linux-kernel - Re: [PATCH V3] acpi: Allow ignoring _OSC CPPC v2 bit via kernel parameter

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Date: Wed, 19 Jun 2024 19:37:55 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Aaron Rainbolt <arainbolt@...cus.org>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>, Mario Limonciello <mario.limonciello@....com>, 
	linux-acpi@...r.kernel.org, linux-kernel@...r.kernel.org, lenb@...nel.org, 
	mmikowski@...cus.org, Perry.Yuan@....com, 
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Subject: Re: [PATCH V3] acpi: Allow ignoring _OSC CPPC v2 bit via kernel parameter

On Wed, Jun 19, 2024 at 7:34 PM Aaron Rainbolt <arainbolt@...cus.org> wrote:
>
> On Wed, Jun 19, 2024 at 07:09:35PM +0200, Rafael J. Wysocki wrote:
> > On Wed, Jun 19, 2024 at 6:33 AM Aaron Rainbolt <arainbolt@...cus.org> wrote:
> > >
> > > acpi: Allow ignoring _OSC CPPC v2 bit via kernel parameter
> > >
> > > The _OSC is supposed to contain a bit indicating whether the hardware
> > > supports CPPC v2 or not. This bit is not always set, causing CPPC v2 to
> > > be considered absent. This results in severe single-core performance
> > > issues with the EEVDF scheduler on heterogenous-core Intel processors.
> >
> > While some things can be affected by this, I don't immediately see a
> > connection between CPPC v2, Intel hybrid processors and EEVDF.
> >
> > In particular, why would EEVDF alone be affected?
> >
> > Care to explain this?
>
> From what I understand, the EEVDF scheduler requires ITMT (which in turn
> requires CPPC v2) in order to determine which cores are performance cores
> and which cores are efficiency cores. When CPPC v2 is missing, ITMT is
> also missing, and the scheduler no longer can figure out which cores are
> which. Thus on a system with many efficiency cores and a few performance
> cores, there's a pretty decent chance the scheduler will put an intensive
> single-threaded load on an efficiency core rather than on a performance
> core, which has obvious performance implications since efficiency cores
> are slower than performance cores by design.

So the above information should go into the changelog of your patch.

> A good example of someone else hitting this issue can be seen here:
> https://bugzilla.kernel.org/show_bug.cgi?id=218195 This issue was brought
> onto the LKML here:
> https://lore.kernel.org/lkml/a106fb4733d0a3f0d6d5792705cdb5cee13731f8.camel@linux.intel.com/T/
> Srinivas would have more info here, but I do not have his email so I can't
> CC him on this.

OK, CCed.

Now, please check the patch I've just posted in this thread:

https://lore.kernel.org/linux-acpi/12457165.O9o76ZdvQC@rjwysocki.net/