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

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Message-ID: <20090226104008.GJ11421@balbir.in.ibm.com>
Date:	Thu, 26 Feb 2009 16:10:08 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.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>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpuacct: add a branch prediction

* Li Zefan <lizf@...fujitsu.com> [2009-02-26 16:41:20]:

> 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. ;)
>

I've seen that
 
> 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.
>

My point is the likely/unlikely in this case depends on turning on/off
CONFIG_CGROUP_CPUACCT? Am I missing something?


-- 
	Balbir
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