Re: [RFC REPOST] cgroup: removing css reference drain wait during cgroup removal

[Glauber Costa <glommer@parallels.com>]

On 03/15/2012 04:16 AM, KAMEZAWA Hiroyuki wrote:
> (2012/03/14 18:46), Glauber Costa wrote:
>
>> On 03/14/2012 04:28 AM, KAMEZAWA Hiroyuki wrote:
>>> IIUC, in general, even in the processes are in a tree, in major case
>>> of servers, their workloads are independent.
>>> I think FLAT mode is the dafault. 'heararchical' is a crazy thing which
>>> cannot be managed.
>>
>> Better pay attention to the current overall cgroups discussions being
>> held by Tejun then. ([RFD] cgroup: about multiple hierarchies)
>>
>> The topic of whether of adapting all cgroups to be hierarchical by
>> deafult is a recurring one.
>>
>> I personally think that it is not unachievable to make res_counters
>> cheaper, therefore making this less of a problem.
>>
>
>
> I thought of this a little yesterday. Current my idea is applying following
> rule for res_counter.
>
> 1. All res_counter is hierarchical. But behavior should be optimized.
>
> 2. If parent res_counter has UNLIMITED limit, 'usage' will not be propagated
>    to its parent at _charge_.

That doesn't seem to make much sense. If you are unlimited, but your 
parent is limited,
he has a lot more interest to know about the charge than you do. So the 
logic should rather be the opposite: Don't go around getting locks and 
all that if you are unlimited. Your parent might, though.

I am trying to experiment a bit with billing to percpu counters for 
unlimited res_counters. But their inexact nature is giving me quite a 
headache.

> 3. If a res_counter has UNLIMITED limit, at reading usage, it must visit
>     all children and returns a sum of them.
>
> Then,
> 	/cgroup/
> 		memory/                       (unlimited)
> 			libivirt/             (unlimited)
> 				 qeumu/       (unlimited)
> 				        guest/(limited)
>
> All dir can show hierarchical usage and the guest will not have
> any lock contention at runtime.

If we are okay with summing it up at read time, we may as well
keep everything in percpu counters at all times.
>
> By this
>   1. no runtime overhead if the parent has unlimited limit.
>   2. All res_counter can show aggregate resource usage of children.
>
> To do this
>   1. res_coutner should have children list by itself.
>
> Implementation problem
>   - What should happens when a user set new limit to a res_counter which have
>     childrens ? Shouldn't we allow it ? Or take all locks of children and
>     update in atomic ?
Well, increasing the limit should be always possible.

As for the kids, how about:

- ) Take their locks
- ) scan through them seeing if their usage is bellow the new allowance
     -) if it is, then ok
     -) if it is not, then try to reclaim (*). Fail if it is not possible.

(*) May be hard to implement, because we already have the res_counter 
lock taken, and the code may get nasty. So maybe it is better just fail 
if any of your kids usage is over the new allowance...



>   - memory.use_hierarchy should be obsolete ?
If we're going fully hierarchical, yes.

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