Re: [PATCH 1/1] mm, oom_adj: don't loop through tasks in __set_oom_adj when not necessary

[Eric W. Biederman]

Michal Hocko <mhocko@xxxxxxxx> writes:

> On Thu 20-08-20 07:34:41, Eric W. Biederman wrote:
>> Suren Baghdasaryan <surenb@xxxxxxxxxx> writes:
>> 
>> > Currently __set_oom_adj loops through all processes in the system to
>> > keep oom_score_adj and oom_score_adj_min in sync between processes
>> > sharing their mm. This is done for any task with more that one mm_users,
>> > which includes processes with multiple threads (sharing mm and signals).
>> > However for such processes the loop is unnecessary because their signal
>> > structure is shared as well.
>> > Android updates oom_score_adj whenever a tasks changes its role
>> > (background/foreground/...) or binds to/unbinds from a service, making
>> > it more/less important. Such operation can happen frequently.
>> > We noticed that updates to oom_score_adj became more expensive and after
>> > further investigation found out that the patch mentioned in "Fixes"
>> > introduced a regression. Using Pixel 4 with a typical Android workload,
>> > write time to oom_score_adj increased from ~3.57us to ~362us. Moreover
>> > this regression linearly depends on the number of multi-threaded
>> > processes running on the system.
>> > Mark the mm with a new MMF_PROC_SHARED flag bit when task is created with
>> > CLONE_VM and !CLONE_SIGHAND. Change __set_oom_adj to use MMF_PROC_SHARED
>> > instead of mm_users to decide whether oom_score_adj update should be
>> > synchronized between multiple processes. To prevent races between clone()
>> > and __set_oom_adj(), when oom_score_adj of the process being cloned might
>> > be modified from userspace, we use oom_adj_mutex. Its scope is changed to
>> > global and it is renamed into oom_adj_lock for naming consistency with
>> > oom_lock. Since the combination of CLONE_VM and !CLONE_SIGHAND is rarely
>> > used the additional mutex lock in that path of the clone() syscall should
>> > not affect its overall performance. Clearing the MMF_PROC_SHARED flag
>> > (when the last process sharing the mm exits) is left out of this patch to
>> > keep it simple and because it is believed that this threading model is
>> > rare. Should there ever be a need for optimizing that case as well, it
>> > can be done by hooking into the exit path, likely following the
>> > mm_update_next_owner pattern.
>> > With the combination of CLONE_VM and !CLONE_SIGHAND being quite rare, the
>> > regression is gone after the change is applied.
>> 
>> So I am confused.
>> 
>> Is there any reason why we don't simply move signal->oom_score_adj to
>> mm->oom_score_adj and call it a day?
>
> Yes. Please read through 44a70adec910 ("mm, oom_adj: make sure processes
> sharing mm have same view of oom_score_adj")

That explains why the scores are synchronized.

It doesn't explain why we don't do the much simpler thing and move
oom_score_adj from signal_struct to mm_struct. Which is my question.

Why not put the score where we need it to ensure that the oom score
is always synchronized?  AKA on the mm_struct, not the signal_struct.

Eric