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

[Michal Hocko <mhocko@suse.cz>]

On Wed 20-03-13 12:34:01, Glauber Costa wrote:
> On 03/20/2013 12:18 PM, Michal Hocko wrote:
> > 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. 
> 
> It is the goal, yes. As I said: I want the root-only case to keep
> providing userspace with meaningful statistics,

Sure, statistics need to stay at the place. I am not objecting on that.

> therefore the bypass.

I am just arguing about bypassing root even when there are children and
use_hierarchy == 1 because it adds more code to maintain.

> But since the machinery is in place, it is trivial to keep bypassing for
> use_hierarchy = 1 at the root level. If you believe it would be simpler,
> I could refrain from doing it.

I am all for "the simple the better" and add more optimizations on top.
We have a real issue now and we should eliminate it. My original plan
was to look at the bottlenecks and eliminate them one after another in
smaller steps. But all the work I have on the plate is preempting me
from looking into that...
-- 
Michal Hocko
SUSE Labs

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>