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

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Date:	Thu, 26 Feb 2009 16:35:33 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...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

KAMEZAWA Hiroyuki wrote:
> On Thu, 26 Feb 2009 16:17:31 +0800
> Li Zefan <lizf@...fujitsu.com> wrote:
> 
>> KAMEZAWA Hiroyuki wrote:
>>> On Thu, 26 Feb 2009 15:40:15 +0800
>>> Li Zefan <lizf@...fujitsu.com> wrote:
>>>
>>>> cpuacct_charge() is in fast-path, and checking of !cpuacct_susys.active
>>>> always returns false after cpuacct has been initialized at system boot.
>>>>
>>>> Signed-off-by: Li Zefan <lizf@...fujitsu.com>
>>>> ---
>>>>  kernel/sched.c |    2 +-
>>>>  1 files changed, 1 insertions(+), 1 deletions(-)
>>>>
>>>> diff --git a/kernel/sched.c b/kernel/sched.c
>>>> index 410eec4..fd2f7fc 100644
>>>> --- a/kernel/sched.c
>>>> +++ b/kernel/sched.c
>>>> @@ -9589,7 +9589,7 @@ static void cpuacct_charge(struct task_struct *tsk, u64 cputime)
>>>>  	struct cpuacct *ca;
>>>>  	int cpu;
>>>>  
>>>> -	if (!cpuacct_subsys.active)
>>>> +	if (unlikely(!cpuacct_subsys.active))
>>>>  		return;
>>>>  
>>> (Just curious)
>>> I wonder "ca = task_ca(tsk)" will return NULL if cpuacct subsys is not initalized.
>> Yes, it will be NULL, and that's why we need this check.
>>
>>> Then, can we just remove this check ?
>>>
>> cpuacct_charge() can be called before cpuacct is initialized, so we have to check this
>> case here.
>>
> My point is, 
> 

Ah, I see.

> ca = task_ca(tsk)
> for (; ca; ca->parent) {
>    ...
> }
> 

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);

> What is problem even if ca is NULL.
> 
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