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Message-ID: <95ba1931-e9c9-45c3-b080-d28f2ad368a7@redhat.com>
Date:   Mon, 31 Jan 2022 12:09:09 -0500
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Subject: Re: [PATCH 1/3] mm, memcg: Don't put offlined memcg into local stock
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To:     Roman Gushchin <guro@fb.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>, linux-kernel@vger.kernel.org,
        cgroups@vger.kernel.org, linux-mm@kvack.org,
        Shakeel Butt <shakeelb@google.com>,
        Muchun Song <songmuchun@bytedance.com>
References: <20211001190938.14050-1-longman@redhat.com>
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 <f7026256-4086-6632-569e-5b13594cb3fc@redhat.com>
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From:   Waiman Long <longman@redhat.com>
In-Reply-To: <YfgV2iiaVlR0hozD@carbon.dhcp.thefacebook.com>
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On 1/31/22 12:01, Roman Gushchin wrote:
> On Sun, Jan 30, 2022 at 10:55:56PM -0500, Waiman Long wrote:
>> On 10/1/21 19:51, 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.
>>>
>>> Thanks!
>> I was notified of a lockdep splat that this patch may help to prevent.
> Would you mind to test this patch:
> https://www.spinics.net/lists/cgroups/msg31244.html ?
>
> It should address this dependency.

Thanks for the pointer. I believe that your patch should be able to 
address this circular locking dependency.

Feel free to add my

Reviewed-by: Waiman Long <longman@redhat.com>

Cheers,
Longman