RE: [PATCH] cpuidle: use high confidence factors only when considering polling

["Doug Smythies" <dsmythies@telus.net>]

Sorry for the delay in my reply / test. The patch e-mail went
to my junk folder for some reason.

On 2106.03.17 17:46 Rafael J. Wysocki wrote:
> On Wednesday, March 16, 2016 12:14:00 PM Rik van Riel wrote:
>> The menu governor uses five different factors to pick the
>> idle state:
>> - the user configured latency_req
>> - the time until the next timer (next_timer_us)
>> - the typical sleep interval, as measured recently
>> - an estimate of sleep time by dividing next_timer_us by an observed factor
>> - a load corrected version of the above, divided again by load
>> 
>> Only the first three items are known with enough confidence that
>> we can use them to consider polling, instead of an actual CPU
>> idle state, because the cost of being wrong about polling can be
>> excessive power use.
>> 
>> The latter two are used in the menu governor's main selection
>> loop, and can result in choosing a shallower idle state when
>> the system is expected to be busy again soon.
>> 
>> This pushes a busy system in the "performance" direction of
>> the performance<>power tradeoff, when choosing between idle
>> states, but stays more strictly on the "power" state when
>> deciding between polling and C1.
>> 
>> Signed-off-by: Rik van Riel <riel@redhat.com>

For my part of it, this patch seems to be not O.K.
(reference rvr5 = this patch)

Aggregate idle for the 2000 second test. All in minutes.
(old tests re-stated)

State	k45rc7-rjw10	k45rc7-rjw10-reverted	k45rc7-rjw10-rvr5
0.00	18.07			0.92				18.67
1.00	12.35			19.51				12.82
2.00	3.96			4.28				3.97
3.00	1.55			1.53				1.58
4.00	138.96		141.99			143.80
			
total	174.90		168.24			180.84

Energy:
>> Kernel 4.5-rc7-rjw10: 61983 Joules
>> Kernel 4.5-rc7-rjw10-reverted: 48409 Joules (test 2 was 55040 Joules)
Kernel 4.5-rc7-rjw10-rvr5: 62243 Joules

I did acquire trace data with this test, but haven't post processed it yet.

... Doug