Re: [RFC] blk-cgroup: Allow creation of hierarchical cgroups

[Jens Axboe <jaxboe@fusionio.com>]

On 2010-11-15 16:28, Vivek Goyal wrote:
> On Tue, Nov 02, 2010 at 06:20:30PM -0400, Vivek Goyal wrote:
>> o Allow hierarchical cgroup creation for blkio controller
>>
>> o Currently we disallow it as both the io controller policies (throttling
>>   as well as proportion bandwidth) do not support hierarhical accounting
>>   and control. But the flip side is that blkio controller can not be used with
>>   libvirt as libvirt creates a cgroup hierarchy deeper than 1 level.
>>
>>   <top-level-cgroup-dir>/<controller>/libvirt/qemu/<virtual-machine-groups>
>>
>> o So this patch will allow creation of cgroup hierarhcy but at the backend
>>   everything will be treated as flat. So if somebody created a an hierarchy
>>   like as follows.
>>
>> 			root	
>> 			/  \
>> 		     test1 test2
>> 			|
>> 		     test3
>>
>>   CFQ and throttling will practically treat all groups at same level.
>> 			
>> 				pivot
>> 			     /  |   \  \
>> 			root  test1 test2  test3
>>
>> o Once we have actual support for hierarchical accounting and control
>>   then we can introduce another cgroup tunable file "blkio.use_hierarchy"
>>   which will be 0 by default but if user wants to enforce hierarhical
>>   control then it can be set to 1. This way there should not be any
>>   ABI problems down the line.
>>
>> o The only not so pretty part is introduction of extra file "use_hierarchy"
>>   down the line. Kame-san had mentioned that hierarhical accounting is
>>   expensive in memory controller hence they keep it off by default. I
>>   suspect same will be the case for IO controller also as for each IO
>>   completion we shall have to account IO through hierarchy up to the root..
>>   if yes, then it probably is not a very bad idea to introduce this extra
>>   file so that it will be used only when somebody needs it and some people
>>   might enable hierarchy only in part of the hierarchy. 
>>
>> o This is how basically memory controller also uses "use_hierarhcy" and
>>   they also allowed creation of hierarchies when actual backend support
>>   was not available.
>>
>> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
>> ---
> 
> Hi Jens,
> 
> Do you have any concerns about this patch? If not, can you please apply
> it.

Applied to for-2.6.38/rc2-holder, it'll be merged into for-2.6.38/core
once -rc2 has been tagged (and I can pull in the conflicting bits).

-- 
Jens Axboe