linux-kernel - Re: [PATCH 1/3] mm, memcg: Don't put offlined memcg into local stock

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Date:   Fri, 1 Oct 2021 21:54:14 -0400
From:   Waiman Long <llong@...hat.com>
To:     Roman Gushchin <guro@...com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <songmuchun@...edance.com>
Subject: Re: [PATCH 1/3] mm, memcg: Don't put offlined memcg into local stock

On 10/1/21 7:51 PM, Roman Gushchin wrote:
> On Fri, Oct 01, 2021 at 03:09:36PM -0400, Waiman Long wrote:
>> When freeing a page associated with an offlined memcg, refill_stock()
>> will put it into local stock delaying its demise until another memcg
>> comes in to take its place in the stock. To avoid that, we now check
>> for offlined memcg and go directly in this case to the slowpath for
>> the uncharge via the repurposed cancel_charge() function.
> Hi Waiman!
>
> I'm afraid it can make a cleanup of a dying cgroup slower: for every
> released page we'll potentially traverse the whole cgroup tree and
> decrease atomic page counters.
>
> I'm not sure I understand the benefits we get from this change which
> do justify the slowdown on the cleanup path.

I am debugging a problem where some dying memcgs somehow stay around for 
a long time leading to gradual increase in memory consumption over time. 
I see the per-cpu stock as one of the places where a reference to a 
dying memcg may be present. Anyway, I agree that it may not help much. I 
am going to drop it if you think it is not a good idea.

Cheers,
Longman