linux-kernel - Re: [RFC PATCH 2/4] sched: Consolidate nohz cpu load prelude code

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Date:	Thu, 20 Jun 2013 14:01:07 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Li Zhong <zhong@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...en8.de>, Alex Shi <alex.shi@...el.com>,
	Paul Turner <pjt@...gle.com>, Mike Galbraith <efault@....de>,
	Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [RFC PATCH 2/4] sched: Consolidate nohz cpu load prelude code

On Thu, Jun 20, 2013 at 10:45:39PM +0200, Frederic Weisbecker wrote:
> Gather the common code that computes the pending idle cpu load
> to decay.
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Li Zhong <zhong@...ux.vnet.ibm.com>
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Borislav Petkov <bp@...en8.de>
> Cc: Alex Shi <alex.shi@...el.com>
> Cc: Paul Turner <pjt@...gle.com>
> Cc: Mike Galbraith <efault@....de>
> Cc: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>  kernel/sched/proc.c |   40 ++++++++++++++--------------------------
>  1 files changed, 14 insertions(+), 26 deletions(-)
> 
> diff --git a/kernel/sched/proc.c b/kernel/sched/proc.c
> index bb3a6a0..030528a 100644
> --- a/kernel/sched/proc.c
> +++ b/kernel/sched/proc.c
> @@ -470,11 +470,14 @@ decay_load_missed(unsigned long load, unsigned long missed_updates, int idx)
>   * scheduler tick (TICK_NSEC). With tickless idle this will not be called
>   * every tick. We fix it up based on jiffies.
>   */
> -static void __update_cpu_load(struct rq *this_rq, unsigned long this_load,
> -			      unsigned long pending_updates)
> +static void __update_cpu_load(struct rq *this_rq, unsigned long this_load)
>  {
> +	unsigned long curr_jiffies = ACCESS_ONCE(jiffies);

Isn't jiffies declared volatile?  (Looks that way to me.)  If so, there
is no need for ACCESS_ONCE().

> +	unsigned long pending_updates;
>  	int i, scale;
> 
> +	pending_updates = curr_jiffies - this_rq->last_load_update_tick;
> +	this_rq->last_load_update_tick = curr_jiffies;
>  	this_rq->nr_load_updates++;
> 
>  	/* Update our load: */
> @@ -521,20 +524,15 @@ static void __update_cpu_load(struct rq *this_rq, unsigned long this_load,
>   */
>  void update_idle_cpu_load(struct rq *this_rq)
>  {
> -	unsigned long curr_jiffies = ACCESS_ONCE(jiffies);
>  	unsigned long load = this_rq->load.weight;
> -	unsigned long pending_updates;
> 
>  	/*
>  	 * bail if there's load or we're actually up-to-date.
>  	 */
> -	if (load || curr_jiffies == this_rq->last_load_update_tick)
> +	if (load || jiffies == this_rq->last_load_update_tick)
>  		return;
> 
> -	pending_updates = curr_jiffies - this_rq->last_load_update_tick;
> -	this_rq->last_load_update_tick = curr_jiffies;
> -
> -	__update_cpu_load(this_rq, load, pending_updates);
> +	__update_cpu_load(this_rq, load);
>  }
> 
>  /*
> @@ -543,22 +541,16 @@ void update_idle_cpu_load(struct rq *this_rq)
>  void update_cpu_load_nohz(void)
>  {
>  	struct rq *this_rq = this_rq();
> -	unsigned long curr_jiffies = ACCESS_ONCE(jiffies);
> -	unsigned long pending_updates;
> 
> -	if (curr_jiffies == this_rq->last_load_update_tick)
> +	if (jiffies == this_rq->last_load_update_tick)
>  		return;
> 
>  	raw_spin_lock(&this_rq->lock);
> -	pending_updates = curr_jiffies - this_rq->last_load_update_tick;
> -	if (pending_updates) {
> -		this_rq->last_load_update_tick = curr_jiffies;
> -		/*
> -		 * We were idle, this means load 0, the current load might be
> -		 * !0 due to remote wakeups and the sort.
> -		 */
> -		__update_cpu_load(this_rq, 0, pending_updates);
> -	}
> +	/*
> +	 * We were idle, this means load 0, the current load might be
> +	 * !0 due to remote wakeups and the sort.
> +	 */
> +	__update_cpu_load(this_rq, 0);
>  	raw_spin_unlock(&this_rq->lock);
>  }
>  #endif /* CONFIG_NO_HZ */
> @@ -568,11 +560,7 @@ void update_cpu_load_nohz(void)
>   */
>  void update_cpu_load_active(struct rq *this_rq)
>  {
> -	/*
> -	 * See the mess around update_idle_cpu_load() / update_cpu_load_nohz().
> -	 */
> -	this_rq->last_load_update_tick = jiffies;
> -	__update_cpu_load(this_rq, this_rq->load.weight, 1);
> +	__update_cpu_load(this_rq, this_rq->load.weight);
> 
>  	calc_load_account_active(this_rq);
>  }
> -- 
> 1.7.5.4
> 

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