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Subject: Re: cgroup pointed by sock is leaked on mode switch
To:     Tejun Heo <tj@kernel.org>
CC:     <linux-kernel@vger.kernel.org>, <cgroups@vger.kernel.org>,
        <netdev@vger.kernel.org>,
        "Libin (Huawei)" <huawei.libin@huawei.com>, <guofan5@huawei.com>,
        <wangkefeng.wang@huawei.com>, <lizefan@huawei.com>
References: <03dab6ab-0ffe-3cae-193f-a7f84e9b14c5@huawei.com>
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From:   Yang Yingliang <yangyingliang@huawei.com>
Message-ID: <c9879fd2-cb91-2a08-8293-c6a436b5a539@huawei.com>
Date:   Wed, 6 May 2020 09:50:43 +0800
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On 2020/5/6 0:06, Tejun Heo wrote:
> Hello, Yang.
>
> On Sat, May 02, 2020 at 06:27:21PM +0800, Yang Yingliang wrote:
>> I find the number nr_dying_descendants is increasing:
>> linux-dVpNUK:~ # find /sys/fs/cgroup/ -name cgroup.stat -exec grep
>> '^nr_dying_descendants [^0]'  {} +
>> /sys/fs/cgroup/unified/cgroup.stat:nr_dying_descendants 80
>> /sys/fs/cgroup/unified/system.slice/cgroup.stat:nr_dying_descendants 1
>> /sys/fs/cgroup/unified/system.slice/system-hostos.slice/cgroup.stat:nr_dying_descendants
>> 1
>> /sys/fs/cgroup/unified/lxc/cgroup.stat:nr_dying_descendants 79
>> /sys/fs/cgroup/unified/lxc/5f1fdb8c54fa40c3e599613dab6e4815058b76ebada8a27bc1fe80c0d4801764/cgroup.stat:nr_dying_descendants
>> 78
>> /sys/fs/cgroup/unified/lxc/5f1fdb8c54fa40c3e599613dab6e4815058b76ebada8a27bc1fe80c0d4801764/system.slice/cgroup.stat:nr_dying_descendants
>> 78
> Those numbers are nowhere close to causing oom issues. There are some
> aspects of page and other cache draining which is being improved but unless
> you're seeing numbers multiple orders of magnitude higher, this isn't the
> source of your problem.
>
>> The situation is as same as the commit bd1060a1d671 ("sock, cgroup: add
>> sock->sk_cgroup") describes.
>> "On mode switch, cgroup references which are already being pointed to by
>> socks may be leaked."
> I'm doubtful that you're hitting that issue. Mode switching means memcg
> being switched between cgroup1 and cgroup2 hierarchies, which is unlikely to
> be what's happening when you're launching docker containers.
>
> The first step would be identifying where memory is going and finding out
> whether memcg is actually being switched between cgroup1 and 2 - look at the
> hierarchy number in /proc/cgroups, if that's switching between 0 and
> someting not zero, it is switching.
>
> Thanks.
>