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

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Date:   Sat, 17 Aug 2019 19:15:23 +0000
From:   Roman Gushchin <guro@...com>
To:     Greg KH <greg@...ah.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Yafang Shao <laoar.shao@...il.com>
Subject: Re: [PATCH] Partially revert "mm/memcontrol.c: keep local VM counters
 in sync with the hierarchical ones"

On Sat, Aug 17, 2019 at 08:36:16AM +0200, Greg KH 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@...com>
> > Cc: Yafang Shao <laoar.shao@...il.com>
> > Cc: Johannes Weiner <hannes@...xchg.org>
> > ---
> >  mm/memcontrol.c | 8 +++-----
> >  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> <formletter>
> 
> This is not the correct way to submit patches for inclusion in the
> stable kernel tree.  Please read:
>     https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
> for how to do this properly.

Oh, I'm sorry, will read and follow next time. Thanks!

> 
> </formletter>