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

Eric W. Biederman
2020-08-20T13:00:28Z

ebiederm@xmission.com (Eric W. Biederman) writes:

2> Michal Hocko <mhocko@suse.com> writes:
>
>> On Thu 20-08-20 07:34:41, Eric W. Biederman wrote:
>>> Suren Baghdasaryan <surenb@google.com> 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.

Apologies. That 44a70adec910 does describe that some people have seen
vfork users set oom_score. No details unfortunately.

I will skip the part where posix calls this undefined behavior. It
breaks userspace to change.

It still seems like the code should be able to buffer oom_adj during
vfork, and only move the value onto mm_struct during exec.

Eric