linux-kernel - Re: [PATCH 3/3 staging-next] mm: Remove RCU and tasklocks from lmk

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Date:   Thu, 9 Feb 2017 21:05:07 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     peter enderborg <peter.enderborg@...ymobile.com>
Cc:     devel@...verdev.osuosl.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org,
        Arve Hjønnevåg <arve@...roid.com>,
        Riley Andrews <riandrews@...roid.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm@...ck.org
Subject: Re: [PATCH 3/3 staging-next] mm: Remove RCU and tasklocks from lmk

On Thu 09-02-17 14:21:52, peter enderborg wrote:
> Fundamental changes:
> 1 Does NOT take any RCU lock in shrinker functions.
> 2 It returns same result for scan and counts, so  we dont need to do
>   shinker will know when it is pointless to call scan.
> 3 It does not lock any other process than the one that is
>   going to be killed.
> 
> Background.
> The low memory killer scans for process that can be killed to free
> memory. This can be cpu consuming when there is a high demand for
> memory. This can be seen by analysing the kswapd0 task work.
> The stats function added in earler patch adds a counter for waste work.
> 
> How it works.
> This patch create a structure within the lowmemory killer that caches
> the user spaces processes that it might kill. It is done with a
> sorted rbtree so we can very easy find the candidate to be killed,
> and knows its properies as memory usage and sorted by oom_score_adj
> to look up the task with highest oom_score_adj. To be able to achive
> this it uses oom_score_notify events.
> 
> This patch also as a other effect, we are now free to do other
> lowmemorykiller configurations.  Without the patch there is a need
> for a tradeoff between freed memory and task and rcu locks. This
> is no longer a concern for tuning lmk. This patch is not intended
> to do any calculation changes other than we do use the cache for
> calculate the count values and that makes kswapd0 to shrink other
> areas.

I have to admit I really do not understand big part of the above
paragraph as well as how this all is supposed to work. A quick glance
over the implementation. __lmk_task_insert seems to be only called from
the oom_score notifier context. If nobody updates the value then no task
will get into the tree. Or am I missing something really obvious here?
Moreover oom scores tend to be mostly same for tasks. That means that
your sorted tree will become sorted by pids in most cases. I do not see
any sorting based on the rss nor any updates that would reflect updates
of rss. How can this possibly work?
-- 
Michal Hocko
SUSE Labs