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

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Message-ID: <20170801145435.GN15774@dhcp22.suse.cz>
Date:   Tue, 1 Aug 2017 16:54:35 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Roman Gushchin <guro@...com>
Cc:     linux-mm@...ck.org, Vladimir Davydov <vdavydov.dev@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        David Rientjes <rientjes@...gle.com>,
        Tejun Heo <tj@...nel.org>, kernel-team@...com,
        cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v4 2/4] mm, oom: cgroup-aware OOM killer

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