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

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Message-Id: <20090226174033.094e4834.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 26 Feb 2009 17:40:33 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	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

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 {
> 	...
> } 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

void charge_statistics(tsk, cputime)
{
   ca = task_ca(tsk)
   /* ca can be null while init */
   if (likely(ca)) {
     u64 *cpuusage = percpu_ptr(ca->cpuusage, task_cpu(tsk))
     *cpuusage += cputime
   }
}
(Need changes to read-side and create/destroy of cpuacct cgroups.)

Thanks,
-Kame

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