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

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Date:   Wed, 23 Aug 2017 19:04:50 +0100
From:   Roman Gushchin <guro@...com>
To:     Johannes Weiner <hannes@...xchg.org>
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

On Wed, Aug 23, 2017 at 01:24:41PM -0400, Johannes Weiner wrote:
> 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.

Sorry, haven't noticed this.

> > > >  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).

Why do you call it implementation, it's definitely an user's intention
"if a cgroup is under OOM, all belonging processes should be killed".

How it can be implemented differently?

> 
> 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'.

I don't think it's a good argument here: realistically, nobody will type
the knob's name often. Your option is shorter only by 3 characters :)

Anyway, I'm ok with memory.oom_group too, if everybody else prefer it.
Michal, David?
What's your opinion?

Thanks!