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

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Message-ID: <Zs7Bwh6T3HCGlR9C@gpd3>
Date: Wed, 28 Aug 2024 08:20:50 +0200
From: Andrea Righi <andrea.righi@...ux.dev>
To: "Gautham R. Shenoy" <gautham.shenoy@....com>
Cc: Mario Limonciello <mario.limonciello@....com>,
	Mario Limonciello <superm1@...nel.org>,
	Borislav Petkov <bp@...en8.de>, Perry Yuan <perry.yuan@....com>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
	"Rafael J . Wysocki" <rafael@...nel.org>,
	"open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <linux-kernel@...r.kernel.org>,
	"open list:ACPI" <linux-acpi@...r.kernel.org>,
	"open list:CPU FREQUENCY SCALING FRAMEWORK" <linux-pm@...r.kernel.org>,
	changwoo@...lia.com, David Vernet <void@...ifault.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 8/8] cpufreq: amd-pstate: Drop some uses of
 cpudata->hw_prefcore

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).

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).

Thanks,
-Andrea