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Date:   Wed, 24 Feb 2021 12:53:35 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Tim Chen <tim.c.chen@linux.intel.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Ying Huang <ying.huang@intel.com>, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 2/3] mm: Force update of mem cgroup soft limit tree on
 usage excess
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On Mon 22-02-21 11:48:37, Tim Chen wrote:
> 
> 
> On 2/22/21 11:09 AM, Michal Hocko wrote:
> 
> >>
> >> I actually have tried adjusting the threshold but found that it doesn't work well for
> >> the case with unenven memory access frequency between cgroups.  The soft
> >> limit for the low memory event cgroup could creep up quite a lot, exceeding
> >> the soft limit by hundreds of MB, even
> >> if I drop the SOFTLIMIT_EVENTS_TARGET from 1024 to something like 8.
> > 
> > What was the underlying reason? Higher order allocations?
> > 
> 
> Not high order allocation.
> 
> The reason was because the run away memcg asks for memory much less often, compared
> to the other memcgs in the system.  So it escapes the sampling update and
> was not put onto the tree and exceeds the soft limit
> pretty badly.  Even if it was put onto the tree and gets page reclaimed below the
> limit, it could escape the sampling the next time it exceeds the soft limit.

I am sorry but I really do not follow. Maybe I am missing something
obvious but the the rate of events (charge/uncharge) shouldn't be really
important. There is no way to exceed the limit without charging memory
(either a new or via task migration in v1 and immigrate_on_move). If you
have SOFTLIMIT_EVENTS_TARGET 8 then you should be 128 * 8 events to
re-evaluate. Huge pages can make the runaway much bigger but how it
would be possible to runaway outside of that bound.

Btw. do we really need SOFTLIMIT_EVENTS_TARGET at all? Why cannot we
just stick with a single threshold? mem_cgroup_update_tree can be made
a effectivelly a noop when there is no soft limit in place so overhead
shouldn't matter for the vast majority of workloads.
-- 
Michal Hocko
SUSE Labs