Re: [PATCH] mm: memcontrol: Pass NULL memcg for oom_badness() check.

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

Michal Hocko wrote:
> On Fri 19-02-16 22:51:38, Tetsuo Handa wrote:
> > Currently, mem_cgroup_out_of_memory() is calling
> > oom_scan_process_thread(&oc, task, totalpages) which includes
> > a call to oom_unkillable_task(task, NULL, NULL) and then is
> > calling oom_badness(task, memcg, NULL, totalpages) which includes
> > a call to oom_unkillable_task(task, memcg, NULL).
> > 
> > Since for_each_mem_cgroup_tree() iterates on only tasks from the given
> > memcg hierarchy, there is no point with passing non-NULL memcg argument
> > to oom_unkillable_task() via oom_badness().
> > 
> > Replace memcg argument with NULL in order to save a call to
> > task_in_mem_cgroup(task, memcg) in oom_unkillable_task()
> > which is always true.
> 
> yes this is true but oom_badness is called from super slow path here so
> I am not sure this change will buy anything. It makes the code little
> bit more confusing because now you have to think twice (or git blame) to
> see why the memcg == NULL is really OK.
> 
> So I do not think this is an improvement. If anything wouldn't it be
> cleaner to remove memcg parameter from oom_badness altogether and
> instead do the task_in_mem_cgroup check where it is really needed?
> In other words do the check in oom_kill_process when evaluating children
> to sacrifice them?

This patch is a clarification before proposing
http://lkml.kernel.org/r/1455892411-7611-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp
which converts two oom_unkillable_task() calls into one and
fixes infinite loop which will occur after we merge the OOM reaper.

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