On Fri, May 15, 2020 at 8:00 AM Roman Gushchin <g...@fb.com> wrote: > > On Fri, May 15, 2020 at 06:44:44AM -0700, Shakeel Butt wrote: > > On Fri, May 15, 2020 at 6:24 AM Johannes Weiner <han...@cmpxchg.org> wrote: > > > > > > On Fri, May 15, 2020 at 10:29:55AM +0200, Michal Hocko wrote: > > > > On Sat 09-05-20 07:06:38, Shakeel Butt wrote: > > > > > On Fri, May 8, 2020 at 2:44 PM Johannes Weiner <han...@cmpxchg.org> > > > > > wrote: > > > > > > > > > > > > On Fri, May 08, 2020 at 10:06:30AM -0700, Shakeel Butt wrote: > > > > > > > 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. > > > > > > > > Can we have those subtle differences documented please? > > > > > > > > > > > > > > > > > > Generally having the fully recursive memory.stat at the root > > > > > > > level could help a broader range of usecases. > > > > > > > > > > > > The changelog begs the question why we don't just "fix" the > > > > > > system-level stats. It may be useful to include the conclusions from > > > > > > that discussion, and why there is value in keeping the stats this > > > > > > way. > > > > > > > > > > > > > > > > Right. Andrew, can you please add the following para to the changelog? > > > > > > > > > > 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. > > > > > > > > > > > > Signed-off-by: Shakeel Butt <shake...@google.com> > > > > > > > Suggested-by: Johannes Weiner <han...@cmpxchg.org> > > > > > > > > > > > > Acked-by: Johannes Weiner <han...@cmpxchg.org> > > > > > > > > > > Thanks a lot. > > > > > > > > I was quite surprised that the patch is so simple TBH. For some reason > > > > I've still had memories that we do not account for root memcg (likely > > > > because mem_cgroup_is_root(memcg) bail out in the try_charge. But stats > > > > are slightly different here. > > > > > > Yep, we skip the page_counter for root, but keep in mind that cgroup1 > > > *does* have a root-level memory.stat, so (for the most part) we've > > > been keeping consumer stats for the root level the whole time. > > > > > > > counters because they are not really all the same. E.g. > > > > - mem_cgroup_charge_statistics accounts for each memcg > > > > > > Yep, that's heritage from cgroup1. > > > > > > > - memcg_charge_kernel_stack relies on pages being associated with a > > > > memcg and that in turn relies on __memcg_kmem_charge_page which bails > > > > out on root memcg > > > > > > You're right. It should only bypass the page_counter, but still set > > > page->mem_cgroup = root_mem_cgroup, just like user pages. > > What about kernel threads? We consider them belonging to the root memory > cgroup. Should their memory consumption being considered in root-level stats? > > I'm not sure we really want it, but I guess we need to document how > kernel threads are handled. >
What will be the cons of updating root-level stats for kthreads? > > > > > > This counter also doesn't get exported on cgroup1, so it would indeed > > > be a new bug. It needs to be fixed before this patch here. > > > > > > > - memcg_charge_slab (NR_SLAB*) skips over root memcg as well > > > > > > Same thing with these two. > > > > Yes, we skip page_counter for root but not the stats. I will go over > > all the stats and make sure we are not skipping the stats for root.
