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

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Date:   Thu, 3 Feb 2022 13:10:43 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Waiman Long <longman@...hat.com>, 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 2/2/22 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.

> Cheers,
> Longman
>