linux-kernel - Re: [v5 2/4] mm, oom: cgroup-aware OOM killer

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Message-ID: <20170823172441.GA29085@cmpxchg.org>
Date:   Wed, 23 Aug 2017 13:24:41 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Roman Gushchin <guro@...com>
Cc:     linux-mm@...ck.org, Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        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: [v5 2/4] mm, oom: cgroup-aware OOM killer

Hi,

On Wed, Aug 23, 2017 at 05:20:31PM +0100, Roman Gushchin wrote:
> On Tue, Aug 22, 2017 at 01:03:44PM -0400, Johannes Weiner wrote:
> > > +	css_task_iter_start(&memcg->css, 0, &it);
> > > +	while ((task = css_task_iter_next(&it))) {
> > > +		/*
> > > +		 * If there are no tasks, or all tasks have oom_score_adj set
> > > +		 * to OOM_SCORE_ADJ_MIN and oom_kill_all_tasks is not set,
> > > +		 * don't select this memory cgroup.
> > > +		 */
> > > +		if (!elegible &&
> > > +		    (memcg->oom_kill_all_tasks ||
> > > +		     task->signal->oom_score_adj != OOM_SCORE_ADJ_MIN))
> > > +			elegible = 1;
> > 
> > This is a little awkward to read. How about something like this:
> > 
> > 	/*
> > 	 * When killing individual tasks, we respect OOM score adjustments:
> > 	 * at least one task in the group needs to be killable for the group
> > 	 * to be oomable.
> > 	 *
> > 	 * Also check that previous OOM kills have finished, and abort if
> > 	 * there are any pending OOM victims.
> > 	 */
> > 	oomable = memcg->oom_kill_all_tasks;
> > 	while ((task = css_task_iter_next(&it))) {
> > 		if (!oomable && task->signal_oom_score_adj != OOM_SCORE_ADJ_MIN)
> > 			oomable = 1;
> > 
> > > +		if (tsk_is_oom_victim(task) &&
> > > +		    !test_bit(MMF_OOM_SKIP, &task->signal->oom_mm->flags)) {
> > > +			elegible = -1;
> > > +			break;
> > > +		}
> > > +	}
> > > +	css_task_iter_end(&it);
> 
> We ignore oom_score_adj if oom_kill_all_tasks is set, it's
> not reflected in your version. Anyway, I've moved the comments block
> outside and rephrased it to make more clear.

Yes it is...? We only respect the score if !oomable, which is set to
oom_kill_all_tasks.

> > > +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) {
> > 
> > Please add comments to document the special returns. Maybe #defines
> > would be clearer, too.
> > 
> > > +			oc->chosen_memcg = (void *)-1UL;
> > > +			mem_cgroup_iter_break(root, iter);
> > > +			return;
> > > +		}
> > > +
> > > +		if (!iter->oom_score)
> > > +			continue;
> > 
> > Same here.
> > 
> > Maybe a switch would be suitable to handle the abort/no-score cases. 
> 
> Not sure about switch/defines, but I've added several comment blocks
> to describe possible return values, as well as their handling.
> Hope, it will be enough.

Sounds good.

> > >  static int memory_events_show(struct seq_file *m, void *v)
> > >  {
> > >  	struct mem_cgroup *memcg = mem_cgroup_from_css(seq_css(m));
> > > @@ -5310,6 +5512,12 @@ static struct cftype memory_files[] = {
> > >  		.write = memory_max_write,
> > >  	},
> > >  	{
> > > +		.name = "oom_kill_all_tasks",
> > > +		.flags = CFTYPE_NOT_ON_ROOT,
> > > +		.seq_show = memory_oom_kill_all_tasks_show,
> > > +		.write = memory_oom_kill_all_tasks_write,
> > > +	},
> > 
> > This name is quite a mouthful and reminiscent of the awkward v1
> > interface names. It doesn't really go well with the v2 names.
> > 
> > How about memory.oom_group?
> 
> I'd prefer to have something more obvious. I've renamed
> memory.oom_kill_all_tasks to memory.oom_kill_all, which was earlier suggested
> by Vladimir. Are you ok with it?

No, we should be striving for short and sweet mnemonics that express a
concept (oom applies to group, not member tasks) instead of underscore
sentences that describe an implementation (upon oom, kill all tasks in
the group).

It's better to have newbies consult the documentation once than making
everybody deal with long and cumbersome names for the rest of time.

Like 'ls' being better than 'read_and_print_directory_contents'.