Messages in this thread | | | Date | Sun, 11 Nov 2018 16:40:17 +0100 | From | Peter Zijlstra <> | Subject | Re: [RFC/RFT][PATCH v5] cpuidle: New timer events oriented governor for tickless systems |
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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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