linux-kernel - Re: [PATCH 00/10] OOM Debug print selection and additional information

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Date:   Thu, 29 Aug 2019 23:09:08 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Edward Chron <echron@...sta.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>,
        Johannes Weiner <hannes@...xchg.org>,
        David Rientjes <rientjes@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Ivan Delalande <colona@...sta.com>
Subject: Re: [PATCH 00/10] OOM Debug print selection and additional
 information

On 2019/08/29 20:56, Michal Hocko wrote:
>> But please be aware that, I REPEAT AGAIN, I don't think neither eBPF nor
>> SystemTap will be suitable for dumping OOM information. OOM situation means
>> that even single page fault event cannot complete, and temporary memory
>> allocation for reading from kernel or writing to files cannot complete.
> 
> And I repeat that no such reporting is going to write to files. This is
> an OOM path afterall.

The process who fetches from e.g. eBPF event cannot involve page fault.
The front-end for iovisor/bcc is a python userspace process. But I think
that such process can't run under OOM situation.

> 
>> Therefore, we will need to hold all information in kernel memory (without
>> allocating any memory when OOM event happened). Dynamic hooks could hold
>> a few lines of output, but not all lines we want. The only possible buffer
>> which is preallocated and large enough would be printk()'s buffer. Thus,
>> I believe that we will have to use printk() in order to dump OOM information.
>> At that point,
> 
> Yes, this is what I've had in mind.

Probably I incorrectly shortcut.

Dynamic hooks could hold a few lines of output, but dynamic hooks can not hold
all lines when dump_tasks() reports 32000+ processes. We have to buffer all output
in kernel memory because we can't complete even a page fault event triggered by
the python process monitoring eBPF event (and writing the result to some log file
or something) while out_of_memory() is in flight.

And "set /proc/sys/vm/oom_dump_tasks to 0" is not the right reaction. What I'm
saying is "we won't be able to hold output from dump_tasks() if output from
dump_tasks() goes to buffer preallocated for dynamic hooks". We have to find
a way that can handle the worst case.