linux-kernel - Re: [PATCH v13 3/7] arm64: smp: Remove dedicated wakeup IPI

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Date:   Mon, 25 Sep 2023 17:39:32 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Sumit Garg <sumit.garg@...aro.org>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Marc Zyngier <maz@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Lecopzer Chen <lecopzer.chen@...iatek.com>,
        Chen-Yu Tsai <wens@...e.org>,
        Tomohiro Misono <misono.tomohiro@...itsu.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Masayoshi Mizuma <msys.mizuma@...il.com>,
        Stephane Eranian <eranian@...gle.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        kgdb-bugreport@...ts.sourceforge.net,
        Stephen Boyd <swboyd@...omium.org>,
        linux-perf-users@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>, ito-yuichi@...itsu.com,
        Chen-Yu Tsai <wenst@...omium.org>, jpoimboe@...nel.org,
        keescook@...omium.org, linux-kernel@...r.kernel.org,
        philmd@...aro.org, samitolvanen@...gle.com,
        scott@...amperecomputing.com, vschneid@...hat.com
Subject: Re: [PATCH v13 3/7] arm64: smp: Remove dedicated wakeup IPI

Mark,

On Wed, Sep 6, 2023 at 9:06 AM Douglas Anderson <dianders@...omium.org> wrote:
>
> +#ifdef CONFIG_ARM64_ACPI_PARKING_PROTOCOL
> +void arch_send_wakeup_ipi(unsigned int cpu)
> +{
> +       /*
> +        * We use a scheduler IPI to wake the CPU as this avoids the need for a
> +        * dedicated IPI and we can safely handle spurious scheduler IPIs.
> +        */
> +       arch_smp_send_reschedule(cpu);

I was backporting this to our ChromeOS kernels and our build test bot
noticed that arch_smp_send_reschedule() didn't exist in older kernels.
That's fine--I can always adjust this patch when backporting or
cherry-pick extra patches, but it made me wonder. Is there a reason
you chose to use arch_smp_send_reschedule() directly here instead of
smp_send_reschedule()? I guess the only difference is that you're
bypassing the tracing. Is that on purpose? Should we add a comment
about it, or change this to smp_send_reschedule()?

Thanks!

-Doug