[Bug 210993] Intel frequency scaling causes electrical noise on 10th gen CPUs

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=210993

Zhang Rui (rui.zhang@intel.com) changed:

           What    |Removed                     |Added
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             Status|NEW                         |NEEDINFO
                 CC|                            |rui.zhang@intel.com

--- Comment #2 from Zhang Rui (rui.zhang@intel.com) ---

(In reply to Karol Herbst from comment #0)
> One thing I noticed with my i7-10850H CPU is, that if there are cores with
> huge differences in their frequencies (e.g. one at 900 MHz another at 4.9
> GHz) some electrical becomes noticable.
> 
> The noise can be reduced by limiting the available range the CPUs frequency
> can be scaled to. Sadly the scaling_min_freq property is ignored outright
> and only matters as long as the CPU stays idle, but as long as one core gets
> some load, another one drops below scaling_min_freq, so I couldn't test if
> limiting the lower bound even helps.

Usually, if we didn't take Pe into account, the CPU frequency should not drop
below scaling_min_freq. This sounds like a bug to me.
please give turbostat output when this happens.

> 
> Putting one core at max load with stress -c 1 effectively eliminates all
> noise.

please give turbostat output when this happens.

> 
> This issue is even more annoying on my laptop as it generally runs with the
> fans turned off.
> 
> intel_pstate is used in active mode, but putting it in passive mode,
> disabling HWP or even using the ACPI freq scaling didn't really change
> anything in this regard. The powersave governor and default
> energy_performance_preference is used.
> 
> Is there anything else which could be done to mitigate the problem? Just
> putting a finger on the touchpad makes the noise quite noticeable as cores
> get scaled to the max.
> 
I'd prefer this is the fan noise rather than CPU electrical noise.

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