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

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Aug 25, 2020 at 4:15 AM Christian Brauner
<christian.brauner@ubuntu.com> 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@google.com>
> > Debugged-by: Minchan Kim <minchan@kernel.org>
> > Suggested-by: Michal Hocko <mhocko@kernel.org>
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> >
> > v2:
> > - Implemented proposal from Michal Hocko in:
> > https://lore.kernel.org/linux-fsdevel/20200820124109.GI5033@dhcp22.suse.cz/
> > - Updated description to reflect the change
> >
> > v1:
> > - https://lore.kernel.org/linux-mm/20200820002053.1424000-1-surenb@google.com/
> >
> >  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
cloning a kernel thread. Also IMHO this is a more obvious way to
protect from dereferencing a NULL tsk->mm, which is the important
point here. I can change or remove the comment if that makes things
more clear. Please LMK if that would help.

>
> Christian