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

[Michal Hocko <mhocko@suse.com>]

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")
-- 
Michal Hocko
SUSE Labs