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

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Date:	Mon, 18 Apr 2016 15:35:06 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Byungchul Park <byungchul.park@....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 Mon, Apr 18, 2016 at 06:17:21PM +0900, Byungchul Park wrote:
> 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.

load[0] won't change because it's set by cpu_load_update_nohz_start().
But all the other load[idx] need to be decayed further.

> 
> 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.
> 

Thanks for your review!