linux-kernel - Re: [PATCH 2/3] mm: memcg: add a helper for non-unified stats flushing

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Date:   Tue, 22 Aug 2023 09:00:06 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Ivan Babrou <ivan@...udflare.com>, Tejun Heo <tj@...nel.org>,
        linux-mm@...ck.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] mm: memcg: add a helper for non-unified stats flushing

On Tue, Aug 22, 2023 at 6:01 AM Michal Koutný <mkoutny@...e.com> wrote:
>
> On Mon, Aug 21, 2023 at 08:54:57PM +0000, Yosry Ahmed <yosryahmed@...gle.com> wrote:
> > +static void do_stats_flush(struct mem_cgroup *memcg)
> > +{
> > +     cgroup_rstat_flush(memcg->css.cgroup);
>         if(memcg == root_mem_cgroup)
>                 atomic_set(&stats_flush_threshold, 0);
> > +}
> > +
> >  /*
> >   * do_unified_stats_flush - do a unified flush of memory cgroup statistics
> >   *
> > @@ -656,7 +667,7 @@ static void do_unified_stats_flush(void)
> >
> >       WRITE_ONCE(flush_next_time, jiffies_64 + 2*FLUSH_TIME);
> >
> > -     cgroup_rstat_flush(root_mem_cgroup->css.cgroup);
> > +     do_stats_flush(root_mem_cgroup);
> >
>   -     atomic_set(&stats_flush_threshold, 0);
> >       atomic_set(&stats_flush_ongoing, 0);
>
> You may reset stats_flush_threshold to save the unified flushers some
> work.

We can probably also kick FLUSH_TIME forward as well. Perhaps I can
move both into do_stats_flush() then. If I understand correctly this
is what you mean?

static void do_stats_flush(struct mem_cgroup *memcg)
{
       if (mem_cgroup_is_root(memcg))
               WRITE_ONCE(flush_next_time, jiffies_64 + 2*FLUSH_TIME);

       cgroup_rstat_flush(memcg->css.cgroup);

       if (mem_cgroup_is_root(memcg))
               atomic_set(&stats_flush_threshold, 0);
}

  static void do_unified_stats_flush(void)
  {
          if (atomic_read(&stats_flush_ongoing) ||
              atomic_xchg(&stats_flush_ongoing, 1))
                  return;

          do_stats_flush(root_mem_cgroup);
          atomic_set(&stats_flush_ongoing, 0);
  }

, or simplify it further by just resetting stats_flush_threshold
before we flush as well:

static void do_stats_flush(struct mem_cgroup *memcg)
{
       /* for unified flushing, root non-unified flushing can help as well */
       if (mem_cgroup_is_root(memcg)) {
               WRITE_ONCE(flush_next_time, jiffies_64 + 2*FLUSH_TIME);
               atomic_set(&stats_flush_threshold, 0);
       }

       cgroup_rstat_flush(memcg->css.cgroup);
}

  static void do_unified_stats_flush(void)
  {
          if (atomic_read(&stats_flush_ongoing) ||
              atomic_xchg(&stats_flush_ongoing, 1))
                  return;

          do_stats_flush(root_mem_cgroup);
          atomic_set(&stats_flush_ongoing, 0);
  }


What do you think?