RE: cpufreq: intel_pstate: HWP mode issue

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

On 2020.06.30 11:41 Doug Smythies wrote:
> 
> Hi Srinivas,
> 
> O.K. let's try this again, starting a new thread, with address list similar to a few weeks ago.
> I believe I have untangled my multiple issues, such that this e-mail should be only about
> the single issue of HWP capable processors incorrectly deciding to lower the CPU frequency
> under some conditions. Also, my previous assertion as to the issue was indeed incorrect.
> 
> I now:
> . never use x86_energy_perf_policy.
> . For HWP disabled: never change from active to passive or via versa, but rather do it via boot.
> . after boot always check and reset the various power limit log bits that are set.
> . never compile the kernel (well, until after any tests), which will set those bits again.
> . never run prime95 high heat torture test, which will set those bits again.
> . Note that the tests done for this e-mail never ever set those bits again.
> . Invented an entirely new way to manifest, demonstrate, and exploit the issue (also mentioned June
> 6th).
> . All tests were repeated on another HWP capable computer, so a i5-9600K and a i5-6200U.
> 
> New method (old was periodic workflow):
> 
> Long busy, short gap, busy but taking loop time samples so as to estimate CPU frequency.
> I am calling it an inverse impulse response test.
> 
> Assertion:
> 
> If the short sleep is somehow simultaneous with some sort of 5.0 millisecond (200 Hertz)
> periodic event (either in HWP itself, or via the driver, I am unable to determine which,
> but think it is inside the black box that is HWP),

I have been attempting to characterise the "black box" that is HWP.
In terms of system response verses EPP, I only observe the HWP loop time as the
response variable.

0 <= EPP <= 1 : My test can not measure loop time.
2 <= EPP <= 39 : HWP servo loop time 2 milliseconds
40 <= EPP <= 55 : HWP servo loop time 3 milliseconds
56 <= EPP <= 79 : HWP servo loop time 4 milliseconds
80 <= EPP <= 133 : HWP servo loop time 5 milliseconds
134 <= EPP <= 143 : HWP servo loop time 6 milliseconds
144 <= EPP <= 154 : HWP servo loop time 7 milliseconds
155 <= EPP <= 175 : HWP servo loop time 8 milliseconds
176 <= EPP <= 255 : HWP servo loop time 9 milliseconds

If there are other system response differences within
those groups, I haven't been able to detect them,
but would be grateful for any further insight.

Otherwise, in future, I do not see a need to test anything
other than 9 values of EPP, one from each group.

> then there is a possibility that the
> CPU frequency will drop significantly and will take an excessive amount of time to recover.
> Frequency step ups are exactly on 5.0 millisecond boundaries +/- the short gap time.
> 
> . The probability is somewhat inconsistent and a function of whatever else the computer is doing.
> . The time to recover is a function of EPP, and if EPP is low enough my test never fails.
> . These tests were all done with default settings.
> . The "5.0" mSec is only for those default settings, it actually depends on EPP.
>   . Crude step boundaries, mSec: EPP=32, 2; EPP=64, 4; EPP=128, 5.00; EPP=196, 9

Now fully understood, as listed above.

> . High level: i5-9600K: 2453 tests, 60 failures, 2.45% fail rate. (HWP - powersave)
> . High level: i5-6200U: 4134 tests, 128 failures, 3.1% fail rate. (HWP - powersave)
> . Low level (capture waveforms): i5-9600K: 1842 captured failure waveforms. See graph.
> . Low level (capture waveforms): i5-6200U: 458 captured failure waveforms. See graph.
> . Verify acpi-cpufreq/ondemand works fine: i5-9600K: 8975 tests. 0 failures.
> . Verify acpi-cpufreq/ondemand works fine: i5-6200U: 8575 tests. 0 failures.

The tests were all done using the teo idle governor.
While the menu governor does not fail for this particular test, it fails
in other scenarios.

I have yet to find a failure scenario when idle state 2 is disabled.
I have captured and analyzed about 400 megabytes of trace data,
and have not been able to isolate an exact correlation.

> 
> The short gap was 842 uSeconds for all these tests, and for no particular reason.
> 
> While I have not re-done the bounds investigation, I have no reason to doubt
> my previous work, re-stated below:
> 
> > Gap definition:
> > lower limit not known, but < 747 uSeconds.
> > Upper limit is between 952 and 955 uSeconds (there will be some overhead uncertainties).

The only new information I have is that the upper bound is bigger.

> > Must be preceded by busy time spanning a couple of HWP sampling boundaries
> > or jiffy boundaries or something (I don't actually know how HWP does stuff).
> 
> Rather than point to graphs, which nobody seems to look at, they are attached,
> and so might get striped for some of you.
> 
> ... Doug
> 
> Addendum: Some of the MSRs you have requested in the past:
> 
> i5-9600K (HWP - powersave after test):
> 
> root@s18:/home/doug# /home/doug/c/msr-decoder
> 8.) 0x198: IA32_PERF_STATUS     : CPU 0-5 :   8 :   8 :   8 :   8 :   8 :   8 :
> B.) 0x770: IA32_PM_ENABLE: 1 : HWP enable
> 1.) 0x19C: IA32_THERM_STATUS: 88480000
> 2.) 0x1AA: MSR_MISC_PWR_MGMT: 401CC0 EIST enabled Coordination enabled OOB Bit 8 reset OOB Bit 18
> reset
> 3.) 0x1B1: IA32_PACKAGE_THERM_STATUS: 88460000
> 4.) 0x64F: MSR_CORE_PERF_LIMIT_REASONS: 0
> A.) 0x1FC: MSR_POWER_CTL: 3C005D : C1E disable : EEO disable : RHO disable
> 5.) 0x771: IA32_HWP_CAPABILITIES (performance): 108252E : high 46 : guaranteed 37 : efficient 8 :
> lowest 1
> 6.) 0x774: IA32_HWP_REQUEST:    CPU 0-5 :
>     raw: 80002E08 : 80002E08 : 80002E08 : 80002E08 : 80002E08 : 80002E08 :
>     min:        8 :        8 :        8 :        8 :        8 :        8 :
>     max:       46 :       46 :       46 :       46 :       46 :       46 :
>     des:        0 :        0 :        0 :        0 :        0 :        0 :
>     epp:      128 :      128 :      128 :      128 :      128 :      128 :
>     act:        0 :        0 :        0 :        0 :        0 :        0 :
> 7.) 0x777: IA32_HWP_STATUS: 0 : high 0 : guaranteed 0 : efficient 0 : lowest 0
> 
> i5-9600K (no HWP - acpi-cpufreq/ondemand after test):
> 
> root@s18:/home/doug/c# /home/doug/c/msr-decoder
> 8.) 0x198: IA32_PERF_STATUS     : CPU 0-5 :   8 :   8 :   8 :   8 :   8 :   8 :
> B.) 0x770: IA32_PM_ENABLE: 0 : HWP disable
> 9.) 0x199: IA32_PERF_CTL        : CPU 0-5 :   8 :   8 :   8 :   8 :   8 :   8 :
> C.) 0x1B0: IA32_ENERGY_PERF_BIAS: CPU 0-5 :   6 :   6 :   6 :   6 :   6 :   6 :
> 1.) 0x19C: IA32_THERM_STATUS: 88480000
> 2.) 0x1AA: MSR_MISC_PWR_MGMT: 401CC0 EIST enabled Coordination enabled OOB Bit 8 reset OOB Bit 18
> reset
> 3.) 0x1B1: IA32_PACKAGE_THERM_STATUS: 88460000
> 4.) 0x64F: MSR_CORE_PERF_LIMIT_REASONS: 0
> A.) 0x1FC: MSR_POWER_CTL: 3C005D : C1E disable : EEO disable : RHO disable
> 
> i5-6200U (HWP - powersave after test):
> 
> 8.) 0x198: IA32_PERF_STATUS : CPU 0-3 : 19 : 19 : 19 : 19 :
> B.) 0x770: IA32_PM_ENABLE: 1 : HWP enable
> 1.) 0x19C: IA32_THERM_STATUS: 88430000
> 2.) 0x1AA: MSR_MISC_PWR_MGMT: 4018C0 EIST enabled Coordination enabled OOB Bit 8 reset OOB Bit 18
> reset
> 3.) 0x1B1: IA32_PACKAGE_THERM_STATUS: 88420000
> 4.) 0x64F: MSR_CORE_PERF_LIMIT_REASONS: 0
> A.) 0x1FC: MSR_POWER_CTL: 24005D : C1E disable : EEO enable : RHO enable
> 5.) 0x771: IA32_HWP_CAPABILITIES (performance): 105171C : high 28 : guaranteed 23 : efficient 5 :
> lowest 1
> 6.) 0x774: IA32_HWP_REQUEST: CPU 0-3 :
>     raw: 80001B04 : 80001B04 : 80001B04 : 80001B04 :
>     min:        4 :        4 :        4 :        4 :
>     max:       27 :       27 :       27 :       27 :
>     des:        0 :        0 :        0 :        0 :
>     epp:      128 :      128 :      128 :      128 :
>     act:        0 :        0 :        0 :        0 :
> 7.) 0x777: IA32_HWP_STATUS: 4 : high 4 : guaranteed 0 : efficient 0 : lowest 0
> 
> i5-6200U (no HWP - acpi-cpufreq/ondemand after test):
> 
> 8.) 0x198: IA32_PERF_STATUS     : CPU 0-3 :  23 :  23 :  23 :  23 :
> B.) 0x770: IA32_PM_ENABLE: 0 : HWP disable
> 9.) 0x199: IA32_PERF_CTL        : CPU 0-3 :  11 :   5 :   5 :   5 :
> C.) 0x1B0: IA32_ENERGY_PERF_BIAS: CPU 0-3 :   6 :   6 :   6 :   6 :
> 1.) 0x19C: IA32_THERM_STATUS: 88440000
> 2.) 0x1AA: MSR_MISC_PWR_MGMT: 4018C0 EIST enabled Coordination enabled OOB Bit 8 reset OOB Bit 18
> reset
> 3.) 0x1B1: IA32_PACKAGE_THERM_STATUS: 88430000
> 4.) 0x64F: MSR_CORE_PERF_LIMIT_REASONS: 0
> A.) 0x1FC: MSR_POWER_CTL: 24005D : C1E disable : EEO enable : RHO enable