Messages in this thread | | | Date | Fri, 26 Apr 2024 12:45:39 +0200 | From | Beata Michalska <> | Subject | Re: Re: [PATCH v4 4/4] cpufreq: Use arch specific feedback for cpuinfo_cur_freq |
| |
On Wed, Apr 17, 2024 at 02:38:58PM -0700, Vanshidhar Konda wrote: > 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. >
@Viresh:
It seems that we might need to revisit the discussion we've had around scaling_cur_freq and cpuinfo_cur_freq and the use of arch_freq_get_on_cpu. As Vanshi has raised, having both utilizing arch specific feedback for getting current frequency is bit problematic and might be confusing at best. As arch_freq_get_on_cpu is already used by show_scaling_cur_freq there are not many options we are left with, if we want to kee all archs aligned: we can either try to rework show_scaling_cur_freq and it's use of arch_freq_get_on_cpu, and move it to cpuinfo_cur_freq, which would align with relevant docs, though that will not work for x86, or we keep it only there and skip updating cpuinfo_cur_freq, going against the guidelines. Other options, purely theoretical, would involve making arch_freq_get_on_cpu aware of type of the info requested (hw vs sw) or adding yet another arch-specific implementation, and those are not really appealing alternatives to say at least. What's your opinion on this one ?
--- BR Beata
> 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
| |