Re: [PATCH 12/12] cfq-iosched: add hierarchical cfq_group statistics

[Vivek Goyal <vgoyal@redhat.com>]

On Tue, Dec 18, 2012 at 11:14:25AM -0800, Tejun Heo wrote:
> Hello, Vivek.
> 
> On Tue, Dec 18, 2012 at 02:11:17PM -0500, Vivek Goyal wrote:
> > All these stats needs to be mentioned in blkio-controller.txt file to 
> > keep that file uptodate.
> > 
> > I think it also needs another word about nature of hierarchical stats.
> > That is they represent current view of the system and don't store the
> > history. So if a cgroup was created, did some IO and it was removed, we
> > lost that history. Deleted cgroup's parent will have no history of
> > stats of deleted cgroup.
> 
> Haven't thought about that.  That's nasty.
> 
> > Hence these stats can't be used for things like billing purposes.
> > 
> > IIRC, this is different from the way we collect hierarhical stats for
> > memory controller.
> > 
> > But I kind of like this because stat update overhead does not increase
> > with depth of hierarchy. Primarily stat reader pays the price of
> > traversing through all the stats.
> 
> Yeah, hmmm, maybe we should add another set of counters to carry stats
> from dead ones?  Avoiding hierarchical accounting overhead in hot path
> while remembering by-gones shouldn't be that hard.  Will work on that.

So are you planning to migrate the stats to parent when a cgroup is being
deleted?  That should make sure we don't do hierarhical update.

Thanks
Vivek