Re: [PATCH] mm,oom: Remove wake_oom_reaper().

[Michal Hocko <mhocko@kernel.org>]

On Fri 03-08-18 22:19:10, Tetsuo Handa wrote:
> Currently, wake_oom_reaper() is checking whether an OOM victim thread is
> already chained to the OOM victim list. But chaining one thread from each
> OOM victim process is sufficient.
> 
> Since mark_oom_victim() sets signal->oom_mm for one thread from each OOM
> victim process, we can chain that OOM victim thread there. Since oom_lock
> is held during __oom_kill_process(), by replacing oom_reaper_lock with
> oom_lock, it becomes safe to use mark_oom_victim() even if MMF_OOM_SKIP is
> later set due to is_global_init() case.

The changelog doesn't explain _why_ would we want to merge this. What is
the actual advantage of replacing a dedicated lock by a more scoped one
and make additional dependency here?

> Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> ---
>  mm/oom_kill.c | 41 ++++++++++-------------------------------
>  1 file changed, 10 insertions(+), 31 deletions(-)
> 
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 0e10b86..dad0409 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -486,7 +486,6 @@ bool process_shares_mm(struct task_struct *p, struct mm_struct *mm)
>  static struct task_struct *oom_reaper_th;
>  static DECLARE_WAIT_QUEUE_HEAD(oom_reaper_wait);
>  static struct task_struct *oom_reaper_list;
> -static DEFINE_SPINLOCK(oom_reaper_lock);
>  
>  bool __oom_reap_task_mm(struct mm_struct *mm)
>  {
> @@ -607,7 +606,7 @@ static void oom_reap_task(struct task_struct *tsk)
>  	 */
>  	set_bit(MMF_OOM_SKIP, &mm->flags);
>  
> -	/* Drop a reference taken by wake_oom_reaper */
> +	/* Drop a reference taken by mark_oom_victim(). */
>  	put_task_struct(tsk);
>  }
>  
> @@ -617,12 +616,12 @@ static int oom_reaper(void *unused)
>  		struct task_struct *tsk = NULL;
>  
>  		wait_event_freezable(oom_reaper_wait, oom_reaper_list != NULL);
> -		spin_lock(&oom_reaper_lock);
> +		mutex_lock(&oom_lock);
>  		if (oom_reaper_list != NULL) {
>  			tsk = oom_reaper_list;
>  			oom_reaper_list = tsk->oom_reaper_list;
>  		}
> -		spin_unlock(&oom_reaper_lock);
> +		mutex_unlock(&oom_lock);
>  
>  		if (tsk)
>  			oom_reap_task(tsk);
> @@ -631,32 +630,12 @@ static int oom_reaper(void *unused)
>  	return 0;
>  }
>  
> -static void wake_oom_reaper(struct task_struct *tsk)
> -{
> -	/* tsk is already queued? */
> -	if (tsk == oom_reaper_list || tsk->oom_reaper_list)
> -		return;
> -
> -	get_task_struct(tsk);
> -
> -	spin_lock(&oom_reaper_lock);
> -	tsk->oom_reaper_list = oom_reaper_list;
> -	oom_reaper_list = tsk;
> -	spin_unlock(&oom_reaper_lock);
> -	trace_wake_reaper(tsk->pid);
> -	wake_up(&oom_reaper_wait);
> -}
> -
>  static int __init oom_init(void)
>  {
>  	oom_reaper_th = kthread_run(oom_reaper, NULL, "oom_reaper");
>  	return 0;
>  }
>  subsys_initcall(oom_init)
> -#else
> -static inline void wake_oom_reaper(struct task_struct *tsk)
> -{
> -}
>  #endif /* CONFIG_MMU */
>  
>  /**
> @@ -682,6 +661,13 @@ static void mark_oom_victim(struct task_struct *tsk)
>  	if (!cmpxchg(&tsk->signal->oom_mm, NULL, mm)) {
>  		mmgrab(tsk->signal->oom_mm);
>  		set_bit(MMF_OOM_VICTIM, &mm->flags);
> +#ifdef CONFIG_MMU
> +		get_task_struct(tsk);
> +		tsk->oom_reaper_list = oom_reaper_list;
> +		oom_reaper_list = tsk;
> +		trace_wake_reaper(tsk->pid);
> +		wake_up(&oom_reaper_wait);
> +#endif
>  	}
>  
>  	/*
> @@ -833,7 +819,6 @@ static void __oom_kill_process(struct task_struct *victim)
>  {
>  	struct task_struct *p;
>  	struct mm_struct *mm;
> -	bool can_oom_reap = true;
>  
>  	p = find_lock_task_mm(victim);
>  	if (!p) {
> @@ -883,7 +868,6 @@ static void __oom_kill_process(struct task_struct *victim)
>  		if (same_thread_group(p, victim))
>  			continue;
>  		if (is_global_init(p)) {
> -			can_oom_reap = false;
>  			set_bit(MMF_OOM_SKIP, &mm->flags);
>  			pr_info("oom killer %d (%s) has mm pinned by %d (%s)\n",
>  					task_pid_nr(victim), victim->comm,
> @@ -900,9 +884,6 @@ static void __oom_kill_process(struct task_struct *victim)
>  	}
>  	rcu_read_unlock();
>  
> -	if (can_oom_reap)
> -		wake_oom_reaper(victim);
> -
>  	mmdrop(mm);
>  	put_task_struct(victim);
>  }
> @@ -941,7 +922,6 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
>  	task_lock(p);
>  	if (task_will_free_mem(p)) {
>  		mark_oom_victim(p);
> -		wake_oom_reaper(p);
>  		task_unlock(p);
>  		put_task_struct(p);
>  		return;
> @@ -1071,7 +1051,6 @@ bool out_of_memory(struct oom_control *oc)
>  	 */
>  	if (task_will_free_mem(current)) {
>  		mark_oom_victim(current);
> -		wake_oom_reaper(current);
>  		return true;
>  	}
>  
> -- 
> 1.8.3.1

-- 
Michal Hocko
SUSE Labs