Re: [PATCH v2] memcg: use ratelimited stats flush in the reclaim

[Yosry Ahmed <yosryahmed@google.com>]

On Wed, Aug 14, 2024 at 5:19 PM Nhat Pham <nphamcs@gmail.com> wrote:
>
> On Wed, Aug 14, 2024 at 4:49 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> >
> >
> > We can also use such atomic counters in obj_cgroup_may_zswap() and
> > eliminate the rstat flush there as well. Same for zswap_current_read()
> > probably.
>
> zswap/zswapped are subtree-cumulative counters. Would that be a problem?

For obj_cgroup_may_zswap() we iterate the parents anyway, so I think
it should be fine. We will also iterate the nodes on each level, but
this is usually a small number and is probably cheaper than an rstat
flush (but I can easily be wrong).

For zswap_current_read() we need to iterate the children, not the
parents. At this point the rstat flush is probably much better, so we
can leave this one alone. It's a userspace read anyway, so it
shouldn't be causing problems.

>
> >
> > Most in-kernel flushers really only need a few stats, so I am
> > wondering if it's better to incrementally move these ones outside of
> > the rstat framework and completely eliminate in-kernel flushers. For
> > instance, MGLRU does not require the flush that reclaim does as
> > Shakeel pointed out.
> >
> > This will solve so many scalability problems that all of us have
> > observed at some point or another and tried to optimize. I believe
> > using rstat for userspace reads was the original intention anyway.
>
> But yeah, the fewer in-kernel flushers we have, the fewer
> scalability/lock contention issues there will be. Not an expert in
> this area, but sounds like a worthwhile direction to pursue.