Re: [PATCH] Partially revert "mm/memcontrol.c: keep local VM counters in sync with the hierarchical ones"

[Roman Gushchin <guro@fb.com>]

On Sun, Aug 18, 2019 at 08:30:15AM +0800, Yafang Shao wrote:
> On Sun, Aug 18, 2019 at 3:14 AM Roman Gushchin <guro@fb.com> wrote:
> >
> > On Sat, Aug 17, 2019 at 11:33:57AM +0800, Yafang Shao wrote:
> > > On Sat, Aug 17, 2019 at 8:47 AM Roman Gushchin <guro@fb.com> wrote:
> > > >
> > > > Commit 766a4c19d880 ("mm/memcontrol.c: keep local VM counters in sync
> > > > with the hierarchical ones") effectively decreased the precision of
> > > > per-memcg vmstats_local and per-memcg-per-node lruvec percpu counters.
> > > >
> > > > That's good for displaying in memory.stat, but brings a serious regression
> > > > into the reclaim process.
> > > >
> > > > One issue I've discovered and debugged is the following:
> > > > lruvec_lru_size() can return 0 instead of the actual number of pages
> > > > in the lru list, preventing the kernel to reclaim last remaining
> > > > pages. Result is yet another dying memory cgroups flooding.
> > > > The opposite is also happening: scanning an empty lru list
> > > > is the waste of cpu time.
> > > >
> > > > Also, inactive_list_is_low() can return incorrect values, preventing
> > > > the active lru from being scanned and freed. It can fail both because
> > > > the size of active and inactive lists are inaccurate, and because
> > > > the number of workingset refaults isn't precise. In other words,
> > > > the result is pretty random.
> > > >
> > > > I'm not sure, if using the approximate number of slab pages in
> > > > count_shadow_number() is acceptable, but issues described above
> > > > are enough to partially revert the patch.
> > > >
> > > > Let's keep per-memcg vmstat_local batched (they are only used for
> > > > displaying stats to the userspace), but keep lruvec stats precise.
> > > > This change fixes the dead memcg flooding on my setup.
> > > >
> > >
> > > That will make some misunderstanding if the local counters are not in
> > > sync with the hierarchical ones
> > > (someone may doubt whether there're something leaked.).
> >
> > Sure, but the actual leakage is a much more serious issue.
> >
> > > If we have to do it like this, I think we should better document this behavior.
> >
> > Lru size calculations can be done using per-zone counters, which is
> > actually cheaper, because the number of zones is usually smaller than
> > the number of cpus. I'll send a corresponding patch on Monday.
> >
> 
> Looks like a good idea.
> 
> > Maybe other use cases can also be converted?
> 
> We'd better keep the behavior the same across counters. I think you
> can have a try.

As I said, consistency of counters is important, but not nearly as important
as the real behavior of the system. Especially because we talk about
per-node memcg statistics, which I believe is mostly used for debugging.

So for now I think the right thing to do is to revert the change to fix
the memory reclaim process. And then we can discuss how to get counters
right.

Thanks!