static int menu_select()

in governors/menu.c [265:426]

• 93 lines of code
• 32 McCabe index (conditional complexity)

static int menu_select(struct cpuidle_driver *drv, struct cpuidle_device *dev,
		       bool *stop_tick)
{
	struct menu_device *data = this_cpu_ptr(&menu_devices);
	s64 latency_req = cpuidle_governor_latency_req(dev->cpu);
	unsigned int predicted_us;
	u64 predicted_ns;
	u64 interactivity_req;
	unsigned int nr_iowaiters;
	ktime_t delta, delta_tick;
	int i, idx;

	if (data->needs_update) {
		menu_update(drv, dev);
		data->needs_update = 0;
	}

	/* determine the expected residency time, round up */
	delta = tick_nohz_get_sleep_length(&delta_tick);
	if (unlikely(delta < 0)) {
		delta = 0;
		delta_tick = 0;
	}
	data->next_timer_ns = delta;

	nr_iowaiters = nr_iowait_cpu(dev->cpu);
	data->bucket = which_bucket(data->next_timer_ns, nr_iowaiters);

	if (unlikely(drv->state_count <= 1 || latency_req == 0) ||
	    ((data->next_timer_ns < drv->states[1].target_residency_ns ||
	      latency_req < drv->states[1].exit_latency_ns) &&
	     !dev->states_usage[0].disable)) {
		/*
		 * In this case state[0] will be used no matter what, so return
		 * it right away and keep the tick running if state[0] is a
		 * polling one.
		 */
		*stop_tick = !(drv->states[0].flags & CPUIDLE_FLAG_POLLING);
		return 0;
	}

	/* Round up the result for half microseconds. */
	predicted_us = div_u64(data->next_timer_ns *
			       data->correction_factor[data->bucket] +
			       (RESOLUTION * DECAY * NSEC_PER_USEC) / 2,
			       RESOLUTION * DECAY * NSEC_PER_USEC);
	/* Use the lowest expected idle interval to pick the idle state. */
	predicted_ns = (u64)min(predicted_us,
				get_typical_interval(data, predicted_us)) *
				NSEC_PER_USEC;

	if (tick_nohz_tick_stopped()) {
		/*
		 * If the tick is already stopped, the cost of possible short
		 * idle duration misprediction is much higher, because the CPU
		 * may be stuck in a shallow idle state for a long time as a
		 * result of it.  In that case say we might mispredict and use
		 * the known time till the closest timer event for the idle
		 * state selection.
		 */
		if (predicted_ns < TICK_NSEC)
			predicted_ns = data->next_timer_ns;
	} else {
		/*
		 * Use the performance multiplier and the user-configurable
		 * latency_req to determine the maximum exit latency.
		 */
		interactivity_req = div64_u64(predicted_ns,
					      performance_multiplier(nr_iowaiters));
		if (latency_req > interactivity_req)
			latency_req = interactivity_req;
	}

	/*
	 * Find the idle state with the lowest power while satisfying
	 * our constraints.
	 */
	idx = -1;
	for (i = 0; i < drv->state_count; i++) {
		struct cpuidle_state *s = &drv->states[i];

		if (dev->states_usage[i].disable)
			continue;

		if (idx == -1)
			idx = i; /* first enabled state */

		if (s->target_residency_ns > predicted_ns) {
			/*
			 * Use a physical idle state, not busy polling, unless
			 * a timer is going to trigger soon enough.
			 */
			if ((drv->states[idx].flags & CPUIDLE_FLAG_POLLING) &&
			    s->exit_latency_ns <= latency_req &&
			    s->target_residency_ns <= data->next_timer_ns) {
				predicted_ns = s->target_residency_ns;
				idx = i;
				break;
			}
			if (predicted_ns < TICK_NSEC)
				break;

			if (!tick_nohz_tick_stopped()) {
				/*
				 * If the state selected so far is shallow,
				 * waking up early won't hurt, so retain the
				 * tick in that case and let the governor run
				 * again in the next iteration of the loop.
				 */
				predicted_ns = drv->states[idx].target_residency_ns;
				break;
			}

			/*
			 * If the state selected so far is shallow and this
			 * state's target residency matches the time till the
			 * closest timer event, select this one to avoid getting
			 * stuck in the shallow one for too long.
			 */
			if (drv->states[idx].target_residency_ns < TICK_NSEC &&
			    s->target_residency_ns <= delta_tick)
				idx = i;

			return idx;
		}
		if (s->exit_latency_ns > latency_req)
			break;

		idx = i;
	}

	if (idx == -1)
		idx = 0; /* No states enabled. Must use 0. */

	/*
	 * Don't stop the tick if the selected state is a polling one or if the
	 * expected idle duration is shorter than the tick period length.
	 */
	if (((drv->states[idx].flags & CPUIDLE_FLAG_POLLING) ||
	     predicted_ns < TICK_NSEC) && !tick_nohz_tick_stopped()) {
		*stop_tick = false;

		if (idx > 0 && drv->states[idx].target_residency_ns > delta_tick) {
			/*
			 * The tick is not going to be stopped and the target
			 * residency of the state to be returned is not within
			 * the time until the next timer event including the
			 * tick, so try to correct that.
			 */
			for (i = idx - 1; i >= 0; i--) {
				if (dev->states_usage[i].disable)
					continue;

				idx = i;
				if (drv->states[i].target_residency_ns <= delta_tick)
					break;
			}
		}
	}

	return idx;
}