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

Andrea Righi
2024-08-29T15:17:01Z

On Thu, Aug 29, 2024 at 08:01:48AM -0500, Mario Limonciello wrote:
> 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.

Ok.

>
> 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.

So, if I move to 6.9+ I should see the same max frequency across all the
CPUs and I can use amd_pstate_precore_ranking to determine the subset of
fast cores.

Thanks for the clarification.

-Andrea