linux-kernel - Re: [PATCH] oom_reaper: switch to struct list

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Date:   Wed, 15 Feb 2017 20:01:33 +1100
From:   Aleksa Sarai <asarai@...e.de>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, cyphar@...har.com
Subject: Re: [PATCH] oom_reaper: switch to struct list_head for reap queue

>>> This is an extra pointer to task_struct and more lines of code to
>>> accomplish the same thing. Why would we want to do that?
>>
>> I don't think it's more "actual" lines of code (I think the wrapping is
>> inflating the line number count),
>
> I too think it doesn't make sense to blow task_struct and the generated code.
> And to me this patch doesn't make the source code more clean.
>
>> but switching it means that it's more in
>> line with other queues in the kernel (it took me a bit to figure out what
>> was going on with oom_reaper_list beforehand).
>
> perhaps you can turn oom_reaper_list into llist_head then. This will also
> allow to remove oom_reaper_lock. Not sure this makes sense too.

Actually, I just noticed that the original implementation is a stack not 
a queue. So the reaper will always reap the *most recent* task to get 
OOMed as opposed to the least recent. Since select_bad_process() will 
always pick worse processes first, this means that the reaper will reap 
"less bad" processes (lower oom score) before it reaps worse ones 
(higher oom score).

While it's not a /huge/ deal (N is going to be small in most OOM cases), 
is this something that we should consider?

RE: llist_head, the problem with that is that appending to the end is an 
O(n) operation. Though, as I said, n is not going to be very large in 
most cases.

-- 
Aleksa Sarai
Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/