[Devel] Re: How to draw values for /proc/stat

[Peter Zijlstra]

On Mon, 2011-12-05 at 07:32 -0200, Glauber Costa wrote:
> Hi,
> 
> Specially Peter and Paul, but all the others:
> 
> As you can see in https://lkml.org/lkml/2011/12/4/178, and in my answer 
> to that, there is a question - one I've asked before but without that 
> much of an audience - of whether /proc files read from process living on 
> cgroups should display global or per-cgroup resources.
> 
> In the past, I was arguing for a knob to control that, but I recently 
> started to believe that a knob here will only overcomplicate matters:
> if you live in a cgroup, you should display only the resources you can 
> possibly use. Global is for whoever is in the main cgroup.
> 
> Now, it comes two questions:
> 1) Do you agree with that, for files like /proc/stat ? I think the most 
> important part is to be consistent inside the system, regardless of what 
> is done

Personally I don't give a rats arse about (/proc vs) cgroups :-)
Currently /proc is unaffected by whatever cgroup you happen to be in and
that seems to make some sort of sense.

Namespaces seem to be about limiting visibility, cgroups about
controlling resources.

The two things are hopelessly disjoint atm, but I believe someone was
looking at this mess.

IOW a /proc namespace coupled to cgroup scope would do what you want.
Now my head hurts..

> 2) Will cpuacct stay? 

I really want to kill it. Balbir seems to want to retain a control-less
accounting capability, but I really don't see the point in that. Nor is
anybody selling it convincingly.

So I think I'll simply deprecate it and schedule it for removal and
anybody wanting something like this is free to send patches to implement
what they want in the cpu controller and convince me its worth the pain.

> I think if it does, that becomes almost mandatory 

You're referring to #1 here, right?

> (at least the bind mount idea is pretty much over here), because drawing 
> value for /proc/stat becomes quite complex.
> The cpuacct cgroup can provide user, sys, etc values. But we also have:
> 
> * nr_context_switches,
> * jiffies since boot,
> * total_forks,
> * nr_running,
> * nr_iowait,
> 
> Now I doubt any of us want to see /proc/stat extended to accommodate 
> things like nr_context_switches, or even worse, nr_running. The way I 
> see it, there are two options here:

Why would we want to display all those anyway?

>   a) moving everything to cpu cgroup so we keep all values being drawn
>      from the same place

This is /cgroup/$controller.stat, right?

>   b) Collect that info from multiple places in a transparent way. ctx,
>      nr_running and nr_iowait will probably come from cpu. jiffies can
>      come from wherever, and maybe we can even draw total_forks
>      from Frederic's and avoid counting it twice.

trouble with that is 'drawing from someplace' is a total nightmare, what
happens if that fork muck from Frederic isn't co-mounted with the cpu
controller?