Re: kernel/timer: avoid spurious ksoftirqd wakeups

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Apr 02, 2015 at 09:58:26AM -0400, Rik van Riel wrote:
> On 04/01/2015 09:44 PM, Marcelo Tosatti wrote:
> 
> > +++ 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;
> 
> You may want to initialize this to false. Nothing else
> in the code ever seems to set it to false.
> 
> It may work in your test due to that address on the stack
> already being zeroed out due to a lucky coincidence, but
> that is not a guarantee.
> 
> > --- a/kernel/time/timer.c
> > +++ b/kernel/time/timer.c
> > @@ -1343,7 +1343,7 @@ static unsigned long cmp_next_hrtimer_event(unsigned long now,
> >   * get_next_timer_interrupt - return the jiffy of the next pending timer
> >   * @now: current time (in jiffies)
> >   */
> > -unsigned long get_next_timer_interrupt(unsigned long now)
> > +unsigned long get_next_timer_interrupt(unsigned long now, bool *raise_softirq)
> >  {
> >  	struct tvec_base *base = __this_cpu_read(tvec_bases);
> >  	unsigned long expires = now + NEXT_TIMER_MAX_DELTA;
> > @@ -1357,6 +1357,7 @@ unsigned long get_next_timer_interrupt(unsigned long now)
> >  
> >  	spin_lock(&base->lock);
> >  	if (base->active_timers) {
> > +		*raise_softirq = true;
> >  		if (time_before_eq(base->next_timer, base->timer_jiffies))
> >  			base->next_timer = __next_timer_interrupt(base);
> >  		expires = base->next_timer;
> 
> Given that run_timer_softirq() only actually does something
> if the timer has expired, would it make sense to only raise
> the softirq after the timer has expired?

jiffies might be increased by tick_nohz_stop_sched_tick.

So you'd have to test again after increasing jiffies.