On Thu, May 22, 2025 at 11:46 AM Shashank Balaji <shashank.mahadasyam@xxxxxxxx> wrote: > > Hi Russell, > > On Thu, May 22, 2025 at 03:50:55AM -0500, Russell Haley wrote: > > If the user asks for the frequency to be set from userspace, the > > frequency had damn well better be set from userspace. > > First of all, I agree with you. In fact, before sending this patch, I > was considering adding CPUFREQ_GOV_STRICT_TARGET to the userspace > governor. intel_pstate should handle the rest of it. This wouldn't work the way you expect, though. It would cause the driver to always set the frequency to policy->max. > > 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 intel_pstate is left in "active" mode, then userspace can't use any > of the other governors. Moreover, intel_pstate's min and max frequencies > apply to all the cpus. That's not true. scaling_min_freq and scaling_max_freq is per CPU, but the values from there are subject to hardware coordination. > Whereas, the userspace governor can be set on a per-cpu basis. This is also subject to hardware coordination. > Let's say this is "fixed" by adding CPUFREQ_GOV_STRICT_TARGET flag to > the userspace governor. Then userspace has no way to get back the > current behavior where the hardware automagically increases frequency > beyond the target frequency. At least not without a new interface. > > With the current behaviour, userspace can have it both ways: > - actual frequency = target frequency > - actual frequency >= target frequency > > And the occasional higher frequency shouldn't hurt performance, right? > But if they still want exact equality, with the current interface, they > can do that too. > > This consideration is what led me to document the "actual freq >= target > freq" rather than patch it so that "actual freq = target freq". The documentation can be adjusted by replacing "set" with "request" in the userspace governor description and adding a clarification to it that the requested frequency is between the policy min and max levels. With HWP enabled, the closest to setting the frequency to a specific value one can get is by setting scaling_min_freq and scaling_max_freq to that value.
