From e5eaa4ee8eb2d5914b07a8424eb8e0ee4b1f879c Mon Sep 17 00:00:00 2001 From: Shakeel Butt Date: Wed, 22 Sep 2021 15:49:06 -0700 Subject: [PATCH] memcg: flush lruvec stats in the refault MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit Prior to the commit 8c9787b569d0 ("memcg: switch lruvec stats to rstat") and the commit 8604faa3af29 ("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 8604faa3af29 moved the lruvec stats to rstat infrastructure and the commit 8c9787b569d0 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 8604faa3af29, 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 8604faa3af29 and 8c9787b569d0 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 8604faa3af29 ("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: 8604faa3af29 ("memcg: infrastructure to flush memcg stats") Signed-off-by: Shakeel Butt Tested-by: Michael Larabel Cc: Johannes Weiner Cc: Roman Gushchin Cc: Feng Tang Cc: Michal Hocko Cc: Hillf Danton , Cc: Michal Koutný Cc: Andrew Morton , Signed-off-by: Linus Torvalds --- mm/memcontrol.c | 10 ---------- mm/workingset.c | 1 + 2 files changed, 1 insertion(+), 10 deletions(-) diff --git a/mm/memcontrol.c b/mm/memcontrol.c index b762215d73eb9..6da5020a8656d 100644 --- a/mm/memcontrol.c +++ b/mm/memcontrol.c @@ -106,9 +106,6 @@ static bool do_memsw_account(void) /* memcg and lruvec stats flushing */ static void flush_memcg_stats_dwork(struct work_struct *w); static DECLARE_DEFERRABLE_WORK(stats_flush_dwork, flush_memcg_stats_dwork); -static void flush_memcg_stats_work(struct work_struct *w); -static DECLARE_WORK(stats_flush_work, flush_memcg_stats_work); -static DEFINE_PER_CPU(unsigned int, stats_flush_threshold); static DEFINE_SPINLOCK(stats_flush_lock); #define THRESHOLDS_EVENTS_TARGET 128 @@ -682,8 +679,6 @@ void __mod_memcg_lruvec_state(struct lruvec *lruvec, enum node_stat_item idx, /* Update lruvec */ __this_cpu_add(pn->lruvec_stats_percpu->state[idx], val); - if (!(__this_cpu_inc_return(stats_flush_threshold) % MEMCG_CHARGE_BATCH)) - queue_work(system_unbound_wq, &stats_flush_work); } /** @@ -5361,11 +5356,6 @@ static void flush_memcg_stats_dwork(struct work_struct *w) queue_delayed_work(system_unbound_wq, &stats_flush_dwork, 2UL*HZ); } -static void flush_memcg_stats_work(struct work_struct *w) -{ - mem_cgroup_flush_stats(); -} - static void mem_cgroup_css_rstat_flush(struct cgroup_subsys_state *css, int cpu) { struct mem_cgroup *memcg = mem_cgroup_from_css(css); diff --git a/mm/workingset.c b/mm/workingset.c index d4268d8e9a82d..d5b81e4f4cbe8 100644 --- a/mm/workingset.c +++ b/mm/workingset.c @@ -352,6 +352,7 @@ void workingset_refault(struct page *page, void *shadow) inc_lruvec_state(lruvec, WORKINGSET_REFAULT_BASE + file); + mem_cgroup_flush_stats(); /* * Compare the distance to the existing workingset size. We * don't activate pages that couldn't stay resident even if -- 2.39.5