Hi, Butt, Shakeel Butt <shakeelb@xxxxxxxxxx> writes: > On Wed, Mar 10, 2021 at 4:47 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: >> >> From: Huang Ying <ying.huang@xxxxxxxxx> >> >> In shrink_node(), to determine whether to enable cache trim mode, the >> LRU size is gotten via lruvec_page_state(). That gets the value from >> a per-CPU counter (mem_cgroup_per_node->lruvec_stat[]). The error of >> the per-CPU counter from CPU local counting and the descendant memory >> cgroups may cause some issues. We run into this in 0-Day performance >> test. >> >> 0-Day uses the RAM file system as root file system, so the number of >> the reclaimable file pages is very small. In the swap testing, the >> inactive file LRU list will become almost empty soon. But the size of >> the inactive file LRU list gotten from the per-CPU counter may keep a >> much larger value (say, 33, 50, etc.). This will enable cache trim >> mode, but nothing can be scanned in fact. The following pattern >> repeats for long time in the test, >> >> priority inactive_file_size cache_trim_mode >> 12 33 0 >> 11 33 0 >> ... >> 6 33 0 >> 5 33 1 >> ... >> 1 33 1 >> >> That is, the cache_trim_mode will be enabled wrongly when the scan >> priority decreases to 5. And the problem will not be recovered for >> long time. >> >> It's hard to get the more accurate size of the inactive file list >> without much more overhead. And it's hard to estimate the error of >> the per-CPU counter too, because there may be many descendant memory >> cgroups. But after the actual scanning, if nothing can be scanned >> with the cache trim mode, it should be wrong to enable the cache trim >> mode. So we can retry with the cache trim mode disabled. This patch >> implement this policy. > > Instead of playing with the already complicated heuristics, we should > improve the accuracy of the lruvec stats. Johannes already fixed the > memcg stats using rstat infrastructure and Tejun has suggestions on > how to use rstat infrastructure efficiently for lruvec stats at > https://lore.kernel.org/linux-mm/YCFgr300eRiEZwpL@xxxxxxxxxxxxxxx/. Thanks for your information! It should be better if we can improve the accuracy of lruvec stats without much overhead. But that may be not a easy task. If my understanding were correct, what Tejun suggested is to add a fast read interface to rstat to be used in hot path. And its accuracy is similar as that of traditional per-CPU counter. But if we can regularly update the lruvec rstat with something like vmstat_update(), that should be OK for the issue described in this patch. Best Regards, Huang, Ying
