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

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Message-Id: <20090226172234.a931931f.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 26 Feb 2009 17:22:34 +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 16:17:31 +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, 

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

What is problem even if ca is NULL.

Thanks,
-Kame





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