From: Shakeel Butt <shakeelb@xxxxxxxxxx> Subject: memcg: expose root cgroup's memory.stat One way to measure the efficiency of memory reclaim is to look at the ratio (pgscan+pfrefill)/pgsteal. However at the moment these stats are not updated consistently at the system level and the ratio of these are not very meaningful. The pgsteal and pgscan are updated for only global reclaim while pgrefill gets updated for global as well as cgroup reclaim. Please note that this difference is only for system level vmstats. The cgroup stats returned by memory.stat are actually consistent. The cgroup's pgsteal contains number of reclaimed pages for global as well as cgroup reclaim. So, one way to get the system level stats is to get these stats from root's memory.stat, so, expose memory.stat for the root cgroup. from Johannes Weiner: There are subtle differences between /proc/vmstat and memory.stat, and cgroup-aware code that wants to watch the full hierarchy currently has to know about these intricacies and translate semantics back and forth. Generally having the fully recursive memory.stat at the root level could help a broader range of usecases. Why not fix the stats by including both the global and cgroup reclaim activity instead of exposing root cgroup's memory.stat? The reason is the benefit of having metrics exposing the activity that happens purely due to machine capacity rather than localized activity that happens due to the limits throughout the cgroup tree. Additionally there are userspace tools like sysstat(sar) which reads these stats to inform about the system level reclaim activity. So, we should not break such use-cases. Link: http://lkml.kernel.org/r/20200508170630.94406-1-shakeelb@xxxxxxxxxx Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx> Suggested-by: Johannes Weiner <hannes@xxxxxxxxxxx> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx> Acked-by: Yafang Shao <laoar.shao@xxxxxxxxx> Acked-by: Chris Down <chris@xxxxxxxxxxxxxx> Cc: Mel Gorman <mgorman@xxxxxxx> Cc: Roman Gushchin <guro@xxxxxx> Cc: Michal Hocko <mhocko@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/memcontrol.c | 1 - 1 file changed, 1 deletion(-) --- a/mm/memcontrol.c~memcg-expose-root-cgroups-memorystat +++ a/mm/memcontrol.c @@ -6228,7 +6228,6 @@ static struct cftype memory_files[] = { }, { .name = "stat", - .flags = CFTYPE_NOT_ON_ROOT, .seq_show = memory_stat_show, }, { _
