On Tue, Jun 26, 2012 at 11:04:51AM -0700, Tejun Heo wrote:
> On Tue, Jun 26, 2012 at 07:47:14PM +0400, Glauber Costa wrote:
> > Okay, so after recent discussions, I am proposing the following
> > patch. It won't remove hierarchy, or anything like that. Just default
> > to true in the root cgroup, and print a warning once if you try
> > to set it back to 0.
> >
> > I am not adding it to feature-removal-schedule.txt because I don't
> > view it as a consensus. Rather, changing the default would allow us
> > to give it a time around in the open, and see if people complain
> > and what we can learn about that.
> >
> > Signed-off-by: Glauber Costa <glommer@xxxxxxxxxxxxx>
> > Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> > CC: Michal Hocko <mhocko@xxxxxxx>
> > CC: Kamezawa Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> > CC: Tejun Heo <tj@xxxxxxxxxx>
> > ---
> > mm/memcontrol.c | 5 +++++
> > 1 file changed, 5 insertions(+)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 85f7790..c37e4c1 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -3993,6 +3993,10 @@ static int mem_cgroup_hierarchy_write(struct cgroup *cont, struct cftype *cft,
> > if (memcg->use_hierarchy == val)
> > goto out;
> >
> > + WARN_ONCE(!parent_memcg && memcg->use_hierarchy,
> > + "Non-hierarchical memcg is considered for deprecation\n"
> > + "Please consider reorganizing your tree to work with hierarchical accounting\n"
> > + "If you have any reason not to, let us know at cgroups@xxxxxxxxxxxxxxx\n");
> > /*
> > * If parent's use_hierarchy is set, we can't make any modifications
> > * in the child subtrees. If it is unset, then the change can
> > @@ -5221,6 +5225,7 @@ mem_cgroup_create(struct cgroup *cont)
> > INIT_WORK(&stock->work, drain_local_stock);
> > }
> > hotcpu_notifier(memcg_cpu_hotplug_callback, 0);
> > + memcg->use_hierarchy = true;
> > } else {
> > parent = mem_cgroup_from_cont(cont->parent);
> > memcg->use_hierarchy = parent->use_hierarchy;
>
> So, ummm, I don't think we can do this. We CAN NOT silently flip the
> default behavior like this. Hell, no.
Why?
This is not flipping a 50/50 switch and forcing half the userbase to
something that fits us better right now. It's about changing a
default that most setups either don't care about or enable manually.
We expect zero setups to functionally rely on nesting the directories
without nesting the cgroups they represent.
> What we can do is something like the following.
>
> 1. Make .use_hierarchy a global property and convert .use_hierarchy
> file to reject writes to the setting which is different from the
> global one. Rip out partial hierarchy related code (how little
> they may be). Note that the default should still be flat
> hierarchy.
>
> 2. Mark flat hierarchy deprecated and produce a warning message if
> memcg is mounted w/o hierarchy option for a year or two.
I think most of us assume that the common case is either not nesting
directories or still working with hierarchy support actually enabled.
I would hate if people had to jump through hoops to get the only
behaviour we want to end up supporting and to not get yelled at, it
sends all the wrong signals.
> 3. After the existing users had enough chance to move away from flat
> hierarchy, rip out flat hierarchy code and error if hierarchy
> option is not specified.
This description sounds much more sane than what we are actually
trying to ban, which is not a flat structure, but treating groups with
nested directories as equal siblings.
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