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

[Christian Brauner]

On Wed, Aug 19, 2020 at 05:20:53PM -0700, 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_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);

This seems fine.

> +			/* 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;

But this seems wrong to me.
copy_signal() should be the only place where ->signal is set. Just from
a pure conceptual perspective. The copy_*() should be as self-contained
as possible imho.
Also, now I have to remember/look for two different locations where
oom_score_adj{_min} is initialized during fork. And this also creates a
dependency between copy_signal() and copy_mm() that doesn't need to be
there imho. I'm not a fan.

Also, you're in a branch where you're not sharing signal struct. So
after copy_signal() why would the parent changing their
oom_score_adj{_min} value matter? If that "race" is really important to
handle than you need to verify that the child has expected oom settings
after process creation anyway and should probably set it again from
userspace to make sure. This seems like an unnecessary optimization that
just makes the code that tiny bit more complex than it needs to be.

I'd really just do:

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);
	mutex_unlock(&oom_adj_lock);
}

makes this look way cleaner too imho.

Christian