Re: [PATCH tip/core/timers 2/4] timers: Reduce __run_timers() latency for empty list

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Jan 15, 2014 at 06:03:10PM +0100, Oleg Nesterov wrote:
> On 01/14, Paul E. McKenney wrote:
> >
> > The __run_timers() function currently steps through the list one jiffy at
> > a time
> 
> And this is very suboptimal if jiffies - timer_jiffies is huge. Looks
> like, we should rework base->tv* structures, or (perhaps) optimize
> the "cascade" logic so that __run_timers() can increment timer_jiffies
> and move all the expired timers into work_list at one step. And the
> ->next_timer logic is obviously very suboptimal.
> 
> But this is almost off-topic, I agree that in the short term these
> changes make sense.
> 
> > +static bool catchup_timer_jiffies(struct tvec_base *base)
> > +{
> > +#ifdef CONFIG_NO_HZ_FULL
> > +	if (!base->all_timers) {
> > +		base->timer_jiffies = jiffies;
> > +		return 1;
> > +	}
> > +#endif /* #ifdef CONFIG_NO_HZ_FULL */
> > +	return 0;
> > +}
> > +
> >  static void
> >  __internal_add_timer(struct tvec_base *base, struct timer_list *timer)
> >  {
> > @@ -1150,6 +1161,10 @@ static inline void __run_timers(struct tvec_base *base)
> >  	struct timer_list *timer;
> >  
> >  	spin_lock_irq(&base->lock);
> > +	if (catchup_timer_jiffies(base)) {
> > +		spin_unlock_irq(&base->lock);
> > +		return;
> > +	}
> 
> 
> This is really minor, but perhaps it would be better to modify
> run_timer_softirq() to call catchup_timer_jiffies() lockless along
> with another fast-path time_after_eq() check.

Given that this is at best a temporary solution, I would like to avoid
the complexity of this sort of optimization unless it turns out to be
a major performance issue.

> Better yet, it would be nice to avoid raise_softirq(TIMER_SOFTIRQ),
> but this is not simple due to hrtimer_run_pending().

And I do want to keep this pretty simple!

							Thanx, Paul