linux-kernel - Re: [PATCH 2/3] sched: Correctly handle nohz ticks cpu load accounting

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Date:	Mon, 18 Apr 2016 17:22:41 +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>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 2/3] sched: Correctly handle nohz ticks cpu load
 accounting

On Wed, Apr 13, 2016 at 03:56:51PM +0200, Frederic Weisbecker wrote:
> @@ -4524,12 +4523,12 @@ decay_load_missed(unsigned long load, unsigned long missed_updates, int idx)
>   *   load[i]_n = (1 - 1/2^i)^n * load[i]_0
>   *
>   * see decay_load_misses(). For NOHZ_FULL we get to subtract and add the extra
> - * term. See the @active paramter.
> + * term.
>   */
> -static void __cpu_load_update(struct rq *this_rq, unsigned long this_load,
> -			      unsigned long pending_updates, int active)
> +static void cpu_load_update(struct rq *this_rq, unsigned long this_load,
> +			    unsigned long pending_updates)
>  {
> -	unsigned long tickless_load = active ? this_rq->cpu_load[0] : 0;
> +	unsigned long tickless_load = this_rq->cpu_load[0];

Hello,

Good for my humble code to be fixed so we can write it like this here.

> @@ -4618,26 +4617,56 @@ static void cpu_load_update_idle(struct rq *this_rq)
>  	if (weighted_cpuload(cpu_of(this_rq)))
>  		return;
>  
> -	__cpu_load_update_nohz(this_rq, READ_ONCE(jiffies), 0, 0);
> +	cpu_load_update_nohz(this_rq, READ_ONCE(jiffies), 0);
>  }
>  
>  /*
> - * Called from tick_nohz_idle_exit() -- try and fix up the ticks we missed.
> + * Record CPU load on nohz entry so we know the tickless load to account
> + * on nohz exit. cpu_load[0] happens then to be updated more frequently
> + * than other cpu_load[idx] but it should be fine as cpu_load readers
> + * shouldn't rely into synchronized cpu_load[*] updates.
>   */
> -void cpu_load_update_nohz(int active)
> +void cpu_load_update_nohz_start(void)
>  {
>  	struct rq *this_rq = this_rq();
> +
> +	/*
> +	 * This is all lockless but should be fine. If weighted_cpuload changes
> +	 * concurrently we'll exit nohz. And cpu_load write can race with
> +	 * cpu_load_update_idle() but both updater would be writing the same.
> +	 */
> +	this_rq->cpu_load[0] = weighted_cpuload(cpu_of(this_rq));

Like it.

> +/*
> + * Account the tickless load in the end of a nohz frame.
> + */
> +void cpu_load_update_nohz_stop(void)
> +{
>  	unsigned long curr_jiffies = READ_ONCE(jiffies);
> -	unsigned long load = active ? weighted_cpuload(cpu_of(this_rq)) : 0;
> +	struct rq *this_rq = this_rq();
> +	unsigned long load;
>  
>  	if (curr_jiffies == this_rq->last_load_update_tick)
>  		return;
>  
> +	load = weighted_cpuload(cpu_of(this_rq));

Like it.

> @@ -4645,11 +4674,11 @@ void cpu_load_update_nohz(int active)
>  void cpu_load_update_active(struct rq *this_rq)
>  {
>  	unsigned long load = weighted_cpuload(cpu_of(this_rq));
> -	/*
> -	 * See the mess around cpu_load_update_idle() / cpu_load_update_nohz().
> -	 */
> -	this_rq->last_load_update_tick = jiffies;
> -	__cpu_load_update(this_rq, load, 1, 1);
> +
> +	if (tick_nohz_tick_stopped())
> +		cpu_load_update_nohz(this_rq, READ_ONCE(jiffies), load);
> +	else
> +		cpu_load_update_periodic(this_rq, load);

Oh! We have missed it until now. Terrible.. :-(

Thank you,
Byungchul