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

[Oleg Nesterov <oleg@redhat.com>]

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.

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

Oleg.