Re: [PATCH v3 1/1] mm, oom_adj: don't loop through tasks in __set_oom_adj when not necessary

[Christian Brauner <christian.brauner@ubuntu.com>]

On Wed, Sep 02, 2020 at 07:53:02AM +0200, Michal Hocko wrote:
> On Tue 01-09-20 18:25:58, Suren Baghdasaryan wrote:
> > 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_MULTIPROCESS flag bit when task is created with
> > (CLONE_VM && !CLONE_THREAD && !CLONE_VFORK). Change __set_oom_adj to use
> > MMF_MULTIPROCESS 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. The combination of
> > (CLONE_VM && !CLONE_THREAD) is rarely used except for the case of vfork().
> > To prevent performance regressions of vfork(), we skip taking oom_adj_mutex
> > and setting MMF_MULTIPROCESS when CLONE_VFORK is specified. Clearing the
> > MMF_MULTIPROCESS 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 && !CLONE_THREAD && !CLONE_VFORK) being
> > quite rare, the regression is gone after the change is applied.
> > 
> > Fixes: 44a70adec910 ("mm, oom_adj: make sure processes sharing mm have same view of oom_score_adj")
> > Reported-by: Tim Murray <timmurray@google.com>
> > Debugged-by: Minchan Kim <minchan@kernel.org>
> > Suggested-by: Michal Hocko <mhocko@kernel.org>
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> 
> Acked-by: Michal Hocko <mhocko@suse.com>

Acked-by: Christian Brauner <christian.brauner@ubuntu.com>