Re: [patch 1/2] oom: avoid killing a task if a thread sharing its mm cannot be killed

[Oleg Nesterov <oleg@redhat.com>]

Well. I shouldn't try to comment this patch because I do not know
the state of the current code (and I do not understand the changelog).
Still, it looks a bit strange to me.

On 08/14, David Rientjes wrote:
>
> + * Determines whether an mm is unfreeable since a user thread attached to
> + * it cannot be killed.  Kthreads only temporarily assume a thread's mm,
> + * so they are not considered.
> + *
> + * mm need not be protected by task_lock() since it will not be
> + * dereferened.
> + */
> +static bool is_mm_unfreeable(struct mm_struct *mm)
> +{
> +	struct task_struct *g, *q;
> +
> +	do_each_thread(g, q) {
> +		if (q->mm == mm && !(q->flags & PF_KTHREAD) &&
> +		    q->signal->oom_score_adj == OOM_SCORE_ADJ_MIN)
> +			return true;
> +	} while_each_thread(g, q);

do_each_thread() doesn't look good. All sub-threads have the same ->mm.

	for_each_process(p) {
		if (p->flags && PF_KTHREAD)
			continue;
		do {
			if (!t->mm)
				continue;
			if (t->mm != mm)
				break;
			if (t->signal->oom_score_adj == OOM_SCORE_ADJ_MIN)
				return true;
		} while_each_thread(p, t);
	}

	return false;

However, even if is_mm_unfreeable() uses for_each_process(),

> @@ -160,12 +181,7 @@ unsigned int oom_badness(struct task_struct *p, struct mem_cgroup *mem,
>  	p = find_lock_task_mm(p);
>  	if (!p)
>  		return 0;
> -
> -	/*
> -	 * Shortcut check for OOM_SCORE_ADJ_MIN so the entire heuristic doesn't
> -	 * need to be executed for something that cannot be killed.
> -	 */
> -	if (p->signal->oom_score_adj == OOM_SCORE_ADJ_MIN) {
> +	if (is_mm_unfreeable(p->mm)) {

oom_badness() becomes O(n**2), not good.

And, more importantly. This patch makes me think ->oom_score_adj should
be moved from ->signal to ->mm.

Oleg.

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