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

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Message-ID: <49A655B0.9020002@cn.fujitsu.com>
Date:	Thu, 26 Feb 2009 16:41:20 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	balbir@...ux.vnet.ibm.com
CC:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Menage <menage@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpuacct: add a branch prediction

Balbir Singh wrote:
> * Li Zefan <lizf@...fujitsu.com> [2009-02-26 15:40:15]:
> 
>> 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))
> 
> What happens if most distros make cpuacct_subsys active? Can we decide
> on unlikely depending on how the system is configured?
> 

Note, it's ->active but not ->disabled. ;)

And note, I make it "unlikely(!cpuacct_subsys.active)", but not "unlikely(cpuacct_subsys.active)"

->active can't be configured by users. If CONFIG_CGROUP_CPUACCT is enabled,
cpuacct_subsys.active will be set to true after cpuacct is initialized.

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