linux-kernel - Re: [PATCH 1/4] sched: Don't account tickless CPU load on tick

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Date:	Tue, 19 Jan 2016 19:56:47 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
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>
Subject: Re: [PATCH 1/4] sched: Don't account tickless CPU load on tick

On Tue, Jan 19, 2016 at 05:22:11PM +0100, Frederic Weisbecker wrote:
> On Tue, Jan 19, 2016 at 02:08:57PM +0100, Peter Zijlstra wrote:
> > On Wed, Jan 13, 2016 at 05:01:28PM +0100, Frederic Weisbecker wrote:
> > > The cpu load update on tick doesn't care about dynticks and as such is
> > > buggy when occuring on nohz ticks (including idle ticks) as it resets
> > > the jiffies snapshot that was recorded on nohz entry. We eventually
> > > ignore the potentially long tickless load that happened before the
> > > tick.
> > 
> > I don't get it, how can we call scheduler_tick() while
> > tick_nohz_tick_stopped() ?
> 
> tick_nohz_tick_stopped() (which is ts->tick_stopped == 1) doesn't actually
> mean that the tick is really stopped. It just means that the tick fires only
> when it's really needed (timer list expired, RCU stuff, irq_work, ...).

That's insane and broken. Fix _that_.

If RCU, irq_work etc.. needs the tick, do not stop the tick.