Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Message-ID: <c781641d-e5dc-f79b-a9c0-aa91ab228316@redhat.com>
Date:   Mon, 31 Jan 2022 14:01:18 -0500
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.4.0
Subject: Re: [PATCH v2 3/3] mm/page_owner: Dump memcg information
Content-Language: en-US
To:     Michal Hocko <mhocko@suse.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Sergey Senozhatsky <senozhatsky@chromium.org>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        linux-kernel@vger.kernel.org, cgroups@vger.kernel.org,
        linux-mm@kvack.org, Ira Weiny <ira.weiny@intel.com>,
        Rafael Aquini <aquini@redhat.com>
References: <20220129205315.478628-1-longman@redhat.com>
 <20220129205315.478628-4-longman@redhat.com>
 <YfeuK5j7cbgM+Oo+@dhcp22.suse.cz>
From:   Waiman Long <longman@redhat.com>
In-Reply-To: <YfeuK5j7cbgM+Oo+@dhcp22.suse.cz>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk

On 1/31/22 04:38, Michal Hocko wrote:
> On Sat 29-01-22 15:53:15, Waiman Long wrote:
>> It was found that a number of offlined memcgs were not freed because
>> they were pinned by some charged pages that were present. Even "echo
>> 1 > /proc/sys/vm/drop_caches" wasn't able to free those pages. These
>> offlined but not freed memcgs tend to increase in number over time with
>> the side effect that percpu memory consumption as shown in /proc/meminfo
>> also increases over time.
>>
>> In order to find out more information about those pages that pin
>> offlined memcgs, the page_owner feature is extended to dump memory
>> cgroup information especially whether the cgroup is offlined or not.
> It is not really clear to me how this is supposed to be used. Are you
> really dumping all the pages in the system to find out offline memcgs?
> That looks rather clumsy to me. I am not against adding memcg
> information to the page owner output. That can be useful in other
> contexts.

I am just piggybacking on top of the existing page_owner tool to provide 
information for me to find out what pages are pinning the dead memcgs. 
page_owner is a debugging tool that is not turned on by default. We do 
have to add a kernel parameter and  rebooting the system to use that, 
but that is pretty easy to do once we have a reproducer to reproduce the 
problem.

Cheers,
Longman