On Thu, 17 May 2012 14:33:27 -0700 (PDT)
David Rientjes <rientjes@xxxxxxxxxx> wrote:
> The oom_score_adj scale ranges from -1000 to 1000 and represents the
> proportion of memory available to the process at allocation time. This
> means an oom_score_adj value of 300, for example, will bias a process as
> though it was using an extra 30.0% of available memory and a value of
> -350 will discount 35.0% of available memory from its usage.
>
> The oom killer badness heuristic also uses this scale to report the oom
> score for each eligible process in determining the "best" process to
> kill. Thus, it can only differentiate each process's memory usage by
> 0.1% of system RAM.
>
> On large systems, this can end up being a large amount of memory: 256MB
> on 256GB systems, for example.
>
> This can be fixed by having the badness heuristic to use the actual
> memory usage in scoring threads and then normalizing it to the
> oom_score_adj scale for userspace. This results in better comparison
> between eligible threads for kill and no change from the userspace
> perspective.
>
> ...
>
> @@ -198,45 +198,33 @@ unsigned int oom_badness(struct task_struct *p, struct mem_cgroup *memcg,
> }
>
> /*
> - * The memory controller may have a limit of 0 bytes, so avoid a divide
> - * by zero, if necessary.
> - */
> - if (!totalpages)
> - totalpages = 1;
> -
> - /*
> * The baseline for the badness score is the proportion of RAM that each
> * task's rss, pagetable and swap space use.
> */
> - points = get_mm_rss(p->mm) + p->mm->nr_ptes;
> - points += get_mm_counter(p->mm, MM_SWAPENTS);
> -
> - points *= 1000;
> - points /= totalpages;
> + points = get_mm_rss(p->mm) + p->mm->nr_ptes +
> + get_mm_counter(p->mm, MM_SWAPENTS);
> task_unlock(p);
>
> /*
> * Root processes get 3% bonus, just like the __vm_enough_memory()
> * implementation used by LSMs.
> */
> - if (has_capability_noaudit(p, CAP_SYS_ADMIN))
> - points -= 30;
> + if (has_capability_noaudit(p, CAP_SYS_ADMIN) && totalpages)
There doesn't seem much point in testing totalpages here - it's a
micro-optimisation which adds a branch, on a slow path.
> + points -= 30 * totalpages / 1000;
>
> /*
> * /proc/pid/oom_score_adj ranges from -1000 to +1000 such that it may
> * either completely disable oom killing or always prefer a certain
> * task.
> */
> - points += p->signal->oom_score_adj;
> + points += p->signal->oom_score_adj * totalpages / 1000;
And if we *do* want to add that micro-optimisation, we may as well
extend it to cover this expression also:
if (totalpages) { /* reason goes here */
if (has_capability_noaudit(...))
points -= 30 * totalpages / 1000;
p->signal->oom_score_adj * totalpages / 1000;
}
> /*
> * Never return 0 for an eligible task that may be killed since it's
> * possible that no single user task uses more than 0.1% of memory and
> * no single admin tasks uses more than 3.0%.
> */
> - if (points <= 0)
> - return 1;
> - return (points < 1000) ? points : 1000;
> + return points <= 0 ? 1 : points;
`points' is unsigned - testing it for negative looks odd.
> }
>
> /*
> @@ -314,7 +302,7 @@ static struct task_struct *select_bad_process(unsigned int *ppoints,
> {
> struct task_struct *g, *p;
> struct task_struct *chosen = NULL;
> - *ppoints = 0;
> + unsigned long chosen_points = 0;
>
> do_each_thread(g, p) {
> unsigned int points;
> @@ -354,7 +342,7 @@ static struct task_struct *select_bad_process(unsigned int *ppoints,
> */
> if (p == current) {
> chosen = p;
> - *ppoints = 1000;
> + chosen_points = ULONG_MAX;
> } else if (!force_kill) {
> /*
> * If this task is not being ptraced on exit,
> @@ -367,12 +355,13 @@ static struct task_struct *select_bad_process(unsigned int *ppoints,
> }
>
> points = oom_badness(p, memcg, nodemask, totalpages);
> - if (points > *ppoints) {
> + if (points > chosen_points) {
> chosen = p;
> - *ppoints = points;
> + chosen_points = points;
> }
> } while_each_thread(g, p);
>
> + *ppoints = chosen_points * 1000 / totalpages;
So it's up to the select_bad_process() callers to prevent the
divide-by-zero. It is unobvious that they actually do this, and this
important and unobvious caller requirement is undocumented.
> return chosen;
> }
>
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