linux-kernel - Re: [PATCH 2/4] sched: Correctly handle nohz ticks cpu load accounting

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Message-ID: <20160402123205.GB27395@lerouge>
Date:	Sat, 2 Apr 2016 14:32:07 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Byungchul Park <byungchul.park@....com>,
	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 2/4] sched: Correctly handle nohz ticks cpu load
 accounting

On Sat, Apr 02, 2016 at 09:15:20AM +0200, Peter Zijlstra wrote:
> On Fri, Apr 01, 2016 at 03:23:05PM +0200, Frederic Weisbecker wrote:
> > Ticks can happen in the middle of a nohz frame and
> 
> I'm still miffed with that.. And this changelog doesn't even explain why
> and how.

Indeed, it looks like I've cooked sloppy changelogs in this series, I'll
do another pass on all of them.

> 
> > cpu_load_update_active() doesn't handle these correctly. It forgets the
> > whole previous tickless load and just records the current tick, ignoring
> > potentially long idle periods.
> > 
> > In order to solve this, record the load on nohz frame entry so we know
> > what to record in case of nohz interruptions, then use this recorded load
> > to account the tickless load on nohz ticks and nohz frame end.
> 
> 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index f33764d..394f008 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -4527,9 +4527,9 @@ decay_load_missed(unsigned long load, unsigned long missed_updates, int idx)
> >   * term. See the @active paramter.
> 
> ^^^^^^^^^^^^^^
> 
> What active parameter... you need to update that comment.

Yeah, forgot that.

Thanks.