linux-kernel - Re: [PATCH v8 01/16] ACPI: processor: Simplify initial onlining to use same path for cold and hotplug

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Date: Fri, 26 Apr 2024 19:57:48 +0200
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Jonathan Cameron <Jonathan.Cameron@...wei.com>
Cc: Miguel Luis <miguel.luis@...cle.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Peter Zijlstra <peterz@...radead.org>, 
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>, 
	"loongarch@...ts.linux.dev" <loongarch@...ts.linux.dev>, 
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>, 
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, 
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>, 
	"kvmarm@...ts.linux.dev" <kvmarm@...ts.linux.dev>, "x86@...nel.org" <x86@...nel.org>, 
	Russell King <linux@...linux.org.uk>, "Rafael J . Wysocki" <rafael@...nel.org>, 
	James Morse <james.morse@....com>, Salil Mehta <salil.mehta@...wei.com>, 
	Jean-Philippe Brucker <jean-philippe@...aro.org>, Catalin Marinas <catalin.marinas@....com>, 
	Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>, Hanjun Guo <guohanjun@...wei.com>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "linuxarm@...wei.com" <linuxarm@...wei.com>, 
	"justin.he@....com" <justin.he@....com>, "jianyong.wu@....com" <jianyong.wu@....com>, 
	Lorenzo Pieralisi <lpieralisi@...nel.org>, Sudeep Holla <sudeep.holla@....com>
Subject: Re: [PATCH v8 01/16] ACPI: processor: Simplify initial onlining to
 use same path for cold and hotplug

On Fri, Apr 26, 2024 at 7:49 PM Jonathan Cameron
<Jonathan.Cameron@...wei.com> wrote:
>
> On Fri, 26 Apr 2024 17:21:41 +0000
> Miguel Luis <miguel.luis@...cle.com> wrote:
>
> > Hi Jonathan,
> >
> > > On 26 Apr 2024, at 16:05, Miguel Luis <miguel.luis@...cle.com> wrote:
> > >
> > >
> > >
> > >> On 26 Apr 2024, at 13:51, Jonathan Cameron <Jonathan.Cameron@...wei.com> wrote:
> > >>
> > >> Separate code paths, combined with a flag set in acpi_processor.c to
> > >> indicate a struct acpi_processor was for a hotplugged CPU ensured that
> > >> per CPU data was only set up the first time that a CPU was initialized.
> > >> This appears to be unnecessary as the paths can be combined by letting
> > >> the online logic also handle any CPUs online at the time of driver load.
> > >>
> > >> Motivation for this change, beyond simplification, is that ARM64
> > >> virtual CPU HP uses the same code paths for hotplug and cold path in
> > >> acpi_processor.c so had no easy way to set the flag for hotplug only.
> > >> Removing this necessity will enable ARM64 vCPU HP to reuse the existing
> > >> code paths.
> > >>
> > >> Leave noisy pr_info() in place but update it to not state the CPU
> > >> was hotplugged.
> >
> > On a second thought, do we want to keep it? Can't we just assume that no
> > news is good news while keeping the warn right after __acpi_processor_start ?
>
> Good question - my inclination was to keep this in place for now as removing
> it would remove a source of information people may expect on x86 hotplug.
>
> Then maybe propose dropping it as overly noisy kernel as a follow up
> patch after this series is merged.  Felt like a potential rat hole I didn't
> want to go down if I could avoid it.
>
> If any x86 experts want to shout that no one cares then I'll happily drop
> the print.

You can do that I think and see if anyone complains.  I'm not really
expecting this to happen, though.