Re: [PATCH 8/8] cpufreq: amd-pstate: Drop some uses of cpudata->hw_prefcore

Mario Limonciello
2024-08-29T13:01:54Z

On 8/29/2024 07:52, Andrea Righi wrote:
> On Wed, Aug 28, 2024 at 08:27:44PM +0530, Gautham R. Shenoy wrote:
>> Hello Andrea,
>>
>> On Wed, Aug 28, 2024 at 08:20:50AM +0200, Andrea Righi wrote:
>>> On Wed, Aug 28, 2024 at 10:38:45AM +0530, Gautham R. Shenoy wrote:
>>> ...
>>>>> I had thought this was a malfunction in the behavior that it reflected the
>>>>> current status, not the hardware /capability/.
>>>>>
>>>>> Which one makes more sense for userspace? In my mind the most likely
>>>>> consumer of this information would be something a sched_ext based userspace
>>>>> scheduler. They would need to know whether the scheduler was using
>>>>> preferred cores; not whether the hardware supported it.
>>>>
>>>> The commandline parameter currently impacts only the fair sched-class
>>>> tasks since the preference information gets used only during
>>>> load-balancing.
>>>>
>>>> IMO, the same should continue with sched-ext, i.e. if the user has
>>>> explicitly disabled prefcore support via commandline, the no sched-ext
>>>> scheduler should use the preference information to make task placement
>>>> decisions. However, I would like to see what the sched-ext folks have
>>>> to say. Adding some of them to the Cc list.
>>>
>>> IMHO it makes more sense to reflect the real state of prefcore support
>>> from a "system" perspective, more than a "hardware" perspective, so if
>>> it's disabled via boot command line it should show disabled.
>>>
>>> From a user-space scheduler perspective we should be fine either way, as
>>> long as the ABI is clearly documented, since we also have access to
>>> /proc/cmdline and we would be able to figure out if the user has
>>> disabled it via cmdline (however, the preference is still to report the
>>> actual system status).
>>
>> Thank you for confirming this.
>>
>>>
>>> Question: having prefcore enabled affects also the value of
>>> scaling_max_freq? Like an `lscpu -e`, for example, would show a higher
>>> max frequency for the specific preferred cores? (this is another useful
>>> information from a sched_ext scheduler perspective).
>>
>> Since the scaling_max_freq is computed based on the boost-numerator,
>> at least from this patchset, the numerator would be the same across
>> all kinds of cores, and thus the scaling_max_freq reported will be the
>> same across all the cores.
>
> I see, so IIUC from user-space the most reliable way to detect the
> fastest cores is to check amd_pstate_highest_perf / amd_pstate_max_freq,
> right? I'm trying to figure out a way to abstract and generalize the
> concept of "fast cores" in sched_ext.

Right now the best way to do this is to look at the
amd_pstate_precore_ranking file.

In this series there has been some discussion of dropping it though in
favor of looking at the highest perf file. I don't believe we're
concluded one way or another on it yet though.

>
> Also, is this something that has changed recently? I see this on an
> AMD Ryzen Threadripper PRO 7975WX 32-Cores running a 6.8 kernel:
>
> $ uname -r
> 6.8.0-40-generic

You're missing the preferred core patches on this kernel. They landed
in 6.9, it's better to upgrade to 6.10.y or 6.11-rc.