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

Eric W. Biederman
2020-08-20T12:38:58Z

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?

The problem in all of this appears to be that we want the score to be
per mm and we are setting it per ``process'' (aka signal_struct).

To maintained backwards compatibility I expect exec can be taught to
copy the oom_score_adj from one mm to another more simply than
we can synchronize oom_score_adj between all of the threads in the
thread group.

Eric

>
> Fixes: 44a70adec910 ("mm, oom_adj: make sure processes sharing mm have same view of oom_score_adj")
> Reported-by: Tim Murray <timmurray@google.com>
> Suggested-by: Michal Hocko <mhocko@kernel.org>
> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> ---
> 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)
> {