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SubjectRe: [RFC/RFT][PATCH v5] cpuidle: New timer events oriented governor for tickless systems
On Thu, Nov 08, 2018 at 06:25:07PM +0100, Rafael J. Wysocki wrote:
> +unsigned int teo_idle_duration(struct cpuidle_driver *drv,
> + struct teo_cpu *cpu_data,
> + unsigned int sleep_length_us)
> +{
> + u64 range, max_spread, sum, max, min;
> + unsigned int i, count;
> +
> + /*
> + * If the sleep length is below the target residency of idle state 1,
> + * the only viable choice is to select the first available (enabled)
> + * idle state, so return immediately in that case.
> + */
> + if (sleep_length_us < drv->states[1].target_residency)
> + return sleep_length_us;
> +
> + /*
> + * The purpose of this function is to check if there is a pattern of
> + * wakeups indicating that it would be better to select a state
> + * shallower than the deepest one matching the sleep length or the
> + * deepest one at all if the sleep lenght is long. Larger idle duration
> + * values are beyond the interesting range.
> + */
> + range = drv->states[drv->state_count-1].target_residency;
> + range = min_t(u64, sleep_length_us, range + (range >> 2));
> +
> + /*
> + * This is the value to compare with the distance between the average
> + * and the greatest sample to decide whether or not it is small enough.
> + * Take 10 us as the total cap of it.
> + */
> + max_spread = max_t(u64, range >> MAX_SPREAD_SHIFT, 10);
> +
> + /*
> + * First pass: compute the sum of interesting samples, the minimum and
> + * maximum of them and count them.
> + */
> + count = 0;
> + sum = 0;
> + max = 0;
> + min = UINT_MAX;
> +
> + for (i = 0; i < INTERVALS; i++) {
> + u64 val = cpu_data->intervals[i];
> +
> + if (val >= range)
> + continue;
> +
> + count++;
> + sum += val;
> + if (max < val)
> + max = val;
> +
> + if (min > val)
> + min = val;
> + }
> +
> + /* Give up if the number of interesting samples is too small. */
> + if (count <= INTERVALS / 2)
> + return sleep_length_us;
> +
> + /*
> + * If the distance between the max or min and the average is too large,
> + * try to refine by discarding the max, as long as the count is above 3.
> + */
> + while (count > 3 && max > max_spread &&
> + ((max - max_spread) * count > sum ||
> + (min + max_spread) * count < sum)) {
> +
> + range = max;
> +
> + /*
> + * Compute the sum of samples in the interesting range. Count
> + * them and find the maximum of them.
> + */
> + count = 0;
> + sum = 0;
> + max = 0;
> +
> + for (i = 0; i < INTERVALS; i++) {
> + u64 val = cpu_data->intervals[i];
> +
> + if (val >= range)
> + continue;
> +
> + count++;
> + sum += val;
> + if (max < val)
> + max = val;
> + }
> + }
> +
> + return div64_u64(sum, count);
> +}

By always discarding the larger value; you're searching for the first or
shortest peak, right?

While that is always a safe value; it might not be the best value.

Also; I think you can write the whole thing shorter; maybe like:


do {
count = sum = max = 0;
min = UINT_MAX;

for (i = 0; i < INTERVALS; i++) {
u64 val = cpu_data->intervals[i];

if (val >= range)
continue;

count++;
sum += val;
max = max(max, val);
min = min(min, val);
}

range = max;

} while (count > 3 && max > max_spread &&
((max - max_spread) * count > sum ||
(min + max_spread) * count < sum));

per the fact that <= INTERVALS/2 := > 3, without assuming that you need
one more condition in there for the first pass or something.


Anyway; a fair while ago I proposed a different estimator. I've not had
time to dig through the 4 prior versions so I cannot tell if you've
already tried this, but the idea was simple:

- track the last @n wakeup distances in the @idle-states buckets;
- sum the buckets in increasing idle state and pick the state before
you reach 50% of @n.

That is computationally cheaper than what you have; and should allow you
to increase @n without making the computation more expensive.

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