Re: [PATCH v2 2/5] memcg: provide root figures from system totals

[Michal Hocko <mhocko@suse.cz>]

On Wed 20-03-13 12:08:17, Glauber Costa wrote:
> On 03/20/2013 12:03 PM, Michal Hocko wrote:
> > On Wed 20-03-13 11:03:17, Glauber Costa wrote:
> >> On 03/19/2013 04:55 PM, Michal Hocko wrote:
> >>> On Tue 19-03-13 13:46:50, Michal Hocko wrote:
> >>>> On Tue 05-03-13 17:10:55, Glauber Costa wrote:
> >>>>> For the root memcg, there is no need to rely on the res_counters if hierarchy
> >>>>> is enabled The sum of all mem cgroups plus the tasks in root itself, is
> >>>>> necessarily the amount of memory used for the whole system. Since those figures
> >>>>> are already kept somewhere anyway, we can just return them here, without too
> >>>>> much hassle.
> >>>>>
> >>>>> Limit and soft limit can't be set for the root cgroup, so they are left at
> >>>>> RESOURCE_MAX. Failcnt is left at 0, because its actual meaning is how many
> >>>>> times we failed allocations due to the limit being hit. We will fail
> >>>>> allocations in the root cgroup, but the limit will never the reason.
> >>>>
> >>>> I do not like this very much to be honest. It just adds more hackery...
> >>>> Why cannot we simply not account if nr_cgroups == 1 and move relevant
> >>>> global counters to the root at the moment when a first group is
> >>>> created?
> >>>
> >>> OK, it seems that the very next patch does what I was looking for. So
> >>> why all the churn in this patch?
> >>> Why do you want to make root even more special?
> >>
> >> Because I am operating under the assumption that we want to handle that
> >> transparently and keep things working. If you tell me: "Hey, reading
> >> memory.usage_in_bytes from root should return 0!", then I can get rid of
> >> that.
> > 
> > If you simply switch to accounting for root then you do not have to care
> > about this, don't you?
> > 
> Of course not, but the whole point here is *not* accounting root.

I thought the objective was to not account root if there are no
children. I would see the "not account root at all" as another step.
And we are skipping charging it already (do not call
mem_cgroup_do_charge) for root.

> So if we are entirely skipping root account, it, I personally believe
> we need to replace it with something else so we can keep things
> working as much as we can.
> 
> It doesn't need to be perfect, though: There is no way we can have
> max_usage without something like a res_counter that locks memory
> charges. I believe we can live without that. But as for the basic
> statistics and numbers, I believe they should keep working.
 
[...]
-- 
Michal Hocko
SUSE Labs

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