Re: [PATCH] memcg: effective memory.high reclaim for remote charging

[Shakeel Butt]

On Mon, May 11, 2020 at 8:57 AM Johannes Weiner <hannes@xxxxxxxxxxx> wrote:
>
> On Thu, May 07, 2020 at 10:00:07AM -0700, Shakeel Butt wrote:
> > On Thu, May 7, 2020 at 9:47 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
> > >
> > > On Thu 07-05-20 09:33:01, Shakeel Butt wrote:
> > > [...]
> > > > @@ -2600,8 +2596,23 @@ static int try_charge(struct mem_cgroup *memcg, gfp_t gfp_mask,
> > > >                               schedule_work(&memcg->high_work);
> > > >                               break;
> > > >                       }
> > > > -                     current->memcg_nr_pages_over_high += batch;
> > > > -                     set_notify_resume(current);
> > > > +
> > > > +                     if (gfpflags_allow_blocking(gfp_mask))
> > > > +                             reclaim_over_high(memcg, gfp_mask, batch);
> > > > +
> > > > +                     if (page_counter_read(&memcg->memory) <=
> > > > +                         READ_ONCE(memcg->high))
> > > > +                             break;
> > >
> > > I am half way to a long weekend so bear with me. Shouldn't this be continue? The
> > > parent memcg might be still in excess even the child got reclaimed,
> > > right?
> > >
> >
> > The reclaim_high() actually already does this walk up to the root and
> > reclaim from ones who are still over their high limit. Though having
> > 'continue' here is correct too.
>
> If reclaim was weak and failed to bring the child back in line, we
> still do set_notify_resume(). We should do that for ancestors too.
>
> But it seems we keep adding hierarchy walks and it's getting somewhat
> convoluted: page_counter does it, then we check high overage
> recursively, and now we add the call to reclaim which itself is a walk
> up the ancestor line.
>
> Can we hitchhike on the page_counter_try_charge() walk, which already
> has the concept of identifying counters with overage? Rename the @fail
> to @limited and return the first counter that is in excess of its high
> as well, even when the function succeeds?
>
> Then we could ditch the entire high checking loop here and simply
> replace it with
>
> done_restock:
>         ...
>
>         if (*limited) {
>                 if (gfpflags_allow_blocking())
>                         reclaim_over_high(memcg_from_counter(limited));
>                 /* Reclaim may not be able to do much, ... */
>                 set_notify_resume(); // or schedule_work()
>         };
>

I will try to code the above and will give a shot to the following
long-term suggestion as well.

> In the long-term, the best thing might be to integrate memory.high
> reclaim with the regular reclaim that try_charge() is already
> doing. Especially the part where it retries several times - we
> currently give up on memory.high unnecessarily early. Make
> page_counter_try_charge() fail on high and max equally, and after
> several reclaim cycles, instead of invoking the OOM killer, inject the
> penalty sleep and force the charges. OOM killing and throttling is
> supposed to be the only difference between the two, anyway, and yet
> the code diverges far more than that for no apparent reason.
>
> But I also appreciate that this is a cleanup beyond the scope of this
> patch here, so it's up to you how far you want to take it.

Thanks,
Shakeel