Messages in this thread | | | Date | Wed, 17 Apr 2024 14:38:58 -0700 | From | Vanshidhar Konda <> | Subject | Re: Re: [PATCH v4 4/4] cpufreq: Use arch specific feedback for cpuinfo_cur_freq |
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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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