linux-kernel - Re: [PATCH] Add hierarchical accounting to cpu accounting controller

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Message-ID: <661de9470810262357y6c560facl87dcaea3ce35e3ac@mail.gmail.com>
Date:	Mon, 27 Oct 2008 12:27:44 +0530
From:	"Balbir Singh" <balbir@...ux.vnet.ibm.com>
To:	bharata@...ux.vnet.ibm.com
Cc:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>,
	"Paul Menage" <menage@...gle.com>, linux-kernel@...r.kernel.org,
	"Srivatsa Vaddagiri" <vatsa@...ux.vnet.ibm.com>,
	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>,
	"Ingo Molnar" <mingo@...e.hu>
Subject: Re: [PATCH] Add hierarchical accounting to cpu accounting controller

On Mon, Oct 27, 2008 at 10:13 AM, Bharata B Rao
<bharata@...ux.vnet.ibm.com> wrote:
> On Mon, Oct 27, 2008 at 10:17:03AM +0900, KAMEZAWA Hiroyuki wrote:
>> On Sat, 25 Oct 2008 08:38:52 -0700
>> "Paul Menage" <menage@...gle.com> wrote:
>>
>> > On Fri, Oct 24, 2008 at 11:01 PM, Bharata B Rao
>> > <bharata@...ux.vnet.ibm.com> wrote:
>> > >
>> > > Reported-by: Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>
>> > > Signed-off-by: Bharata B Rao <bharata@...ux.vnet.ibm.com>
>> >
>> > Reviewed-by: Paul Menage <menage@...gle.com>
>
> Thanks Paul.
>
>> >
>> > 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.


>> >
>> Hmm..how about having 2 params as "aggregated usage" and "private usage" ?
>>
>>  cpuacct.usage.
>>  cpuacct.all_subtree_usage.
>
> Is there really a need to differentiate between aggregated and private
> usage other than to maintain the current behaviour ?
>

That might be useful to have, but as above it can always be derived.

Balbir
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