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

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Message-ID: <20150423114704.GA22800@lerouge>
Date:	Thu, 23 Apr 2015 13:47:06 +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 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);

 out:
    ...

>  
> > In the end, the possible side effect, at least on low-res, is that timers
> > which are already expired will be handled on the next tick instead of now.
> > But probably it doesn't matter much to have a one-tick delay.
> 
> We really don't care. That stuff has no guarantees aside of the
> guarantee that it does not expire early :)

Good :)

> 
> Thanks,
> 
> 	tglx
> 
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