Messages in this thread Patch in this message | | | From | "Doug Smythies" <> | Subject | RE: [RFC][PATCH 7/7] cpufreq: intel_pstate: Change P-state selection algorithm for Core | Date | Sat, 13 Aug 2016 08:59:01 -0700 |
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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
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