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SubjectRe: kernel/timer: avoid spurious ksoftirqd wakeups (v2)
On Wed, Apr 08, 2015 at 12:12:45AM +0200, Frederic Weisbecker wrote:
> 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.

The problem is the following from -RT:

#ifdef CONFIG_PREEMPT_RT_BASE
if (!hrtimer_rt_defer(timer))
return -ETIME;
#endif

It seems a valid solution for this interrupt is to program
sched_timer to the nearest future possible.

if (expires < now)
expires = now + safe_margin;

program_timer(expires);

(perhaps a for loop increasing safe_margin if program_timer fails...)

Is that what you mean by clean up, Frederic?



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