linux-kernel - Re: [PATCH] cpuacct: reduce one NULL check in fast-path

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Date:	Wed, 11 Mar 2009 10:47:44 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Menage <menage@...gle.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpuacct: reduce one NULL check in fast-path


* Li Zefan <lizf@...fujitsu.com> wrote:

> In cpuacct_charge(), task_ca() will never return NULL, so change
> for(...) to do{}while(...) to save one NULL check.
> 
> Signed-off-by: Li Zefan <lizf@...fujitsu.com>
> ---
>  kernel/sched.c |    5 +++--
>  1 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 9bb6d24..bc8a91e 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -9768,10 +9768,11 @@ static void cpuacct_charge(struct task_struct *tsk, u64 cputime)
>  	cpu = task_cpu(tsk);
>  	ca = task_ca(tsk);
>  
> -	for (; ca; ca = ca->parent) {
> +	do {
>  		u64 *cpuusage = percpu_ptr(ca->cpuusage, cpu);
>  		*cpuusage += cputime;
> -	}
> +		ca = ca->parent;
> +	} while (ca);

This patch does not apply cleanly to the latest scheduler tree:

 http://people.redhat.com/mingo/tip.git/README

	Ingo
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