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

[Michal Hocko]

On Thu 20-08-20 10:46:54, Christian Brauner wrote:
> 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.

Yes, this is not great but we will need it because the __set_oom_adj
might happen between copy_signal and copy_mm. If that happens then
__set_oom_adj misses this newly created task and so it will have a
disagreeing oom_score_adj. 

-- 
Michal Hocko
SUSE Labs