linux-kernel - Re: [patch 18/39] tick: sched: Force tick interrupt and get rid of softirq magic

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Message-ID: <20150423161412.GB22800@lerouge>
Date:	Thu, 23 Apr 2015 18:14:14 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Marcelo Tosatti <mtosatt@...hat.com>,
	John Stultz <john.stultz@...aro.org>,
	Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [patch 18/39] tick: sched: Force tick interrupt and get rid of
 softirq magic

On Thu, Apr 23, 2015 at 03:07:59PM +0200, Thomas Gleixner wrote:
> On Thu, 23 Apr 2015, Frederic Weisbecker wrote: 
> > On Wed, Apr 22, 2015 at 04:32:11PM +0200, Thomas Gleixner wrote:
> > > On Wed, 22 Apr 2015, Frederic Weisbecker wrote:
> > > > But the reprogramming happens only under "if ((long)delta_jiffies >= 1)".
> > > > Probably this condition should go away as well.
> > > 
> > > Errm.
> > > 
> > > 	if (!ts->tick_stopped && delta_jiffies <= 1)
> > > 	     goto out;
> > > 
> > > So if the tick is NOT stopped and delta_jiffies <= 1 we let it tick
> > > and do nothing.
> > > 
> > >         if (delta_jiffies >= 1)
> > > 	     Do the magic nohz stuff
> > > 	else
> > > 	     tick_nohz_restart()
> > > 
> > > We want the distinction here because if the tick IS stopped and the
> > > next event is due we need to kick it into gear again. So the condition
> > > needs to stay. It probably should be if (delta > 1), but that's a
> > > different story.
> > 
> > Yes but what if the tick is stopped already and delta_jiffies < 1? Say the
> > tick was last programmed to fire in 5 seconds. An irq fires and enqueues
> > a timer to fire now. If it's soon enough that delta_jiffies < 1, it seems
> > we are missing the clock reprogramming and even the softirq from that irq exit.
> > 
> > Because we have:
> > 
> >       if (!ts->tick_stopped && delta_jiffies <= 1)
> >           goto out;
> > 
> >       if ((long)delta_jiffies >= 1) {
> >           //do clock reprogramming or restart
> >           ...
> >       }
> > 
> > -     raise_softirq(TIMER_SOFTIRQ);
> 
> Did you actually read what I wrote?
> 
> > >     if (delta_jiffies >= 1)
> > > 	     Do the magic nohz stuff
> > > 	else
> > > 	     tick_nohz_restart()
> 
> So if ticked is stopped AND delta < 1 we call
> tick_nohz_restart(). Which will programm the tick for the next
> possible period to expire. Which will kick the timer softirq and
> expire the timer.

Right, I missed the "else" that moved.
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