906 moved the lruvec stats to rstat infrastructure and the commit 7e1c0d6f5820 bounded the error for all the lruvec stats to (nr_cpus * 32) at worst for at most 2 seconds. More specifically it decoupled the number of stats and the number of cgroups from the error rate.

However this reduction in error comes with the cost of triggering the slowpath of stats update more frequently. Previously in the slowpath the kernel adds the stats up the memcg tree. After aa48e47e3906, the kernel triggers the asyn lruvec stats flush through queue_work(). This causes regression reports from 0day kernel bot [1] as well as from phoronix test suite [2].

We tried two options to fix the regression:

1) Increase the threshold to trigger the slowpath in lruvec stats update codepath from 32 to 512.

2) Remove the slowpath from lruvec stats update codepath and instead flush the stats in the page refault codepath. The assumption is that the kernel timely flush the stats, so, the update tree would be small in the refault codepath to not cause the preformance impact.

Following are the results of will-it-scale/page_fault[1|2|3] benchmark on four settings i.e. (1) 5.15-rc1 as baseline (2) 5.15-rc1 with aa48e47e3906 and 7e1c0d6f5820 reverted (3) 5.15-rc1 with option-1 (4) 5.15-rc1 with option-2.

test (1) (2) (3) (4) pg_f1 368563 406277 (10.23%) 399693 (8.44%) 416398 (12.97%) pg_f2 338399 372133 (9.96%) 369180 (9.09%) 381024 (12.59%) pg_f3 500853 575399 (14.88%) 570388 (13.88%) 576083 (15.02%)

From the above result, it seems like the option-2 not only solves the regression but also improves the performance for at least these benchmarks.

Feng Tang (intel) ran the aim7 benchmark with these two options and confirms that option-1 reduces the regression but option-2 removes the regression.

Michael Larabel (phoronix) ran multiple benchmarks with these options and reported the results at [3] and it shows for most benchmarks option-2 removes the regression introduced by the commit aa48e47e3906 ("memcg: infrastructure to flush memcg stats").

Based on the experiment results, this patch proposed the option-2 as the solution to resolve the regression.

Link: https://lore.kernel.org/all/20210726022421.GB21872@xsang-OptiPlex-9020 [1] Link: https://www.phoronix.com/scan.php?page=article&item=linux515-compile-regress [2] Link: https://openbenchmarking.org/result/2109226-DEBU-LINUX5104 [3] Fixes: aa48e47e3906 ("memcg: infrastructure to flush memcg stats")

1f828223b799 memcg: flush lruvec stats in the refault
mm/memcontrol.c | 10 ----------
mm/workingset.c | 1 +
2 files changed, 1 insertion(+), 10 deletions(-)

Upstream: git.kernel.org


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