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

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Date:	Mon, 18 Apr 2016 18:17:21 +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:
> @@ -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);

Considering it further, I wonder if needing it.
(Sorry if I missed something.)

Case 1. tickless -> (scheduler_tick) -> tickless

	I am not sure for this case if the rq's load can be changed or not,
	especially, if the rq's load can be changed *at this point*.
	Please remind that the load[0] is set here.

Case 2. tickless -> (scheduler_tick) -> restart tick

	Will be done by the tick restart routine when exiting irq.
	-> no problem.

Case 3. tick -> (scheduler_tick) -> tickless

	Same as before.
	-> no problem.

Case 4. tick -> (scheduler_tick) -> tick

	We can rely on regular schedule_tick().
	-> no problem.