Messages in this thread | | | Date | Tue, 16 Apr 2024 17:46:18 +0200 | From | Beata Michalska <> | Subject | Re: [PATCH v4 4/4] cpufreq: Use arch specific feedback for cpuinfo_cur_freq |
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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 .... > > 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 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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