Re: [PATCH v7 19/21] timer: Implement the hierarchical pull model

[Frederic Weisbecker]

Le Wed, May 24, 2023 at 09:06:27AM +0200, Anna-Maria Behnsen a écrit :
> +u64 tmigr_cpu_deactivate(u64 nextexp)
> +{
> +	struct tmigr_cpu *tmc = this_cpu_ptr(&tmigr_cpu);
> +	u64 ret;
> +
> +	if (!is_tmigr_enabled() || !tmc->tmgroup || !tmc->online)
> +		return nextexp;
> +
> +	raw_spin_lock(&tmc->lock);
> +
> +	/*
> +	 * CPU is already deactivated in timer migration
> +	 * hierarchy. tick_nohz_get_sleep_length() calls
> +	 * tick_nohz_next_event() and thereby timer idle path is
> +	 * executed once more. tmc->wakeup holds the first timer, when
> +	 * timer migration hierarchy is completely idle and remote
> +	 * expiry was done.

This can also hold the first timer in the hierarchy on nodes
having no true migrator, but still with upper nodes having potentially
a migrator, right??

> +	 * If there is no new next expiry value
> +	 * handed in which should be inserted into the timer migration
> +	 * hierarchy, wakeup value is returned.
> +	 */
> +	if (tmc->idle) {
> +		ret = tmc->wakeup;
> +
> +		tmc->wakeup = KTIME_MAX;
> +
> +		if (nextexp != KTIME_MAX) {

This seem to assume that tick_nohz_next_event() on the last idle CPU in an idle
hierarchy is always called right after a timer interrupt arriving on time
(ie: right after a call to tmigr_handle_remote()), but this can actually be called
after any interrupt. Can the following happen or am I overlooking something?

do_idle() {
	// ===> <IRQ>
	tmigr_handle_remote() {
		// find some timer in the hierarchy
		// expiring in 2 jiffies
		tmc->wakeup = jiffies + 2
	}
	//<=== </IRQ>
	tick_nohz_get_sleep_length() {
		get_next_timer_interrupt() {
			tmigr_cpu_deactivate() {
				wakeup = tmc->wakeup
				tmc->wakeup = KTIME_MAX
				return wakeup // jiffies + 2
			}
		}
	}
	tick_nohz_idle_stop_tick() {
		// SLEEP 2 jiffies
	}

	// ===> <IRQ>
	// whatever IRQ that is not timer
	// or a timer IRQ firing too early
	//<=== </IRQ>
	
	tick_nohz_get_sleep_length() {
		get_next_timer_interrupt() {
			tmigr_cpu_deactivate() {
				wakeup = tmc->wakeup
				 tmc->wakeup = KTIME_MAX
				return wakeup // KTIME_MAX
			}
		}
	}
	tick_nohz_idle_stop_tick() {
		// SLEEP forever, misses the timer in 2 jiffies
	}
}

Thanks.