FROMGIT: mm, oom_adj: don't loop through tasks in __set_oom_adj when not necessary

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: 44a70ade ("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>
Bug: 168521639
Link: https://lore.kernel.org/linux-fsdevel/20200902012558.2335613-1-surenb@google.com/


[isaacm: upstream bound, no cherry-pick info because patch is in mm tree]
Change-Id: I20352a77815fbc358c36e686bb1503fdeea658ed
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>