Re: [PATCH v2 2/3] mm: Force update of mem cgroup soft limit tree on usage excess
From: Michal Hocko
Date: Fri Feb 26 2021 - 03:53:00 EST
On Thu 25-02-21 14:48:58, Tim Chen wrote:
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On 2/24/21 3:53 AM, Michal Hocko wrote:
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> On Mon 22-02-21 11:48:37, Tim Chen wrote:
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>> On 2/22/21 11:09 AM, Michal Hocko wrote:
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>>>> I actually have tried adjusting the threshold but found that it doesn't work well for
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>>>> the case with unenven memory access frequency between cgroups. The soft
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>>>> limit for the low memory event cgroup could creep up quite a lot, exceeding
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>>>> the soft limit by hundreds of MB, even
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>>>> if I drop the SOFTLIMIT_EVENTS_TARGET from 1024 to something like 8.
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>>> What was the underlying reason? Higher order allocations?
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>> Not high order allocation.
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>> The reason was because the run away memcg asks for memory much less often, compared
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>> to the other memcgs in the system. So it escapes the sampling update and
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>> was not put onto the tree and exceeds the soft limit
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>> pretty badly. Even if it was put onto the tree and gets page reclaimed below the
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>> limit, it could escape the sampling the next time it exceeds the soft limit.
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> I am sorry but I really do not follow. Maybe I am missing something
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> obvious but the the rate of events (charge/uncharge) shouldn't be really
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> important. There is no way to exceed the limit without charging memory
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> (either a new or via task migration in v1 and immigrate_on_move). If you
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> have SOFTLIMIT_EVENTS_TARGET 8 then you should be 128 * 8 events to
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> re-evaluate. Huge pages can make the runaway much bigger but how it
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> would be possible to runaway outside of that bound.
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Michal,
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Let's take an extreme case where memcg 1 always generate the
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first event and memcg 2 generates the rest of 128*8-1 events
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and the pattern repeat.
I do not follow. Events are per-memcg, aren't they?
__this_cpu_read(memcg->vmstats_percpu->targets[target]);
[...]
__this_cpu_write(memcg->vmstats_percpu->targets[target], next);
--
Michal Hocko
SUSE Labs