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

From: Roman Gushchin
Date: Fri Aug 23 2019 - 19:29:21 EST

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@xxxxxx>
> Cc: Yafang Shao <laoar.shao@xxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>

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.