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

[Christian Brauner <christian.brauner@ubuntu.com>]

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?

Christian