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

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Date: Mon, 11 Mar 2024 15:56:37 +0100
From: Marek Szyprowski <m.szyprowski@...sung.com>
To: Mark Rutland <mark.rutland@....com>, "Christoph Lameter (Ampere)"
	<cl@...ux.com>
Cc: 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 11.03.2024 13:12, Mark Rutland wrote:
> 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?

Yes, with $subject reverted and CONFIG_NR_CPUS=512 everything works 
fine, so it must be something else broken.

> 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.

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland