Date:   Thu, 5 Oct 2017 14:58:08 +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>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, kernel-team@...com,
        cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v10 4/6] mm, oom: introduce memory.oom_group

On Thu 05-10-17 13:32:14, Roman Gushchin wrote:
> On Thu, Oct 05, 2017 at 02:06:49PM +0200, Michal Hocko wrote:
> > On Wed 04-10-17 16:46:36, Roman Gushchin wrote:
> > > The cgroup-aware OOM killer treats leaf memory cgroups as memory
> > > consumption entities and performs the victim selection by comparing
> > > them based on their memory footprint. Then it kills the biggest task
> > > inside the selected memory cgroup.
> > > 
> > > But there are workloads, which are not tolerant to a such behavior.
> > > Killing a random task may leave the workload in a broken state.
> > > 
> > > To solve this problem, memory.oom_group knob is introduced.
> > > It will define, whether a memory group should be treated as an
> > > indivisible memory consumer, compared by total memory consumption
> > > with other memory consumers (leaf memory cgroups and other memory
> > > cgroups with memory.oom_group set), and whether all belonging tasks
> > > should be killed if the cgroup is selected.
> > > 
> > > If set on memcg A, it means that in case of system-wide OOM or
> > > memcg-wide OOM scoped to A or any ancestor cgroup, all tasks,
> > > belonging to the sub-tree of A will be killed. If OOM event is
> > > scoped to a descendant cgroup (A/B, for example), only tasks in
> > > that cgroup can be affected. OOM killer will never touch any tasks
> > > outside of the scope of the OOM event.
> > > 
> > > Also, tasks with oom_score_adj set to -1000 will not be killed.
> > 
> > I would extend the last sentence with an explanation. What about the
> > following:
> > "
> > Also, tasks with oom_score_adj set to -1000 will not be killed because
> > this has been a long established way to protect a particular process
> > from seeing an unexpected SIGKILL from the oom killer. Ignoring this
> > user defined configuration might lead to data corruptions or other
> > misbehavior.
> > "
> 
> Added, thanks!
> 
> > 
> > few mostly nit picks below but this looks good other than that. Once the
> > fix mentioned in patch 3 is folded I will ack this.
> > 
> > [...]
> > 
> > >  static void select_victim_memcg(struct mem_cgroup *root, struct oom_control *oc)
> > >  {
> > > -	struct mem_cgroup *iter;
> > > +	struct mem_cgroup *iter, *group = NULL;
> > > +	long group_score = 0;
> > >  
> > >  	oc->chosen_memcg = NULL;
> > >  	oc->chosen_points = 0;
> > >  
> > >  	/*
> > > +	 * If OOM is memcg-wide, and the memcg has the oom_group flag set,
> > > +	 * all tasks belonging to the memcg should be killed.
> > > +	 * So, we mark the memcg as a victim.
> > > +	 */
> > > +	if (oc->memcg && mem_cgroup_oom_group(oc->memcg)) {
> > 
> > we have is_memcg_oom() helper which is esier to read and understand than
> > the explicit oc->memcg check
> 
> It's defined in oom_kill.c and not exported, so I'm not sure.

putting it to oom.h shouldn't be a big deal.
 
> > > +		oc->chosen_memcg = oc->memcg;
> > > +		css_get(&oc->chosen_memcg->css);
> > > +		return;
> > > +	}
> > > +
> > > +	/*
> > >  	 * The oom_score is calculated for leaf memory cgroups (including
> > >  	 * the root memcg).
> > > +	 * Non-leaf oom_group cgroups accumulating score of descendant
> > > +	 * leaf memory cgroups.
> > >  	 */
> > >  	rcu_read_lock();
> > >  	for_each_mem_cgroup_tree(iter, root) {
> > >  		long score;
> > >  
> > > +		/*
> > > +		 * We don't consider non-leaf non-oom_group memory cgroups
> > > +		 * as OOM victims.
> > > +		 */
> > > +		if (memcg_has_children(iter) && !mem_cgroup_oom_group(iter))
> > > +			continue;
> > > +
> > > +		/*
> > > +		 * If group is not set or we've ran out of the group's sub-tree,
> > > +		 * we should set group and reset group_score.
> > > +		 */
> > > +		if (!group || group == root_mem_cgroup ||
> > > +		    !mem_cgroup_is_descendant(iter, group)) {
> > > +			group = iter;
> > > +			group_score = 0;
> > > +		}
> > > +
> > 
> > hmm, I thought you would go with a recursive oom_evaluate_memcg
> > implementation that would result in a more readable code IMHO. It is
> > true that we would traverse oom_group more times. But I do not expect
> > we would have very deep memcg hierarchies in the majority of workloads
> > and even if we did then this is a cold path which should focus on
> > readability more than a performance. Also implementing
> > mem_cgroup_iter_skip_subtree shouldn't be all that hard if this ever
> > turns out a real problem.
> 
> I've tried to go this way, but I didn't like the result. These both
> loops will share a lot of code (e.g. breaking on finding a previous victim,
> skipping non-leaf non-oom-group memcgs, etc), so the result is more messy.
> And actually it's strange to start a new loop to iterate exactly over
> the same sub-tree, which you want to skip in the first loop.

As I've said, I will not insist. It just makes more sense to me to do
the hierarchical behavior in a single place rather than open code it in
the main loop.
 
> > Anyway this is nothing really fundamental so I will leave the decision
> > on you.
> > 
> > > +static bool oom_kill_memcg_victim(struct oom_control *oc)
> > > +{
> > >  	if (oc->chosen_memcg == NULL || oc->chosen_memcg == INFLIGHT_VICTIM)
> > >  		return oc->chosen_memcg;
> > >  
> > > -	/* Kill a task in the chosen memcg with the biggest memory footprint */
> > > -	oc->chosen_points = 0;
> > > -	oc->chosen_task = NULL;
> > > -	mem_cgroup_scan_tasks(oc->chosen_memcg, oom_evaluate_task, oc);
> > > -
> > > -	if (oc->chosen_task == NULL || oc->chosen_task == INFLIGHT_VICTIM)
> > > -		goto out;
> > > -
> > > -	__oom_kill_process(oc->chosen_task);
> > > +	/*
> > > +	 * If memory.oom_group is set, kill all tasks belonging to the sub-tree
> > > +	 * of the chosen memory cgroup, otherwise kill the task with the biggest
> > > +	 * memory footprint.
> > > +	 */
> > > +	if (mem_cgroup_oom_group(oc->chosen_memcg)) {
> > > +		mem_cgroup_scan_tasks(oc->chosen_memcg, oom_kill_memcg_member,
> > > +				      NULL);
> > > +		/* We have one or more terminating processes at this point. */
> > > +		oc->chosen_task = INFLIGHT_VICTIM;
> > 
> > it took me a while to realize we need this because of return
> > !!oc->chosen_task in out_of_memory. Subtle... Also a reason to hate
> > oc->chosen_* thingy. As I've said in other reply, don't worry about this
> > I will probably turn my hate into a patch ;)
> > 
> > > +	} else {
> > > +		oc->chosen_points = 0;
> > > +		oc->chosen_task = NULL;
> > > +		mem_cgroup_scan_tasks(oc->chosen_memcg, oom_evaluate_task, oc);
> > > +
> > > +		if (oc->chosen_task == NULL ||
> > > +		    oc->chosen_task == INFLIGHT_VICTIM)
> > > +			goto out;
> > 
> > How can this happen? There shouldn't be any INFLIGHT_VICTIM in our memcg
> > because we have checked for that already. I can see how we do not find
> > any task because those can terminate by the time we get here but no new
> > oom victim should appear we are under the oom_lock.
> 
> You're probably right, but I would prefer to have this check in place,
> rather then get a panic on attempt to kill an INFLIGHT_VICTIM task one day.

This would be a bug which you just paper over.
-- 
Michal Hocko
SUSE Labs

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