On Thu, May 22, 2025 at 1:54 PM Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote: > > On Thu, May 22, 2025 at 1:15 PM Russell Haley <yumpusamongus@xxxxxxxxx> wrote: > > > > > > > > On 5/22/25 4:47 AM, Rafael J. Wysocki wrote: > > > On Thu, May 22, 2025 at 10:51 AM Russell Haley <yumpusamongus@xxxxxxxxx> wrote: > > >> > > >> > > >> On 5/22/25 3:05 AM, Shashank Balaji wrote: > > >>> The userspace governor does not have the CPUFREQ_GOV_STRICT_TARGET flag, which > > >>> means the requested frequency may not strictly be followed. This is true in the > > >>> case of the intel_pstate driver with HWP enabled. When programming the > > >>> HWP_REQUEST MSR, the min_perf is set to `scaling_setspeed`, and the max_perf > > >>> is set to the policy's max. So, the hardware is free to increase the frequency > > >>> beyond the requested frequency. > > >>> > > >>> This behaviour can be slightly surprising, given the current wording "allows > > >>> userspace to set the CPU frequency". Hence, document this. > > >>> > > >> > > >> In my opinion, the documentation is correct, and it is the > > >> implementation in intel_pstate that is wrong. If the user wanted two > > >> separate knobs that control the minimum and maximum frequencies, they > > >> could leave intel_pstate in "active" mode and change scaling_min_freq > > >> and scaling_max_freq. > > >> > > >> If the user asks for the frequency to be set from userspace, the > > >> frequency had damn well better be set from userspace. > > > > > > The userspace governor requests a frequency between policy->min and > > > policy->max on behalf of user space. In intel_pstate this translates > > > to setting DESIRED_PERF to the requested value which is also the case > > > for the other governors. > > > > Huh. On this Skylake box with kernel 6.14.6, it seems to be setting > > Minimum_Performance, and leaving desired at 0. > > > > > echo userspace | sudo tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor > > userspace > > > echo 1400000 | sudo tee /sys/devices/system/cpu/cpu*/cpufreq/scaling_setspeed > > 1400000 > > > sudo x86_energy_perf_policy &| grep REQ > > cpu0: HWP_REQ: min 14 max 40 des 0 epp 128 window 0x0 (0*10^0us) use_pkg 0 > > cpu1: HWP_REQ: min 14 max 40 des 0 epp 128 window 0x0 (0*10^0us) use_pkg 0 > > cpu2: HWP_REQ: min 14 max 40 des 0 epp 128 window 0x0 (0*10^0us) use_pkg 0 > > cpu3: HWP_REQ: min 14 max 40 des 0 epp 128 window 0x0 (0*10^0us) use_pkg 0 > > cpu4: HWP_REQ: min 14 max 40 des 0 epp 128 window 0x0 (0*10^0us) use_pkg 0 > > cpu5: HWP_REQ: min 14 max 40 des 0 epp 128 window 0x0 (0*10^0us) use_pkg 0 > > cpu6: HWP_REQ: min 14 max 40 des 0 epp 128 window 0x0 (0*10^0us) use_pkg 0 > > cpu7: HWP_REQ: min 14 max 40 des 0 epp 128 window 0x0 (0*10^0us) use_pkg 0 > > OK, let me double check the code. I stand corrected, HWP_MIN_PERF is indeed set in accordance with the target frequency, not HWP_DESIRED_PERF. The reason why is because running at a frequency below the target might cause insufficient performance to be delivered which would break the assumptions of the schedutil governor. However, setting HWP_DESIRED_PERF to 0 may be a mistake because it may cause the CPU to always run above the target frequency which is not desirable from the power perspective. What can be done is to set HWP_MIN_PERF and HWP_DESIRED_PERF to the same value. [Note that intel_cpufreq_adjust_perf() used by the schedutil governor actually sets HWP_DESIRED_PERF in accordance with the target frequency, but it also sets HWP_MIN_PERF to the minimum sufficient perf value supplied by schedutil. Since intel_cpufreq_fast_switch() and intel_cpufreq_target() only get one target frequency, they cannot really say if any frequency below the target will be sufficient.]
