linux-kernel - Re: [PATCH v4 2/3] cacheinfo: Add arm64 early level initializer implementation

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Date:   Thu, 13 Apr 2023 15:45:22 +0100
From:   Will Deacon <will@...nel.org>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     Radu Rendec <rrendec@...hat.com>, linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Pierre Gondois <Pierre.Gondois@....com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 2/3] cacheinfo: Add arm64 early level initializer
 implementation

On Thu, Apr 13, 2023 at 11:22:26AM +0100, Sudeep Holla wrote:
> Hi Will,
> 
> On Wed, Apr 12, 2023 at 02:57:58PM -0400, Radu Rendec wrote:
> > This patch adds an architecture specific early cache level detection
> > handler for arm64. This is basically the CLIDR_EL1 based detection that
> > was previously done (only) in init_cache_level().
> > 
> > This is part of a patch series that attempts to further the work in
> > commit 5944ce092b97 ("arch_topology: Build cacheinfo from primary CPU").
> > Previously, in the absence of any DT/ACPI cache info, architecture
> > specific cache detection and info allocation for secondary CPUs would
> > happen in non-preemptible context during early CPU initialization and
> > trigger a "BUG: sleeping function called from invalid context" splat on
> > an RT kernel.
> > 
> > This patch does not solve the problem completely for RT kernels. It
> > relies on the assumption that on most systems, the CPUs are symmetrical
> > and therefore have the same number of cache leaves. The cacheinfo memory
> > is allocated early (on the primary CPU), relying on the new handler. If
> > later (when CLIDR_EL1 based detection runs again on the secondary CPU)
> > the initial assumption proves to be wrong and the CPU has in fact more
> > leaves, the cacheinfo memory is reallocated, and that still triggers a
> > splat on an RT kernel.
> > 
> > In other words, asymmetrical CPU systems *must* still provide cacheinfo
> > data in DT/ACPI to avoid the splat on RT kernels (unless secondary CPUs
> > happen to have less leaves than the primary CPU). But symmetrical CPU
> > systems (the majority) can now get away without the additional DT/ACPI
> > data and rely on CLIDR_EL1 based detection.
> > 
> 
> If you are okay with the change, can I have your Acked-by, so that I can
> route this via Greg's tree ?

I really dislike the profileration of __weak functions in this file,
rather than the usual approach of having arch-specific static inlines in
a header file but it seems that nobody has the appetite to clean that up :(

So I'm fine for Greg to queue this if he wants to, but I'd be a lot more
excited if somebody tidied things up a bit first.

Will