Re: [RFC][-mm] Memory controller hierarchy support (v1)

[Balbir Singh <balbir@linux.vnet.ibm.com>]

Paul Menage wrote:
> On Fri, Apr 18, 2008 at 10:35 PM, Balbir Singh
> <balbir@linux.vnet.ibm.com> wrote:
>>  1. We need to hold cgroup_mutex while walking through the children
>>    in reclaim. We need to figure out the best way to do so. Should
>>    cgroups provide a helper function/macro for it?
> 
> There's already a function, cgroup_lock(). But it would be nice to
> avoid such a heavy locking here, particularly since memory allocations
> can occur with cgroup_mutex held, which could lead to a nasty deadlock
> if the allocation triggered reclaim.
> 

Hmm.. probably..

> One of the things that I've been considering was to put the
> parent/child/sibling hierarchy explicitly in cgroup_subsys_state. This
> would give subsystems their own copy to refer to, and could use their
> own internal locking to synchronize with callbacks from cgroups that
> might change the hierarchy. Cpusets could make use of this too, since
> it has to traverse hierarchies sometimes.
> 

Very cool! I look forward to that infrastructure. I'll also look at the cpuset
code and see how to traverse the hierarchy.

>>  2. Do not allow children to have a limit greater than their parents.
>>  3. Allow the user to select if hierarchial support is required
> 
> My thoughts on this would be:
> 
> 1) Never attach a first-level child's counter to its parent. As
> Yamamoto points out, otherwise we end up with extra global operations
> whenever any cgroup allocates or frees memory. Limiting the total
> system memory used by all user processes doesn't seem to be something
> that people are going to generally want to do, and if they really do
> want to they can just create a non-root child and move the whole
> system into that.
> 
> The one big advantage that you currently get from having all
> first-level children be attached to the root is that the reclaim logic
> automatically scans other groups when it reaches the top-level - but I
> think that can be provided as a special-case in the reclaim traversal,
> avoiding the overhead of hitting the root cgroup that we have in this
> patch.
> 

I've been doing some thinking along these lines, I'll think more about this.

> 2) Always attach other children's counters to their parents - if the
> user didn't want a hierarchy, they could create a flat grouping rather
> than nested groupings.
> 

Yes, that's a TODO

> Paul


-- 
	Warm Regards,
	Balbir Singh
	Linux Technology Center
	IBM, ISTL