Re: [PATCH] Add hierarchical accounting to cpu accountingcontroller

[KAMEZAWA Hiroyuki]

On Thu, 30 Oct 2008 22:46:22 +0530
Dhaval Giani <dhaval@xxxxxxxxxxxxxxxxxx> wrote:

> On Mon, Oct 27, 2008 at 04:25:01PM +0800, Li Zefan wrote:
> > >>>> So in technical terms this patch looks fine now. There's still the
> > >>>> question of whether it's OK to change the existing API, since it's
> > >>>> been in the kernel in its currently (non-hierarchical) form for
> > >>>> several releases now.
> > >> Hmm... Can we consider this as an API change ? Currently cpuacct.usage
> > >> readers of a parent accounting group are missing the usage contributions
> > >> from its children groups. I would consider this patch as fixing the
> > >> above problem by correctly reflecting the cpu usage for every accounting
> > >> group.
> > >>
> > > 
> > > If a particular application desires to derive the usage of its
> > > immediate tasks and does not care about subcgroups, it is a simple
> > > iteration (after this fix)
> > > 
> > > cpuacct - sigma(cpuacct_child)
> > > 
> > > and currently if we cared about child accounting, we could do
> > > 
> > > cpuacct + recursively(sigma(cpuacct_child))
> > > 
> > > In that sense this fix makes more sense, but like Paul said we need to
> > > figure out if it is an API change. My take is that it is a BUG fix,
> > > since we do care about child subgroups in accounting.
> > > 
> > 
> > cpuacct was designed to count cpu usage of a group of tasks, and now some people
> > want it to also take child group's usage into account, so I think this is a feature
> > request but not a bug fix.
> > 
> 
> I disagree. The child is a part of the parent's hierarchy, and therefore
> its usage should reflect in the parent's usage.
> 

In my point of view, there is no big difference. It's just whether we need a tool
in userland or not. If there is no performance impact, I have no objections.

One request from me is add Documentation/controllers/cpuacct.txt or some to explain
"what we see".

Thanks,
-Kame

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