linux-kernel - Re: [RFC] sched: make update_cpu_load

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Date:	Wed, 9 Sep 2015 15:00:58 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	mingo@...nel.org, peterz@...radead.org
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [RFC] sched: make update_cpu_load_active care more than one tick

On Tue, Aug 18, 2015 at 08:39:27AM +0900, Byungchul Park wrote:
> On Thu, Aug 13, 2015 at 06:35:24PM +0900, byungchul.park@....com wrote:
> > From: Byungchul Park <byungchul.park@....com>
> > 
> > i found do_timer accounts other than one tick, so i made
> > update_cpu_load_active care that.
> > 
> > is it intended because of its overhead?
> 
> hello,
> 
> is there anyone who can tell me any opinion about this concern?
> 
> > 
> > Signed-off-by: Byungchul Park <byungchul.park@....com>
> > ---
> >  kernel/sched/fair.c |    7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index ffa70dc..cd3d98f 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -4506,12 +4506,15 @@ void update_cpu_load_nohz(void)
> >   */
> >  void update_cpu_load_active(struct rq *this_rq)
> >  {
> > +	unsigned long curr_jiffies = READ_ONCE(jiffies);
> > +	unsigned long pending_updates;
> >  	unsigned long load = this_rq->cfs.runnable_load_avg;
> >  	/*
> >  	 * See the mess around update_idle_cpu_load() / update_cpu_load_nohz().
> >  	 */
> > -	this_rq->last_load_update_tick = jiffies;
> > -	__update_cpu_load(this_rq, load, 1);
> > +	pending_updates = curr_jiffies - this_rq->last_load_update_tick;

is it expensive to deal with this correction here?

in my QEMU machine, pending_updates appears to be more than 10, however
current code assumes it to be 1 unconditionally.

if this correction is valid, i will resend this with adding commit msg.

> > +	this_rq->last_load_update_tick = curr_jiffies;
> > +	__update_cpu_load(this_rq, load, pending_updates);
> >  }
> >  
> >  /* Used instead of source_load when we know the type == 0 */
> > -- 
> > 1.7.9.5
> > 
> > --
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