Date: Wed, 17 Apr 2024 14:38:58 -0700
From: Vanshidhar Konda <vanshikonda@...amperecomputing.com>
To: Beata Michalska <beata.michalska@....com>
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	ionela.voinescu@....com, sudeep.holla@....com, will@...nel.org, catalin.marinas@....com, 
	vincent.guittot@...aro.org, sumitg@...dia.com, yang@...amperecomputing.com, 
	lihuisong@...wei.com, viresh.kumar@...aro.org
Subject: Re: Re: [PATCH v4 4/4] cpufreq: Use arch specific feedback for
 cpuinfo_cur_freq

On Tue, Apr 16, 2024 at 05:46:18PM +0200, Beata Michalska wrote:
>On Mon, Apr 15, 2024 at 09:23:10PM -0700, Vanshidhar Konda wrote:
>> On Fri, Apr 05, 2024 at 02:33:19PM +0100, Beata Michalska wrote:
>> > Some architectures provide a way to determine an average frequency over
>> > a certain period of time based on available performance monitors (AMU on
>> > ARM or APERF/MPERf on x86). With those at hand, enroll arch_freq_get_on_cpu
>> > into cpuinfo_cur_freq policy sysfs attribute handler, which is expected to
>> > represent the current frequency of a given CPU, as obtained by the hardware.
>> > This is the type of feedback that counters do provide.
>> >
>>
>> --- snip ---
>>
>> While testing this patch series on AmpereOne system, I simulated CPU
>> frequency throttling when the system is under power or thermal
>> constraints.
>>
>> In this scenario, based on the user guilde, I expect scaling_cur_freq
>> is the frequency the kernel requests from the hardware; cpuinfo_cur_freq
>> is the actual frequency that the hardware is able to run at during the
>> power or thermal constraints.
>There has been a discussion on scaling_cur_freq vs cpuinfo_cur_freq [1].
>The guidelines you are referring here (assuming you mean [2]) are kinda
>out-of-sync already as scaling_cur_freq has been wired earlier to use arch
>specific feedback. As there was no Arm dedicated implementation of
>arch_freq_get_on_cpu, this went kinda unnoticed.
>The conclusion of the above mentioned discussion (though rather unstated
>explicitly) was to keep the current behaviour of scaling_cur_freq and align
>both across different archs: so with the patches, both attributes will provide
>hw feedback on current frequency, when available.
>Note that if we are to adhere to the docs cpuinfo_cur_freq is the place to use
>the counters really.
>
>That change was also requested through [3]
>
>Adding @Viresh in case there was any shift in the tides ....
>

Thank you for the pointer to the discussion in [1]. It makes sense to
bring arm64 behavior in line with x86. The question about whether
modifying the behavior of scaling_cur_freq was a good idea did not get
any response.

>>
>> The AmpereOne system I'm testing on has the following configuration:
>> - Max frequency is 3000000
>> - Support for AMU registers
>> - ACPI CPPC feedback counters use PCC register space
>> - Fedora 39 with 6.7.5 kernel
>> - Fedora 39 with 6.9.0-rc3 + this patch series
>>
>> With 6.7.5 kernel:
>> Core        scaling_cur_freq        cpuinfo_cur_freq
>> ----        ----------------        ----------------
>> 0             3000000                 2593000
>> 1             3000000                 2613000
>> 2             3000000                 2625000
>> 3             3000000                 2632000
>>
>So if I got it right from the info you have provided the numbers above are
>obtained without applying the patches. In that case, scaling_cur_freq will
>use policy->cur (in your case) showing last frequency set, not necessarily
>the actual freq, whereas cpuinfo_cur_freq uses __cpufreq_get and AMU counters.
>
>
>> With 6.9.0-rc3 + this patch series:
>> Core        scaling_cur_freq        cpuinfo_cur_freq
>> ----        ----------------        ----------------
>> 0             2671875                 2671875
>> 1             2589632                 2589632
>> 2             2648437                 2648437
>> 3             2698242                 2698242
>>
>With the patches applied both scaling_cur_freq and cpuinfo_cur_freq will use AMU
>counters, or fie scale factor obtained based on AMU counters to be more precise:
>both should now show similar/same frequency (as discussed in [1])
>I'd say due to existing implementation for scaling_cur_freq (which we cannot
>change at this point) this is unavoidable.
>
>> In the second case we can't identify that the CPU frequency is
>> being throttled by the hardware. I noticed this behavior with
>> or without this patch.
>>
>I am not entirely sure comparing the two should be a way to go about throttling
>(whether w/ or w/o the changes).
>It would probably be best to refer to thermal sysfs and get a hold of cur_state

Throttling could happen due to non-thermal reasons. Or a system may not
even support thermal zones. So on those systems we wouldn't be able to
identify/debug the behavior of the hardware providing less than maximum
performance. The discussion around scaling_cur_freq should probably be
re-visited in a targeted manner I think.

I'll test v5 of the series on AmpereOne and report back on that thread.

Thanks,
Vanshi

>which should indicate current throttle state:
>
> /sys/class/thermal/thermal_zone[0-*]/cdev[0-*]/cur_state
>
>with values above '0' implying ongoing throttling.
>
>The appropriate thermal_zone can be identified through 'type' attribute.
>
>
>Thank you for giving those patches a spin.
>
>---
>BR
>Beata
>---
>[1] https://lore.kernel.org/all/20230609043922.eyyqutbwlofqaddz@vireshk-i7/
>[2] https://elixir.bootlin.com/linux/latest/source/Documentation/admin-guide/pm/cpufreq.rst#L197
>[3] https://lore.kernel.org/lkml/2cfbc633-1e94-d741-2337-e1b0cf48b81b@nvidia.com/
>---
>
>
>> Thanks,
>> Vanshi

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