linux-kernel - Re: [RESEND PATCH] sched: consider missed ticks when updating global cpu load

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Date:	Wed, 30 Sep 2015 12:43:43 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	byungchul.park@....com, mingo@...nel.org,
	linux-kernel@...r.kernel.org, tglx@...utronix.de
Subject: Re: [RESEND PATCH] sched: consider missed ticks when updating global
 cpu load

On Sat, Sep 26, 2015 at 03:14:45PM +0200, Frederic Weisbecker wrote:

> > when the next tick occurs, update_process_times() -> scheduler_tick()
> > -> update_cpu_load_active() is performed, assuming the distance between
> > last tick and current tick is 1 tick! it's wrong in this case. thus,
> > this abnormal case should be considered in update_cpu_load_active().
> > 
> > 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 4d5f97b..829282f 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -4356,12 +4356,15 @@ 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 = weighted_cpuload(cpu_of(this_rq));
> >  	/*
> >  	 * 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;
> > +	this_rq->last_load_update_tick = curr_jiffies;
> > +	__update_cpu_load(this_rq, load, pending_updates);
> >  }
> 
> That's right but __update_cpu_load() doesn't handle correctly pending updates
> with non-zero loads. Currently, pending updates are wheeled through decay_load_missed()
> that assume it's all about idle load.
> 
> But in the cases you've enumerated, as well as in the nohz full case, missed pending
> updates can be about buzy loads.
> 
> I think we need to fix update_cpu_load() to handle that first, or your fix is
> going to make things worse.

Its worse than that, the whole call chain of update_process_times()
fully assumes a single tick, fixing just the one function deep down to
handle more than 1 tick is ass backwards.
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