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

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Date:	Thu, 23 Apr 2015 15:07:59 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Frederic Weisbecker <fweisbec@...il.com>
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, 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.

Thanks,

	tglx


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