On 03/03/2016 05:07 AM, Vincent Guittot wrote:

I mainly want to prevent any useless and periodic frequency switch
because of an utilization that changes with the current frequency (if
frequency invariance is not used) and that can make the formula
selects another frequency than the current one. That what i can see
when testing it .

Sorry for the late reply, i was trying to do some test on my board but
was facing some crash issue (not link with your patchset). So i have
done some tests and i can see such instable behavior. I have generated
a load of 33% at max frequency (3ms runs every 9ms) and i can see the
frequency that toggles without any good reason. Saying that, i can see
similar thing with ondemand.

FWIW I ran some performance numbers on my chromebook 2. Initially I
forgot to bring in the frequency invariance support but that yielded an
opportunity to see the impact of it.

The tests below consist of a periodic workload. The OH (overhead)
numbers show how close the workload got to running as slow as fmin (100%
= as slow as powersave gov, 0% = as fast as perf gov). The OR (overrun)
number is the count of instances where the busy work exceeded the period.

First a comparison of schedutil with and without frequency invariance.
Run and period are in milliseconds.

			scu (no inv)	scu (w/inv)	
run	period	busy %	OR	OH	OR	OH
1	100	1.00%	0	79.72%	0	95.86%
10	1000	1.00%	0	24.52%	0	71.61%
1	10	10.00%	0	21.25%	0	41.78%
10	100	10.00%	0	26.06%	0	47.96%
100	1000	10.00%	0	6.36%	0	26.03%
6	33	18.18%	0	15.67%	0	31.61%
66	333	19.82%	0	8.94%	0	29.46%
4	10	40.00%	0	6.26%	0	12.93%
40	100	40.00%	0	6.93%	2	14.08%
400	1000	40.00%	0	1.65%	0	11.58%
5	9	55.56%	0	3.70%	0	7.70%
50	90	55.56%	1	4.19%	6	8.06%
500	900	55.56%	0	1.35%	5	6.94%
9	12	75.00%	0	1.60%	56	3.59%
90	120	75.00%	0	1.88%	21	3.94%
900	1200	75.00%	0	0.73%	4	4.41%

Frequency invariance causes schedutil overhead to increase noticeably. I
haven't dug into traces or anything. Perhaps this is due to the
algorithm overshooting then overcorrecting etc., I do not yet know.

Here is a comparison, with frequency invariance, of ondemand and
interactive with schedfreq and schedutil. The first two columns (run and
period) are omitted so the table will fit.

	ondemand	interactive	schedfreq	schedutil	
busy %	OR	OH	OR	OH	OR	OH	OR	OH
1.00%	0	68.96%	0	100.04%	0	78.49%	0	95.86%
1.00%	0	25.04%	0	22.59%	0	72.56%	0	71.61%
10.00%	0	21.75%	0	63.08%	0	52.40%	0	41.78%
10.00%	0	12.17%	0	14.41%	0	17.33%	0	47.96%
10.00%	0	2.57%	0	2.17%	0	0.29%	0	26.03%
18.18%	0	12.39%	0	9.39%	0	17.34%	0	31.61%
19.82%	0	3.74%	0	3.42%	0	12.26%	0	29.46%
40.00%	2	6.26%	1	12.23%	0	6.15%	0	12.93%
40.00%	0	0.47%	0	0.05%	0	2.68%	2	14.08%
40.00%	0	0.60%	0	0.50%	0	1.22%	0	11.58%
55.56%	2	4.25%	5	5.97%	0	2.51%	0	7.70%
55.56%	0	1.89%	0	0.04%	0	1.71%	6	8.06%
55.56%	0	0.50%	0	0.47%	0	1.82%	5	6.94%
75.00%	2	1.65%	1	0.46%	0	0.26%	56	3.59%
75.00%	0	1.68%	0	0.05%	0	0.49%	21	3.94%
75.00%	0	0.28%	0	0.23%	0	0.62%	4	4.41%

Aside from the 2nd and 3rd tests schedutil is showing decreased
performance across the board. The fifth test is particularly bad.

The catch is that I do not have power numbers to go with this data, as
I'm not currently equipped to gather them. So more analysis is
definitely needed to capture the full story.

thanks,
Steve