linux-kernel - Re: intel_pstate: Lowest frequency not reached with Intel i7-6700

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Date:   Thu, 13 Dec 2018 13:41:40 +0100
From:   Paul Menzel <pmenzel@...gen.mpg.de>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Doug Smythies <dsmythies@...us.net>, linux-pm@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Len Brown <lenb@...nel.org>
Subject: Re: intel_pstate: Lowest frequency not reached with Intel i7-6700

Dear Rafael,


On 12/13/18 11:39, Rafael J. Wysocki wrote:
> On Thu, Dec 13, 2018 at 10:54 AM Paul Menzel <pmenzel@...gen.mpg.de> wrote:

>> On 12/13/18 00:06, Doug Smythies wrote:
>>> On 2018.12.12 13:40 Paul Menzel wrote:
>>>
>>>> Using *powersave* as P-state selection algorithm, on an idle system
>>>
>>> Define "idle system".
>>> If your computer is running a GUI, or is even a server without a GUI
>>> but with many services running, then "idle" really isn't.
>>> Below is from my test server, with many services disabled, so
>>> "idle" really is quite "idle"
>>>
>>> doug@s15:~/temp$ sudo turbostat --Summary --quiet --show Busy%,Bzy_MHz,PkgTmp,PkgWatt --interval 15
>>> Busy%   Bzy_MHz PkgTmp  PkgWatt
>>> 0.01    1608    27      3.71
>>> 0.01    1619    27      3.71
>>> 0.01    1600    28      3.71
>>> 0.01    1600    28      3.70
>>>
>>> Note that p state 16 (1600 MHz) is the minimum for my older i7-2600k
>>> processor.
>>
>> The thing is, on an Intel Kaby Lake laptop with Ubuntu 18.10 and GNOME
>> running, it goes down to the lowest listed frequency.

Checking the numbers again, I was mistaken. The lowest possible frequency
of the Intel Kaby Lake i7-7500U in that laptop is 400 MHz, and it is
going down to 600 MHz. Busy% from turbostat is 0.3 to 0.4.

> Kaby Lake has hardware-managed P-states (HWP) which is a different mechanism.

Isn’t HWP also available for the 6th generation?

    $ dmesg | grep intel_pstate
    [    2.092456] intel_pstate: Intel P-state driver initializing
    [    2.094820] intel_pstate: HWP enabled

>>>> Shouldn’t it go down until 800 MHz?
>>>
>>> We would need some actual busy information, turbostat is the
>>> recommended tool, to know for sure.
>>
>> Here you go.
>>
>> ```
>> tools/power/x86/turbostat> sudo ./turbostat --Summary --quiet --show Busy%,Bzy_MHz,PkgTmp,PkgWatt --interval 15
>> Busy%    Bzy_MHz    PkgTmp    PkgWatt
>> 3.59    1167    31    1.68
>> 3.21    903    31    1.34
>> 3.21    906    31    1.34
>> 3.27    901    31    1.35
>> 8.23    2715    30    2.32  ← stopping GDM (systemctl stop gdm)
>> 2.95    915    30    1.18
>> 2.91    906    30    1.18
>> 2.92    903    30    1.17
>> 2.90    900    29    1.17
>> 2.89    903    29    1.18
>> 2.91    903    30    1.18
>> 2.89    903    29    1.18
>> 2.89    900    29    1.18
>> 2.90    903    30    1.18
>> 2.90    903    29    1.17
>> 2.90    903    29    1.17
>> 2.90    900    29    1.16
>> 2.90    903    29    1.14
>> 2.90    903    28    1.11
>> 2.90    903    29    1.10
>> 2.91    900    29    1.16
>> 2.91    903    29    1.14
>> 2.90    903    29    1.12
>> 2.90    903    29    1.16
>> 2.90    900    28    1.17
>> 2.92    903    29    1.16
>> 2.90    903    29    1.16
>> 2.90    903    29    1.16
>> ```
>>
>> 800 MHz should be enough to keep GDM running, shouldn’t it?
> 
> Well, depending.
> 
>> Otherwise only SSH was running.
> 
> There obviously is something that causes it to stay at 900 MHz.

It’s not obvious to me, but you have more experience. It’d expect
to at least one core(?) to go down to 800 MHz and not all to stay
at 900 MHz.

> Please check max_perf_pct, min_perf_pct and num_pstates under
> /sys/devices/system/cpu/intel_pstate/ .

    /sys/devices/system/cpu/intel_pstate> cat max_perf_pct min_perf_pct num_pstates
    100
    20
    33

> Also cpuinfo_max_freq, cpuinfo_min_freq, scaling_max_freq,
> scaling_min_freq under /sys/devices/system/cpu/cpufreq/policy0/ .

/sys/devices/system/cpu/cpufreq/policy0> cat cpuinfo_{min,max}_freq scaling_{min,max}_freq
    800000
    4000000
    800000
    4000000

> However, please note that Busy% of 3 isn't particularly low.

Indeed. On the laptop it is around 0.3 to 0.4 even with GNOME
running.

So, to check if everything is working, I boot into initramfs
and check the numbers there?


Kind regards,

Paul


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