linux-kernel - Re: [PATCH v11 3/6] arm64: smp: Remove dedicated wakeup IPI

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Date:   Fri, 25 Aug 2023 17:17:06 -0500
From:   Stephen Boyd <swboyd@...omium.org>
To:     Catalin Marinas <catalin.marinas@....com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Douglas Anderson <dianders@...omium.org>,
        Marc Zyngier <maz@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Sumit Garg <sumit.garg@...aro.org>,
        Will Deacon <will@...nel.org>
Cc:     "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        kgdb-bugreport@...ts.sourceforge.net,
        Lecopzer Chen <lecopzer.chen@...iatek.com>,
        linux-perf-users@...r.kernel.org,
        Masayoshi Mizuma <msys.mizuma@...il.com>,
        Chen-Yu Tsai <wens@...e.org>,
        linux-arm-kernel@...ts.infradead.org, ito-yuichi@...itsu.com,
        Peter Zijlstra <peterz@...radead.org>,
        D Scott Phillips <scott@...amperecomputing.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Philippe Mathieu-Daudé <philmd@...aro.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Valentin Schneider <vschneid@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v11 3/6] arm64: smp: Remove dedicated wakeup IPI

Quoting Douglas Anderson (2023-08-24 08:30:29)
> From: Mark Rutland <mark.rutland@....com>
>
> To enable NMI backtrace and KGDB's NMI cpu roundup, we need to free up
> at least one dedicated IPI.
>
> On arm64 the IPI_WAKEUP IPI is only used for the ACPI parking protocol,
> which itself is only used on some very early ARMv8 systems which
> couldn't implement PSCI.
>
> Remove the IPI_WAKEUP IPI, and rely on the IPI_RESCHEDULE IPI to wake
> CPUs from the parked state. This will cause a tiny amonut of redundant
> work to check the thread flags, but this is miniscule in relation to the
> cost of taking and handling the IPI in the first place. We can safely
> handle redundant IPI_RESCHEDULE IPIs, so there should be no functional
> impact as a result of this change.
>
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Marc Zyngier <maz@...nel.org>
> Cc: Sumit Garg <sumit.garg@...aro.org>
> Cc: Will Deacon <will@...nel.org>
> ---

Reviewed-by: Stephen Boyd <swboyd@...omium.org>