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From:   Yafang Shao <laoar.shao@gmail.com>
Date:   Sat, 24 Aug 2019 11:41:09 +0800
Message-ID: <CALOAHbAXzqGksOOCOfB8ykrMQQjo7g_h7hUexr2WdAQkh3N7zg@mail.gmail.com>
Subject: Re: [PATCH] Partially revert "mm/memcontrol.c: keep local VM counters
 in sync with the hierarchical ones"
To:     Roman Gushchin <guro@fb.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Michal Hocko <mhocko@kernel.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Kernel Team <Kernel-team@fb.com>
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On Sat, Aug 24, 2019 at 6:33 AM Roman Gushchin <guro@fb.com> wrote:
>
> On Fri, Aug 16, 2019 at 05:47:26PM -0700, Roman Gushchin 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.
> >
> > Fixes: 766a4c19d880 ("mm/memcontrol.c: keep local VM counters in sync with the hierarchical ones")
> > Signed-off-by: Roman Gushchin <guro@fb.com>
> > Cc: Yafang Shao <laoar.shao@gmail.com>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
>
> Any other concerns/comments here?
>
> I'd prefer to fix the regression: we're likely leaking several pages
> of memory for each created and destroyed memory cgroup. Plus
> all internal structures, which are measured in hundreds of kb.
>

Hi Roman,

As it really introduces issues, I agree with you that we should fix it first.

So for your fix,
Acked-by: Yafang Shao <laoar.shao@gmail.com>

Thanks
Yafang