linux-kernel - Re: [PATCH v4 6/7] sched: add function nr_running_cpu to expose number of tasks running on cpu

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Date:	Sat, 12 Jul 2014 13:25:42 +0400
From:	Kirill Tkhai <tkhai@...dex.ru>
To:	Tim Chen <tim.c.chen@...ux.intel.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	"H. Peter Anvin" <hpa@...or.com>,
	"David S.Miller" <davem@...emloft.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
CC:	Chandramouli Narayanan <mouli@...ux.intel.com>,
	Vinodh Gopal <vinodh.gopal@...el.com>,
	James Guilford <james.guilford@...el.com>,
	Wajdi Feghali <wajdi.k.feghali@...el.com>,
	Jussi Kivilinna <jussi.kivilinna@....fi>,
	linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 6/7] sched: add function nr_running_cpu to expose number
 of tasks running on cpu

On 12.07.2014 00:33, Tim Chen wrote:
> This function will help a thread decide if it wants to to do work
> that can be delayed, to accumulate more tasks for more efficient
> batch processing later.
> 
> However, if no other tasks are running on the cpu, it can take
> advantgae of the available cpu cycles to complete the tasks
> for immediate processing to minimize delay, otherwise it will yield.
> 
> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> ---
>  include/linux/sched.h | 1 +
>  kernel/sched/core.c   | 8 ++++++++
>  2 files changed, 9 insertions(+)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 7cb07fd..0884250 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -168,6 +168,7 @@ extern int nr_threads;
>  DECLARE_PER_CPU(unsigned long, process_counts);
>  extern int nr_processes(void);
>  extern unsigned long nr_running(void);
> +extern unsigned long nr_running_cpu(int cpu);
>  extern unsigned long nr_iowait(void);
>  extern unsigned long nr_iowait_cpu(int cpu);
>  extern unsigned long this_cpu_load(void);
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 9cae286..d5bb8e6 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2283,6 +2283,14 @@ unsigned long nr_running(void)
>  	return sum;
>  }
>  
> +unsigned long nr_running_cpu(int cpu)
> +{
> +	if (cpumask_test_cpu(cpu, cpu_online_mask))
> +		return cpu_rq(cpu)->nr_running;
> +	else
> +		return 0;
> +}
> +

Offline cpu should have nr_running equal to 0. We park last
enqueued thread (migration_thread) at the end of take_cpu_down().

So, it's enough to return cpu_rq(cpu)->nr_running.

>  unsigned long long nr_context_switches(void)
>  {
>  	int i;
> 

Thanks,
Kirill
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