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

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Message-ID: <Ze9TsQ-qVCZMazfI@arm.com>
Date: Mon, 11 Mar 2024 18:55:45 +0000
From: Catalin Marinas <catalin.marinas@....com>
To: Marek Szyprowski <m.szyprowski@...sung.com>
Cc: "Christoph Lameter (Ampere)" <cl@...two.org>,
	Mark Rutland <mark.rutland@....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 03:01:28PM +0100, Marek Szyprowski wrote:
> On 07.03.2024 02:45, Christoph Lameter (Ampere) wrote:
> > Currently defconfig selects NR_CPUS=256, but some vendors (e.g. Ampere
> > Computing) are planning to ship systems with 512 CPUs. So that all CPUs on
> > these systems can be used with defconfig, we'd like to bump NR_CPUS to 512.
> > Therefore this patch increases the default NR_CPUS from 256 to 512.
> >
> > As increasing NR_CPUS will increase the size of cpumasks, there's a fear that
> > this might have a significant impact on stack usage due to code which places
> > cpumasks on the stack. To mitigate that concern, we can select
> > CPUMASK_OFFSTACK. As that doesn't seem to be a problem today with
> > NR_CPUS=256, we only select this when NR_CPUS > 256.
> >
> > CPUMASK_OFFSTACK configures the cpumasks in the kernel to be
> > dynamically allocated. This was used in the X86 architecture in the
> > past to enable support for larger CPU configurations up to 8k cpus.
[...]
> This patch landed in today's linux-next as commit 0499a78369ad ("ARM64: 
> Dynamically allocate cpumasks and increase supported CPUs to 512"). 
> Unfortunately it triggers the following warning during boot on most of 
> my ARM64-based test boards. Here is an example from Odroid-N2 board:

I spent a big part of this afternoon going through the code paths but
there's nothing obvious that triggered this problem. My suspicion is
some memory corruption, algorithmically I can't see anything that could
go wrong with CPUMASK_OFFSTACK. Unfortunately I could not reproduce it
yet to be able to add some debug info.

So I decided to revert this patch. If we get to the bottom of it during
the merging window, I can still revive it. Otherwise we'll add it to
linux-next post -rc1.

Thanks for reporting it and subsequent debugging.

-- 
Catalin