RE: [RFC][PATCH 7/7] cpufreq: intel_pstate: Change P-state selection algorithm for Core

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

On 2016.08.05 17:02 Rafael J. Wysocki wrote:
>> On 2016.08.03 21:19 Doug Smythies wrote:
>>> On 2016.07.31 16:49 Rafael J. Wysocki wrote:
>>> 
>>> The PID-base P-state selection algorithm used by intel_pstate for
>>> Core processors is based on very weak foundations.
>>
>> ...[cut]...
>> 
>>> +static inline int32_t get_target_pstate_default(struct cpudata *cpu)
>>> +{
>>> +	struct sample *sample = &cpu->sample;
>>> +	int32_t busy_frac;
>>> +	int pstate;
>>> +
>>> +	busy_frac = div_fp(sample->mperf, sample->tsc);
>>> +	sample->busy_scaled = busy_frac * 100;
>>> +
>>> +	if (busy_frac < cpu->iowait_boost)
>>> +		busy_frac = cpu->iowait_boost;
>>> +
>>> +	cpu->iowait_boost >>= 1;
>>> +
>>> +	pstate = cpu->pstate.turbo_pstate;
>>> +	return fp_toint((pstate + (pstate >> 2)) * busy_frac);
>>> +}
>>> +
>>
My previous replies (and see below) have suggested that some filtering
is needed on the target pstate, otherwise, and dependant on the type of
workload, it tends to oscillate.

I added the IIR (Infinite Impulse Response) filter that I have suggested in the past:

diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
index c43ef55..262ec5f 100644
--- a/drivers/cpufreq/intel_pstate.c
+++ b/drivers/cpufreq/intel_pstate.c
@@ -98,6 +98,7 @@ static inline u64 div_ext_fp(u64 x, u64 y)
  * @tsc:               Difference of time stamp counter between last and
  *                     current sample
  * @time:              Current time from scheduler
+ * @target:            target pstate filtered.
  *
  * This structure is used in the cpudata structure to store performance sample
  * data for choosing next P State.
@@ -108,6 +109,7 @@ struct sample {
        u64 aperf;
        u64 mperf;
        u64 tsc;
+       u64 target;
        u64 time;
 };

@@ -1168,6 +1170,7 @@ static void intel_pstate_get_cpu_pstates(struct cpudata *cpu)
                pstate_funcs.get_vid(cpu);

        intel_pstate_set_min_pstate(cpu);
+       cpu->sample.target = int_tofp(cpu->pstate.min_pstate);
 }

 static inline void intel_pstate_calc_avg_perf(struct cpudata *cpu)
@@ -1301,8 +1304,10 @@ static inline int32_t get_target_pstate_use_performance(struct cpudata *cpu)
 static inline int32_t get_target_pstate_default(struct cpudata *cpu)
 {
        struct sample *sample = &cpu->sample;
+       int64_t scaled_gain, unfiltered_target;
        int32_t busy_frac;
        int pstate;
+       u64 duration_ns;

        busy_frac = div_fp(sample->mperf, sample->tsc);
        sample->busy_scaled = busy_frac * 100;
@@ -1313,7 +1318,74 @@ static inline int32_t get_target_pstate_default(struct cpudata *cpu)
        cpu->iowait_boost >>= 1;

        pstate = cpu->pstate.turbo_pstate;
-       return fp_toint((pstate + (pstate >> 2)) * busy_frac);
+       /* To Do: I think the above should be:
+        *
+        * if (limits.no_turbo || limits.turbo_disabled)
+        *      pstate = cpu->pstate.max_pstate;
+        * else
+        *      pstate = cpu->pstate.turbo_pstate;
+        *
+        * figure it out.
+        *
+        * no clamps. Pre-filter clamping was needed in past implementations.
+        * To Do: Is any pre-filter clamping needed here? */
+
+       unfiltered_target = (pstate + (pstate >> 2)) * busy_frac;
+
+       /*
+        * Idle check.
+        * We have a deferrable timer. Very long durations can be
+        * either due to long idle (C0 time near 0),
+        * or due to short idle times that spanned jiffy boundaries
+        * (C0 time not near zero).
+        *
+        * To Do: As of the utilization stuff, I do not think the
+        * spanning jiffy boundaries thing is true anymore.
+        * Check, and fix the comment.
+        *
+        * The very long durations are 0.4 seconds or more.
+        * Either way, a very long duration will effectively flush
+        * the IIR filter, otherwise falling edge load response times
+        * can be on the order of tens of seconds, because this driver
+        * runs very rarely. Furthermore, for higher periodic loads that
+        * just so happen to not be in the C0 state on jiffy boundaries,
+        * the long ago history should be forgotten.
+        * For cases of durations that are a few times the set sample
+        * period, increase the IIR filter gain so as to weight
+        * the current sample more appropriately.
+        *
+        * To Do: sample_time should be forced to be accurate. For
+        * example if the kernel is a 250 Hz kernel, then a
+        * sample_rate_ms of 10 should result in a sample_time of 12.
+        *
+        * To Do: Check that the IO Boost case is not filtered too much.
+        *        It might be that a filter by-pass is needed for the boost case.
+        *        However, the existing gain = f(duration) might be good enough.
+        */
+
+       duration_ns = cpu->sample.time - cpu->last_sample_time;
+
+       scaled_gain = div_u64(int_tofp(duration_ns) *
+               int_tofp(pid_params.p_gain_pct), int_tofp(pid_params.sample_rate_ns));
+       if (scaled_gain > int_tofp(100))
+               scaled_gain = int_tofp(100);
+       /*
+        * This code should not be required,
+        * but short duration times have been observed
+        * To Do: Check if this code is actually still needed. I don't think so.
+        */
+       if (scaled_gain < int_tofp(pid_params.p_gain_pct))
+               scaled_gain = int_tofp(pid_params.p_gain_pct);
+
+       /*
+        * Bandwidth limit the output. For now, re-task p_gain_pct for this purpose.
+        * Use a smple IIR (Infinite Impulse Response) filter.
+        */
+       cpu->sample.target = div_u64((int_tofp(100) - scaled_gain) *
+                       cpu->sample.target + scaled_gain *
+                       unfiltered_target, int_tofp(100));
+
+       return fp_toint(cpu->sample.target + (1 << (FRAC_BITS-1)));
 }

 static inline void intel_pstate_update_pstate(struct cpudata *cpu, int pstate)
@@ -1579,6 +1651,7 @@ static void intel_pstate_stop_cpu(struct cpufreq_policy *policy)
                return;

        intel_pstate_set_min_pstate(cpu);
+       cpu->sample.target = int_tofp(cpu->pstate.min_pstate);
 }

 static int intel_pstate_cpu_init(struct cpufreq_policy *policy)

The filter introduces a trade-off between step function load response time
and the tendency for the target pstate to oscillate.

...[cut]...

>> Several tests were done with this patch set.
>> The patch set would not apply to kernel 4.7, but did apply fine to a 4.7+ kernel
>> (I did as of 7a66ecf) from a few days ago.
>>
>> Test 1: Phoronix ffmpeg test (less time is better):
>> Reason: Because it suffers from rotating amongst CPUs in an odd way, challenging for CPU frequency scaling drivers.
>> This test tends to be an indicator of potential troubles with some games.
>> Criteria: (Dirk Brandewie): Must match or better acpi_cpufreq - ondemand.
>> With patch set: 15.8 Seconds average and 24.51 package watts.
>> Without patch set: 11.61 Seconds average and 27.59 watts.
>> Conclusion: Significant reduction in performance with proposed patch set.

With the filter this become even worse at ~18 seconds.
I used to fix this by moving the response curve to the left.
I have not tested this:

+       unfiltered_target = (pstate + (pstate >> 1)) * busy_frac;

which moves the response curve left a little, yet. I will test it.

...[cut]...

>> Test 9: Doug's_specpower simulator (20% load):
>> Time is fixed, less energy is better.
>> Reason: During the long
>> "[intel-pstate driver regression] processor frequency very high even if in idle"
>> and subsequent https://bugzilla.kernel.org/show_bug.cgi?id=115771
>> discussion / thread(s), some sort of test was needed to try to mimic what Srinivas
>> was getting on his fancy SpecPower test platform. So far at least, this test does that.
>> Only the 20% load case was created, because that was the biggest problem case back then.
>> With patch set: 4 tests at an average of 7197 Joules per test, relatively high CPU frequencies.
>> Without the patch set: 4 tests at an average of 5956 Joules per test, relatively low CPU frequencies.
>> Conclusion: 21% energy regression with the patch set.
>> Note: Newer processors might do better than my older i7-2600K.

Patch set + above and IIR gain = 10%: 5670 Joules.
Conclusion: Energy regression eliminated.

Other gains:

gain =  5%: 5342 Joules; Busy MHz: 2172
gain = 10%: 5670 Joules; Busy MHz: 2285
gain = 20%: 6126 Joules; Busy MHz: 2560
gain = 30%: 6426 Joules; Busy MHz: 2739
gain = 40%: 6674 Joules; Busy MHz: 2912
gain = 70%: 7109 Joules; Busy MHz: 3199

locked at minimum pstate (reference): 4653 Joules; Busy MHz: 1600
Performance mode (reference): 7808 Joules; Busy MHz: 3647

>> Test 10: measure the frequency response curve, fixed work packet method,
>> 75 hertz work / sleep frequency (all CPU, no IOWAIT):
>> Reason: To compare to some older data and observe overall.
>> png graph NOT attached.
>> Conclusions: Tends to oscillate, suggesting some sort of damping is needed.
>> However, any filtering tends to increase the step function load rise time
>> (see test 11 below, I think there is some wiggle room here).
>> See also graph which has: with and without patch set; performance mode (for reference);
>> Philippe Longepe's cpu_load method also with setpoint 40 (for reference); one of my previous
>> attempts at a load related patch set from quite some time ago (for reference).

As expected, the filter damps out the oscillation.
New graphs will be sent to Rafael and Srinivas off-list.

>> 
>> Test 11: Look at the step function load response. From no load to 100% on one CPU (CPU load only, no IO).
>> While there is a graph, it is not attached:
>> Conclusion: The step function response is greatly improved (virtually one sample time max).
>> It would probably be O.K. to slow it down a little with a filter so as to reduce the
>> tendency to oscillate under periodic load conditions (to a point, at least. A low enough frequency will
>> always oscillate) (see the graph for test10).

I haven't done this test yet, but from previous work, a gain setting of 10 to 15% gives a
load step function response time similar to the current PID based filter.

The other tests gave similar results with or without the filter.

... Doug