Re: [RFC 2/3] oom: Do not sacrifice already OOM killed children

[David Rientjes <rientjes@google.com>]

On Tue, 12 Jan 2016, Michal Hocko wrote:

> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index 2b9dc5129a89..8bca0b1e97f7 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -671,6 +671,63 @@ static bool process_shares_mm(struct task_struct *p, struct mm_struct *mm)
>  }
>  
>  #define K(x) ((x) << (PAGE_SHIFT-10))
> +
> +/*
> + * If any of victim's children has a different mm and is eligible for kill,
> + * the one with the highest oom_badness() score is sacrificed for its
> + * parent.  This attempts to lose the minimal amount of work done while
> + * still freeing memory.
> + */
> +static struct task_struct *
> +try_to_sacrifice_child(struct oom_control *oc, struct task_struct *victim,
> +		       unsigned long totalpages, struct mem_cgroup *memcg)
> +{
> +	struct task_struct *child_victim = NULL;
> +	unsigned int victim_points = 0;
> +	struct task_struct *t;
> +
> +	read_lock(&tasklist_lock);
> +	for_each_thread(victim, t) {
> +		struct task_struct *child;
> +
> +		list_for_each_entry(child, &t->children, sibling) {
> +			unsigned int child_points;
> +
> +			/*
> +			 * Skip over already OOM killed children as this hasn't
> +			 * helped to resolve the situation obviously.
> +			 */
> +			if (test_tsk_thread_flag(child, TIF_MEMDIE) ||
> +					fatal_signal_pending(child) ||
> +					task_will_free_mem(child))
> +				continue;
> +

What guarantees that child had time to exit after it has been oom killed 
(better yet, what guarantees that it has even scheduled after it has been 
oom killed)?  It seems like this would quickly kill many children 
unnecessarily.

> +			if (process_shares_mm(child, victim->mm))
> +				continue;
> +
> +			child_points = oom_badness(child, memcg, oc->nodemask,
> +								totalpages);
> +			if (child_points > victim_points) {
> +				if (child_victim)
> +					put_task_struct(child_victim);
> +				child_victim = child;
> +				victim_points = child_points;
> +				get_task_struct(child_victim);
> +			}
> +		}
> +	}
> +	read_unlock(&tasklist_lock);
> +
> +	if (!child_victim)
> +		goto out;
> +
> +	put_task_struct(victim);
> +	victim = child_victim;
> +
> +out:
> +	return victim;
> +}
> +
>  /*
>   * Must be called while holding a reference to p, which will be released upon
>   * returning.
> @@ -680,10 +737,7 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
>  		      struct mem_cgroup *memcg, const char *message)
>  {
>  	struct task_struct *victim = p;
> -	struct task_struct *child;
> -	struct task_struct *t;
>  	struct mm_struct *mm;
> -	unsigned int victim_points = 0;
>  	static DEFINE_RATELIMIT_STATE(oom_rs, DEFAULT_RATELIMIT_INTERVAL,
>  					      DEFAULT_RATELIMIT_BURST);
>  	bool can_oom_reap = true;
> @@ -707,34 +761,7 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
>  	pr_err("%s: Kill process %d (%s) score %u or sacrifice child\n",
>  		message, task_pid_nr(p), p->comm, points);
>  
> -	/*
> -	 * If any of p's children has a different mm and is eligible for kill,
> -	 * the one with the highest oom_badness() score is sacrificed for its
> -	 * parent.  This attempts to lose the minimal amount of work done while
> -	 * still freeing memory.
> -	 */
> -	read_lock(&tasklist_lock);
> -	for_each_thread(p, t) {
> -		list_for_each_entry(child, &t->children, sibling) {
> -			unsigned int child_points;
> -
> -			if (process_shares_mm(child, p->mm))
> -				continue;
> -			/*
> -			 * oom_badness() returns 0 if the thread is unkillable
> -			 */
> -			child_points = oom_badness(child, memcg, oc->nodemask,
> -								totalpages);
> -			if (child_points > victim_points) {
> -				put_task_struct(victim);
> -				victim = child;
> -				victim_points = child_points;
> -				get_task_struct(victim);
> -			}
> -		}
> -	}
> -	read_unlock(&tasklist_lock);
> -
> +	victim = try_to_sacrifice_child(oc, victim, totalpages, memcg);
>  	p = find_lock_task_mm(victim);
>  	if (!p) {
>  		put_task_struct(victim);