Date:	Wed, 8 Apr 2015 00:12:45 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Marcelo Tosatti <mtosatti@...hat.com>,
	linux-kernel@...r.kernel.org, Rik van Riel <riel@...hat.com>
Subject: Re: kernel/timer: avoid spurious ksoftirqd wakeups (v2)

On Tue, Apr 07, 2015 at 11:10:49PM +0200, Thomas Gleixner wrote:
> On Mon, 6 Apr 2015, Marcelo Tosatti wrote:
> > It is only necessary to raise timer softirq
> > in case there are active timers.
> 
> Depends. See below.
>  
> > Limit the ksoftirqd wakeup to that case.
> > 
> > Fixes a latency spike with isolated CPUs and
> > nohz full mode.
> 
> This lacks a proper explanation of the observed issue.
> 
> > --- a/kernel/time/tick-sched.c
> > +++ b/kernel/time/tick-sched.c
> > @@ -568,6 +568,7 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
> >  	unsigned long rcu_delta_jiffies;
> >  	struct clock_event_device *dev = __this_cpu_read(tick_cpu_device.evtdev);
> >  	u64 time_delta;
> > +	bool raise_softirq = false;
> 
> This shadows the function name raise_softirq(). Not pretty.
>   
> >  	time_delta = timekeeping_max_deferment();
> >  
> > @@ -584,7 +585,8 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
> >  		delta_jiffies = 1;
> >  	} else {
> >  		/* Get the next timer wheel timer */
> > -		next_jiffies = get_next_timer_interrupt(last_jiffies);
> > +		next_jiffies = get_next_timer_interrupt(last_jiffies,
> > +							&raise_softirq);
> >  		delta_jiffies = next_jiffies - last_jiffies;
> >  		if (rcu_delta_jiffies < delta_jiffies) {
> >  			next_jiffies = last_jiffies + rcu_delta_jiffies;
> > @@ -703,7 +705,8 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
> >  		 */
> >  		tick_do_update_jiffies64(ktime_get());
> >  	}
> > -	raise_softirq_irqoff(TIMER_SOFTIRQ);
> > +	if (raise_softirq)
> > +		raise_softirq_irqoff(TIMER_SOFTIRQ);
> 
> This breaks when high resolution timers are disabled (compile or
> runtime) because then the hrtimer queues are run from the timer
> softirq.
> 
> Now assume the following situation:
> 
>   Tick is stopped completely with no timers and no hrtimers pending.
> 
>   Interrupt happens and schedules a hrtimer.
> 
>   nohz_stop_sched_tick()
>     get_next_timer_interrupt(..., &raise_softirq);
> 
>       ---> base->active_timers = 0, so raise_softirq is false
> 
>     tick_program_event(expires)
>       clockevents_program_event(expires)
>       
>       ---> Assume expires is already in the past
> 
>         if (expires <= ktime_get())
> 	   return -ETIME;
> 
>     if (raise_softirq)
>        raise_softirq_irqoff(TIMER_SOFTIRQ);
> 
> So because the tick device was not armed you wont get a tick
> interrupt up to the point where tick_nohz_stop_sched_tick() is called
> again which might be far off.
> 
> I can see that the unconditional raise_softirq_irqoff() is suboptimal,
> but it was a rather simple solution to get stuff rolling again because
> it forces the cpu out of the inner idle loop which in turn restarts
> the tick.

Doh, that's the kind of side effect I was worried about, thanks for the
explanation. The necessary exit out of the idle loop implied by this
softirq when the timer fails to be programmed really deserves a comment.

And note how it relies on the magic !in_interrupt() in this piece of
hardirq code, otherwise that would be softirq from hardirq without
reschedule() and thus no exit from idle loop, and thus no tick
reprogramming.

Let's see if I can come up with some solution to clean this up, if
Marcelo doesn't beat me at it.

> 
> We certainly can do better, but that needs more thought than the
> proposed solution.
> 
> Thanks,
> 
> 	tglx
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