Re: [PATCH] mm, oom: remove sleep from under oom_lock

[David Rientjes]

On Mon, 9 Jul 2018, Michal Hocko wrote:

> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> Tetsuo has pointed out that since 27ae357fa82b ("mm, oom: fix concurrent
> munlock and oom reaper unmap, v3") we have a strong synchronization
> between the oom_killer and victim's exiting because both have to take
> the oom_lock. Therefore the original heuristic to sleep for a short time
> in out_of_memory doesn't serve the original purpose.
> 
> Moreover Tetsuo has noticed that the short sleep can be more harmful
> than actually useful. Hammering the system with many processes can lead
> to a starvation when the task holding the oom_lock can block for a
> long time (minutes) and block any further progress because the
> oom_reaper depends on the oom_lock as well.
> 
> Drop the short sleep from out_of_memory when we hold the lock. Keep the
> sleep when the trylock fails to throttle the concurrent OOM paths a bit.
> This should be solved in a more reasonable way (e.g. sleep proportional
> to the time spent in the active reclaiming etc.) but this is much more
> complex thing to achieve. This is a quick fixup to remove a stale code.
> 
> Reported-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>

This reminds me:

mm/oom_kill.c

 54) int sysctl_oom_dump_tasks = 1;
 55) 
 56) DEFINE_MUTEX(oom_lock);
 57) 
 58) #ifdef CONFIG_NUMA

Would you mind documenting oom_lock to specify what it's protecting?