linux-kernel - Re: [PATCH v3] ARM64: Dynamically allocate cpumasks and increase supported CPUs to 512

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Message-ID: <Ze71H4AdY786nSvn@FVFF77S0Q05N>
Date: Mon, 11 Mar 2024 12:12:26 +0000
From: Mark Rutland <mark.rutland@....com>
To: "Christoph Lameter (Ampere)" <cl@...ux.com>
Cc: Marek Szyprowski <m.szyprowski@...sung.com>,
	Catalin Marinas <catalin.marinas@....com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Viresh Kumar <vireshk@...nel.org>, Will Deacon <will@...nel.org>,
	Jonathan.Cameron@...wei.com, Matteo.Carlini@....com,
	Valentin.Schneider@....com, akpm@...ux-foundation.org,
	anshuman.khandual@....com, Eric Mackay <eric.mackay@...cle.com>,
	dave.kleikamp@...cle.com, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux@...linux.org.uk, robin.murphy@....com,
	vanshikonda@...amperecomputing.com, yang@...amperecomputing.com,
	Nishanth Menon <nm@...com>, Stephen Boyd <sboyd@...nel.org>
Subject: Re: [PATCH v3] ARM64: Dynamically allocate cpumasks and increase
 supported CPUs to 512

On Fri, Mar 08, 2024 at 09:08:59AM -0800, Christoph Lameter (Ampere) wrote:
> On Fri, 8 Mar 2024, Marek Szyprowski wrote:
> 
> > > > 
> > > > It looks that cpufreq-dt and/or opp drivers needs some adjustments
> > > > related with this change.
> > > That's strange. Is this with defconfig? I wonder whether NR_CPUS being
> > > larger caused the issue with this specific code. Otherwise
> > > CPUMASK_OFFSTACK may not work that well on arm64.
> 
> cpumask handling must use the accessor functions provided in
> include/linux/cpumask.h for declaring and accessing cpumasks. It is likely
> related to the driver opencoding one of the accessors.

I took a look at both the OPP code and the cpufreq-dt code and it looks like
those are doign the right thing w.r.t. cpumask manipulation (i.e. they only use
the cpumask accessors, and use the cpumask_var_*() functions to dynamically
allocate/free cpumasks). Maybe I've missed something, but superficially those
look right.

Marek, can you try reverting this commit and trying defconfig + NR_CPUS=512?
That'll have CPUMASK_OFFSTACK=n, and:

* If that blows up, we know the problem is independent of CPUMASK_OFFSTACK, and
  has something to do with large cpumasks (either a driver bug, or elsewhere).

* If that doesn't blow up, it suggests the problem is related to
  CPUMASK_OFFSTACK rather than with large cpumasks specifically.

Either way, we probably need to revert this patch for now, as this won't have
enough time to soak in linux-next in time for v6.9.

Mark.