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

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Message-ID: <49A6501B.7040604@cn.fujitsu.com>
Date:	Thu, 26 Feb 2009 16:17:31 +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 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.
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