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

From: Catalin Marinas <catalin.marinas@arm.com>
To: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: "Christoph Lameter (Ampere)" <cl@gentwo.org>,
	Mark Rutland <mark.rutland@arm.com>,
	"linux-pm@vger.kernel.org" <linux-pm@vger.kernel.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Viresh Kumar <vireshk@kernel.org>, Will Deacon <will@kernel.org>,
	Jonathan.Cameron@huawei.com, Matteo.Carlini@arm.com,
	Valentin.Schneider@arm.com, akpm@linux-foundation.org,
	anshuman.khandual@arm.com, Eric Mackay <eric.mackay@oracle.com>,
	dave.kleikamp@oracle.com, linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	linux@armlinux.org.uk, robin.murphy@arm.com,
	vanshikonda@os.amperecomputing.com, yang@os.amperecomputing.com,
	Nishanth Menon <nm@ti.com>, Stephen Boyd <sboyd@kernel.org>
Subject: Re: [PATCH v3] ARM64: Dynamically allocate cpumasks and increase supported CPUs to 512
Date: Mon, 11 Mar 2024 18:55:45 +0000	[thread overview]
Message-ID: <Ze9TsQ-qVCZMazfI@arm.com> (raw)
In-Reply-To: <c1f2902d-cefc-4122-9b86-d1d32911f590@samsung.com>

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