Re: [v4 2/4] mm, oom: cgroup-aware OOM killer

Michal Hocko
2017-08-01T14:54:39Z

On Wed 26-07-17 14:27:16, Roman Gushchin wrote:
[...]
> +static long memcg_oom_badness(struct mem_cgroup *memcg,
> + const nodemask_t *nodemask)
> +{
> + long points = 0;
> + int nid;
> +
> + for_each_node_state(nid, N_MEMORY) {
> + if (nodemask && !node_isset(nid, *nodemask))
> + continue;
> +
> + points += mem_cgroup_node_nr_lru_pages(memcg, nid,
> + LRU_ALL_ANON | BIT(LRU_UNEVICTABLE));
> + }
> +
> + points += memcg_page_state(memcg, MEMCG_KERNEL_STACK_KB) /
> + (PAGE_SIZE / 1024);
> + points += memcg_page_state(memcg, NR_SLAB_UNRECLAIMABLE);
> + points += memcg_page_state(memcg, MEMCG_SOCK);
> + points += memcg_page_state(memcg, MEMCG_SWAP);
> +
> + return points;

I am wondering why are you diverging from the global oom_badness
behavior here. Although doing per NUMA accounting sounds like a better
idea but then you just end up mixing this with non NUMA numbers and the
whole thing is harder to understand without great advantages.

> +static void select_victim_memcg(struct mem_cgroup *root, struct oom_control *oc)
> +{
> + struct mem_cgroup *iter, *parent;
> +
> + for_each_mem_cgroup_tree(iter, root) {
> + if (memcg_has_children(iter)) {
> + iter->oom_score = 0;
> + continue;
> + }
> +
> + iter->oom_score = oom_evaluate_memcg(iter, oc->nodemask);
> + if (iter->oom_score == -1) {
> + oc->chosen_memcg = (void *)-1UL;
> + mem_cgroup_iter_break(root, iter);
> + return;
> + }
> +
> + if (!iter->oom_score)
> + continue;
> +
> + for (parent = parent_mem_cgroup(iter); parent && parent != root;
> + parent = parent_mem_cgroup(parent))
> + parent->oom_score += iter->oom_score;
> + }
> +
> + for (;;) {
> + struct cgroup_subsys_state *css;
> + struct mem_cgroup *memcg = NULL;
> + long score = LONG_MIN;
> +
> + css_for_each_child(css, &root->css) {
> + struct mem_cgroup *iter = mem_cgroup_from_css(css);
> +
> + if (iter->oom_score > score) {
> + memcg = iter;
> + score = iter->oom_score;
> + }
> + }
> +
> + if (!memcg) {
> + if (oc->memcg && root == oc->memcg) {
> + oc->chosen_memcg = oc->memcg;
> + css_get(&oc->chosen_memcg->css);
> + oc->chosen_points = oc->memcg->oom_score;
> + }
> + break;
> + }
> +
> + if (memcg->oom_kill_all_tasks || !memcg_has_children(memcg)) {
> + oc->chosen_memcg = memcg;
> + css_get(&oc->chosen_memcg->css);
> + oc->chosen_points = score;
> + break;
> + }
> +
> + root = memcg;
> + }
> +}

This and the rest of the victim selection code is really hairy and hard
to follow.

I would reap out the oom_kill_process into a separate patch.

> -static void oom_kill_process(struct oom_control *oc, const char *message)
> +static void __oom_kill_process(struct task_struct *victim)

To the rest of the patch. I have to say I do not quite like how it is
implemented. I was hoping for something much simpler which would hook
into oom_evaluate_task. If a task belongs to a memcg with kill-all flag
then we would update the cumulative memcg badness (more specifically the
badness of the topmost parent with kill-all flag). Memcg will then
compete with existing self contained tasks (oom_badness will have to
tell whether points belong to a task or a memcg to allow the caller to
deal with it). But it shouldn't be much more complex than that.

Or is there something that I am missing and that would prevent such a
simple approach?
--
Michal Hocko
SUSE Labs