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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