linux-kernel - Re: [PATCH] cpuacct: add a branch prediction

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Message-ID: <18f6db017e5d44596e828e0753f28e75.squirrel@webmail-b.css.fujitsu.com>
Date:	Thu, 26 Feb 2009 19:28:45 +0900 (JST)
From:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>
To:	"Bharata B Rao" <bharata.rao@...il.com>
Cc:	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>,
	"Li Zefan" <lizf@...fujitsu.com>, "Ingo Molnar" <mingo@...e.hu>,
	"Peter Zijlstra" <a.p.zijlstra@...llo.nl>,
	"Paul Menage" <menage@...gle.com>,
	"Balbir Singh" <balbir@...ux.vnet.ibm.com>,
	"LKML" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpuacct: add a branch prediction

Bharata B Rao：
> On Thu, Feb 26, 2009 at 2:10 PM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@...fujitsu.com> wrote:
>> On Thu, 26 Feb 2009 16:35:33 +0800
>> Li Zefan <lizf@...fujitsu.com> wrote:
>>
>>> ca is not checked before hierarchy support, and it's a side-effect.
>>>
>>> Before cpuacct is initialized, css ==
>>> task->cgroups->subsys[cpuacct_subsys] == NULL,
>>> but ca = task_ca(tsk) is not necessarily NULL, unless struct
>>> cgroup_subsys_state is the
>>> first member of struct cpuacct.
>>>
>>> And the above code actually should be:
>>>
>>> do {
>>> &#160; &#160; &#160; ...
>>> } while (ca->parent);
>>>
>> I'll send no more objections to this patch itself.
>>
>> But IMHO, this loop is tooo bad, I think. The hierarchy information
>> should be gathered by
>> read-side and the total code should be
>
> How do you take care of accounting when a group gets deleted ?.
propagate accounts to the parent or root group if deleted.

> Wouldn't you loose some information due to deleted groups if you want
> to accumulate hierarchical stats only during read ?
>
Taking hierarchy mutex while reading will make read-side stable.
I haven't considered in detail but I wrote CSS ID and scan hierarchy
code to do this kind of "visit all css object under hierarchy" jobs.
I think you can catch what I mean by reading what memory.stat file of mem
cgroup does. (mmotm)

Thanks,
-Kame


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