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

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Message-ID: <20230413102226.yzzd6iccqeamheas@bogus>
Date:   Thu, 13 Apr 2023 11:22:26 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Radu Rendec <rrendec@...hat.com>, Will Deacon <will@...nel.org>
Cc:     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

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 ?

-- 
Regards,
Sudeep