Re: [PATCH v2 1/1] mm, oom_adj: don't loop through tasks in __set_oom_adj when not necessary

[Christian Brauner]

On Tue, Aug 25, 2020 at 07:24:34AM -0700, Suren Baghdasaryan wrote:
> On Tue, Aug 25, 2020 at 4:15 AM Christian Brauner
> <christian.brauner@xxxxxxxxxx> wrote:
> >
> > On Mon, Aug 24, 2020 at 08:30:36AM -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 && !CLONE_THREAD && !CLONE_VFORK). 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. 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_lock
> > > and setting MMF_PROC_SHARED when CLONE_VFORK is specified. 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 && !CLONE_THREAD && !CLONE_VFORK) 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>
> > > Debugged-by: Minchan Kim <minchan@xxxxxxxxxx>
> > > Suggested-by: Michal Hocko <mhocko@xxxxxxxxxx>
> > > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > > ---
> > >
> > > v2:
> > > - Implemented proposal from Michal Hocko in:
> > > https://lore.kernel.org/linux-fsdevel/20200820124109.GI5033@xxxxxxxxxxxxxx/
> > > - Updated description to reflect the change
> > >
> > > v1:
> > > - https://lore.kernel.org/linux-mm/20200820002053.1424000-1-surenb@xxxxxxxxxx/
> > >
> > >  fs/proc/base.c                 |  7 +++----
> > >  include/linux/oom.h            |  1 +
> > >  include/linux/sched/coredump.h |  1 +
> > >  kernel/fork.c                  | 21 +++++++++++++++++++++
> > >  mm/oom_kill.c                  |  2 ++
> > >  5 files changed, 28 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..6fce8ffa9b8b 100644
> > > --- a/kernel/fork.c
> > > +++ b/kernel/fork.c
> > > @@ -1809,6 +1809,25 @@ static __always_inline void delayed_free_task(struct task_struct *tsk)
> > >               free_task(tsk);
> > >  }
> > >
> > > +static void copy_oom_score_adj(u64 clone_flags, struct task_struct *tsk)
> > > +{
> > > +     /* Skip if kernel thread */
> > > +     if (!tsk->mm)
> > > +             return;
> >
> > Hm, wouldn't
> >
> >         if (tsk->flags & PF_KTHREAD)
> >                 return;
> >
> > be clearer and more future proof?
> 
> The check follows a similar pattern from copy_mm to detect when we are

Ah cool. Was mostly interest not me asking for a change. :)
This looks like a simple enough fix now. Fwiw:
Acked-by: Christian Brauner <christian.brauner@xxxxxxxxxx>

Thanks!
Christian