linux-kernel - Re: [PATCH v4 4/4] mm/page_owner: Record task command name

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Message-ID: <88619dad-6d41-6a91-a8d6-72e3aaf3575d@redhat.com>
Date:   Thu, 3 Feb 2022 13:53:10 -0500
From:   Waiman Long <longman@...hat.com>
To:     Vlastimil Babka <vbabka@...e.cz>, Michal Hocko <mhocko@...e.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org, Ira Weiny <ira.weiny@...el.com>,
        Mike Rapoport <rppt@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Roman Gushchin <guro@...com>, Rafael Aquini <aquini@...hat.com>
Subject: Re: [PATCH v4 4/4] mm/page_owner: Record task command name

On 2/3/22 07:10, Vlastimil Babka wrote:
> On 2/2/22 17:53, Waiman Long wrote:
>> On 2/1/22 10:28, Michal Hocko wrote:
>>> Cc Vlastimil
>>>
>>> On Mon 31-01-22 17:03:28, Waiman Long wrote:
>>>> The page_owner information currently includes the pid of the calling
>>>> task. That is useful as long as the task is still running. Otherwise,
>>>> the number is meaningless. To have more information about the allocating
>>>> tasks that had exited by the time the page_owner information is
>>>> retrieved, we need to store the command name of the task.
>>>>
>>>> Add a new comm field into page_owner structure to store the command name
>>>> and display it when the page_owner information is retrieved.
>>> I completely agree that pid is effectivelly useless (if not misleading)
>>> but is comm really telling all that much to compensate for the
>>> additional storage required for _each_ page in the system?
>> Yes, it does add an extra 16 bytes per page overhead. The command name can
>> be useful if one want to find out which userspace command is responsible for
>> a problematic page allocation. Maybe we can remove pid from page_owner to
>> save 8 bytes as you also agree that this number is not that useful.
> Pid could be used to correlate command instances (not perfectly if reuse
> happens), but command name could have a higher chance to be useful. In my
> experience the most useful were the stacktraces and gfp/order etc. anyway.
> So I wouldn't be opposed replacing pid with comm. The mild size increase
> should be acceptable, this is an opt-in feature for debugging sessions with
> known tradeoff for memory and cpu overhead for the extra info.

Thanks for the information.

I floated around dropping pid just as a possible way to reduce overall 
memory overhead. I did not do that in my patch and I am not planning to 
post any patch unless everybody agree.

Cheer,
Longman