Re: [PATCH 3/3] mm, oom: introduce memory.oom.group

[Michal Hocko]

On Tue 31-07-18 18:14:48, Roman Gushchin wrote:
> On Tue, Jul 31, 2018 at 11:07:00AM +0200, Michal Hocko wrote:
> > On Mon 30-07-18 11:01:00, Roman Gushchin wrote:
> > > For some workloads an intervention from the OOM killer
> > > can be painful. Killing a random task can bring
> > > the workload into an inconsistent state.
> > > 
> > > Historically, there are two common solutions for this
> > > problem:
> > > 1) enabling panic_on_oom,
> > > 2) using a userspace daemon to monitor OOMs and kill
> > >    all outstanding processes.
> > > 
> > > Both approaches have their downsides:
> > > rebooting on each OOM is an obvious waste of capacity,
> > > and handling all in userspace is tricky and requires
> > > a userspace agent, which will monitor all cgroups
> > > for OOMs.
> > > 
> > > In most cases an in-kernel after-OOM cleaning-up
> > > mechanism can eliminate the necessity of enabling
> > > panic_on_oom. Also, it can simplify the cgroup
> > > management for userspace applications.
> > > 
> > > This commit introduces a new knob for cgroup v2 memory
> > > controller: memory.oom.group. The knob determines
> > > whether the cgroup should be treated as a single
> > > unit by the OOM killer. If set, the cgroup and its
> > > descendants are killed together or not at all.
> > 
> > I do not want to nit pick on wording but unit is not really a good
> > description. I would expect that to mean that the oom killer will
> > consider the unit also when selecting the task and that is not the case.
> > I would be more explicit about this being a single killable entity
> > because it forms an indivisible workload.
> > 
> > You can reuse http://lkml.kernel.org/r/20180730080357.GA24267@xxxxxxxxxxxxxx
> > if you want.
> 
> Ok, I'll do my best to make it clearer.
> 
> > 
> > [...]
> > > +/**
> > > + * mem_cgroup_get_oom_group - get a memory cgroup to clean up after OOM
> > > + * @victim: task to be killed by the OOM killer
> > > + * @oom_domain: memcg in case of memcg OOM, NULL in case of system-wide OOM
> > > + *
> > > + * Returns a pointer to a memory cgroup, which has to be cleaned up
> > > + * by killing all belonging OOM-killable tasks.
> > 
> > Caller has to call mem_cgroup_put on the returned non-null memcg.
> 
> Added.
> 
> > 
> > > + */
> > > +struct mem_cgroup *mem_cgroup_get_oom_group(struct task_struct *victim,
> > > +					    struct mem_cgroup *oom_domain)
> > > +{
> > > +	struct mem_cgroup *oom_group = NULL;
> > > +	struct mem_cgroup *memcg;
> > > +
> > > +	if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
> > > +		return NULL;
> > > +
> > > +	if (!oom_domain)
> > > +		oom_domain = root_mem_cgroup;
> > > +
> > > +	rcu_read_lock();
> > > +
> > > +	memcg = mem_cgroup_from_task(victim);
> > > +	if (!memcg || memcg == root_mem_cgroup)
> > > +		goto out;
> > 
> > When can we have memcg == NULL? victim should be always non-NULL.
> > Also why do you need to special case the root_mem_cgroup here. The loop
> > below should handle that just fine no?
> 
> Idk, I prefer to keep an explicit root_mem_cgroup check,
> rather than traversing the tree and relying on an inability
> to set oom_group on the root.

I will not insist but this just makes the code harder to read.

[...]
> > > +	if (oom_group) {
> > 
> > we want a printk explaining that we are going to tear down the whole
> > oom_group here.
> 
> Does this looks good?
> Or it's better to remove "memory." prefix?
> 
> [   52.835327] Out of memory: Kill process 1221 (allocate) score 241 or sacrifice child
> [   52.836625] Killed process 1221 (allocate) total-vm:2257144kB, anon-rss:2009128kB, file-rss:4kB, shmem-rss:0kB
> [   52.841431] Tasks in /A1 are going to be killed due to memory.oom.group set

Yes, looks good to me.

> [   52.869439] Killed process 1217 (allocate) total-vm:2052344kB, anon-rss:1704036kB, file-rss:0kB, shmem-rss:0kB
> [   52.875601] Killed process 1218 (allocate) total-vm:106668kB, anon-rss:24668kB, file-rss:0kB, shmem-rss:0kB
> [   52.882914] Killed process 1219 (allocate) total-vm:106668kB, anon-rss:21528kB, file-rss:0kB, shmem-rss:0kB
> [   52.891806] Killed process 1220 (allocate) total-vm:2257144kB, anon-rss:1984120kB, file-rss:4kB, shmem-rss:0kB
> [   52.903770] Killed process 1221 (allocate) total-vm:2257144kB, anon-rss:2009128kB, file-rss:4kB, shmem-rss:0kB
> [   52.905574] Killed process 1222 (allocate) total-vm:2257144kB, anon-rss:2063640kB, file-rss:0kB, shmem-rss:0kB
> [   53.202153] oom_reaper: reaped process 1222 (allocate), now anon-rss:0kB, file-rss:0kB, shmem-rss:0kB
> 
> > 
> > > +		mem_cgroup_scan_tasks(oom_group, oom_kill_memcg_member, NULL);
> > > +		mem_cgroup_put(oom_group);
> > > +	}
> > >  }
> > 
> > Other than that looks good to me. My concern that the previous
> > implementation was more consistent because we were comparing memcgs
> > still holds but if there is no way forward that direction this should be
> > acceptable as well.
> > 
> > After above small things are addressed you can add
> > Acked-by: Michal Hocko <mhocko@xxxxxxxx> 
> 
> Thank you!

-- 
Michal Hocko
SUSE Labs