RE: [PATCH] ACPI: processor_idle: Skip dummy wait for processors based on the Zen microarchitecture

[Limonciello, Mario]

[Public]

> -----Original Message-----
> From: Dave Hansen <dave.hansen@xxxxxxxxx>
> Sent: Thursday, September 22, 2022 13:18
> To: Limonciello, Mario <Mario.Limonciello@xxxxxxx>; Nayak, K Prateek
> <KPrateek.Nayak@xxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx
> Cc: rafael@xxxxxxxxxx; lenb@xxxxxxxxxx; linux-acpi@xxxxxxxxxxxxxxx; linux-
> pm@xxxxxxxxxxxxxxx; dave.hansen@xxxxxxxxxxxxxxx; bp@xxxxxxxxx;
> tglx@xxxxxxxxxxxxx; andi@xxxxxxxx; puwen@xxxxxxxx; peterz@xxxxxxxxxxxxx;
> rui.zhang@xxxxxxxxx; gpiccoli@xxxxxxxxxx; daniel.lezcano@xxxxxxxxxx;
> Narayan, Ananth <Ananth.Narayan@xxxxxxx>; Shenoy, Gautham Ranjal
> <gautham.shenoy@xxxxxxx>; Ong, Calvin <Calvin.Ong@xxxxxxx>;
> stable@xxxxxxxxxxxxxxx; regressions@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH] ACPI: processor_idle: Skip dummy wait for processors
> based on the Zen microarchitecture
> 
> On 9/22/22 10:48, Limonciello, Mario wrote:
> >
> > 2) The title says to limit it to old intel systems, but nothing about this
> actually enforces that.
> > It actually is limited to all Intel systems, but effectively won't be used on
> anything but new
> > ones because of intel_idle.
> >
> > As an idea for #2 you could check for CONFIG_INTEL_IDLE in the Intel case
> and
> > if it's not defined show a pr_notice_once() type of message trying to tell
> people to use
> > Intel Idle instead for better performance.
> 
> What does that have to do with *this* patch, though?

It was just a thought triggered by your commit message title.

> 
> If you've got CONFIG_INTEL_IDLE disabled, you'll be slow before this
> patch.  You'll also be slow after this patch.  It's entirely orthogonal.
> 

Yeah it's orthogonal, but with this discussion happening and the code is
changing /anyway/ then a pr_notice_once() seemed like a nice way to
guide people towards intel_idle at the same time so they didn't trip into
the same problem AMD systems do today.

> I can add a "Practically" to the subject so folks don't confuse it with
> some hard limit that is being enforced:
> 
> 	ACPI: processor idle: Practically limit "Dummy wait" workaround to
> old
> Intel systems

That works.

> 
> BTW, is there seriously a strong technical reason that AMD systems are
> still using this code?  Or is it pure inertia?

Maybe a better question for Ananth and Prateek to comment on.