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

[Suren Baghdasaryan]

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.

Fixes: 44a70adec910 ("mm, oom_adj: make sure processes sharing mm have same view of oom_score_adj")
Reported-by: Tim Murray <timmurray@xxxxxxxxxx>
Suggested-by: Michal Hocko <mhocko@xxxxxxxxxx>
Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
---
 fs/proc/base.c                 | 7 +++----
 include/linux/oom.h            | 1 +
 include/linux/sched/coredump.h | 1 +
 kernel/fork.c                  | 9 +++++++++
 mm/oom_kill.c                  | 2 ++
 5 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 617db4e0faa0..cff1a58a236c 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1055,7 +1055,6 @@ static ssize_t oom_adj_read(struct file *file, char __user *buf, size_t count,
 
 static int __set_oom_adj(struct file *file, int oom_adj, bool legacy)
 {
-	static DEFINE_MUTEX(oom_adj_mutex);
 	struct mm_struct *mm = NULL;
 	struct task_struct *task;
 	int err = 0;
@@ -1064,7 +1063,7 @@ static int __set_oom_adj(struct file *file, int oom_adj, bool legacy)
 	if (!task)
 		return -ESRCH;
 
-	mutex_lock(&oom_adj_mutex);
+	mutex_lock(&oom_adj_lock);
 	if (legacy) {
 		if (oom_adj < task->signal->oom_score_adj &&
 				!capable(CAP_SYS_RESOURCE)) {
@@ -1095,7 +1094,7 @@ static int __set_oom_adj(struct file *file, int oom_adj, bool legacy)
 		struct task_struct *p = find_lock_task_mm(task);
 
 		if (p) {
-			if (atomic_read(&p->mm->mm_users) > 1) {
+			if (test_bit(MMF_PROC_SHARED, &p->mm->flags)) {
 				mm = p->mm;
 				mmgrab(mm);
 			}
@@ -1132,7 +1131,7 @@ static int __set_oom_adj(struct file *file, int oom_adj, bool legacy)
 		mmdrop(mm);
 	}
 err_unlock:
-	mutex_unlock(&oom_adj_mutex);
+	mutex_unlock(&oom_adj_lock);
 	put_task_struct(task);
 	return err;
 }
diff --git a/include/linux/oom.h b/include/linux/oom.h
index f022f581ac29..861f22bd4706 100644
--- a/include/linux/oom.h
+++ b/include/linux/oom.h
@@ -55,6 +55,7 @@ struct oom_control {
 };
 
 extern struct mutex oom_lock;
+extern struct mutex oom_adj_lock;
 
 static inline void set_current_oom_origin(void)
 {
diff --git a/include/linux/sched/coredump.h b/include/linux/sched/coredump.h
index ecdc6542070f..070629b722df 100644
--- a/include/linux/sched/coredump.h
+++ b/include/linux/sched/coredump.h
@@ -72,6 +72,7 @@ static inline int get_dumpable(struct mm_struct *mm)
 #define MMF_DISABLE_THP		24	/* disable THP for all VMAs */
 #define MMF_OOM_VICTIM		25	/* mm is the oom victim */
 #define MMF_OOM_REAP_QUEUED	26	/* mm was queued for oom_reaper */
+#define MMF_PROC_SHARED	27	/* mm is shared while sighand is not */
 #define MMF_DISABLE_THP_MASK	(1 << MMF_DISABLE_THP)
 
 #define MMF_INIT_MASK		(MMF_DUMPABLE_MASK | MMF_DUMP_FILTER_MASK |\
diff --git a/kernel/fork.c b/kernel/fork.c
index 4d32190861bd..9177a76bf840 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1403,6 +1403,15 @@ static int copy_mm(unsigned long clone_flags, struct task_struct *tsk)
 	if (clone_flags & CLONE_VM) {
 		mmget(oldmm);
 		mm = oldmm;
+		if (!(clone_flags & CLONE_SIGHAND)) {
+			/* We need to synchronize with __set_oom_adj */
+			mutex_lock(&oom_adj_lock);
+			set_bit(MMF_PROC_SHARED, &mm->flags);
+			/* Update the values in case they were changed after copy_signal */
+			tsk->signal->oom_score_adj = current->signal->oom_score_adj;
+			tsk->signal->oom_score_adj_min = current->signal->oom_score_adj_min;
+			mutex_unlock(&oom_adj_lock);
+		}
 		goto good_mm;
 	}
 
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index e90f25d6385d..c22f07c986cb 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -64,6 +64,8 @@ int sysctl_oom_dump_tasks = 1;
  * and mark_oom_victim
  */
 DEFINE_MUTEX(oom_lock);
+/* Serializes oom_score_adj and oom_score_adj_min updates */
+DEFINE_MUTEX(oom_adj_lock);
 
 static inline bool is_memcg_oom(struct oom_control *oc)
 {
-- 
2.28.0.297.g1956fa8f8d-goog