linux-kernel - Re: [PATCH 2/4] sched: Consolidate nohz CPU load update code

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Date:	Thu, 14 Jan 2016 14:18:40 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Chris Metcalf <cmetcalf@...hip.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 2/4] sched: Consolidate nohz CPU load update code

On Wed, Jan 13, 2016 at 05:01:29PM +0100, Frederic Weisbecker wrote:
>  #ifdef CONFIG_NO_HZ_COMMON
> +static void __update_cpu_load_nohz(struct rq *this_rq,
> +				   unsigned long curr_jiffies,

Do we need to pass current jiffies as a function parameter?

> +				   unsigned long load,
> +				   int active)
> +{
> +	unsigned long pending_updates;
> +
> +	pending_updates = curr_jiffies - this_rq->last_load_update_tick;
> +	if (pending_updates) {
> +		this_rq->last_load_update_tick = curr_jiffies;
> +		/*
> +		 * In the regular NOHZ case, we were idle, this means load 0.
> +		 * In the NOHZ_FULL case, we were non-idle, we should consider
> +		 * its weighted load.
> +		 */
> +		__update_cpu_load(this_rq, load, pending_updates, active);
> +	}
> +}
> +
>  /*
>   * There is no sane way to deal with nohz on smp when using jiffies because the
>   * cpu doing the jiffies update might drift wrt the cpu doing the jiffy reading
> @@ -4467,22 +4486,15 @@ static unsigned long weighted_cpuload(const int cpu)
>   * Called from nohz_idle_balance() to update the load ratings before doing the
>   * idle balance.
>   */
> -static void update_idle_cpu_load(struct rq *this_rq)
> +static void update_cpu_load_idle(struct rq *this_rq)
>  {
> -	unsigned long curr_jiffies = READ_ONCE(jiffies);
> -	unsigned long load = weighted_cpuload(cpu_of(this_rq));
> -	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 (weighted_cpuload(cpu_of(this_rq)))
>  		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, 0);
> +	__update_cpu_load_nohz(this_rq, READ_ONCE(jiffies), 0, 0);

This question is not directly related to this patch but I am just
curious about... Should we use READ_ONCE on jiffies which is already
volatile type?