linux-kernel - Re: [PATCH v9 4/7] arm64: smp: Assign and setup the debug IPI

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Date:   Mon, 7 Aug 2023 11:17:45 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     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>,
        linux-perf-users@...r.kernel.org, ito-yuichi@...itsu.com,
        Chen-Yu Tsai <wens@...e.org>, Ard Biesheuvel <ardb@...nel.org>,
        Stephen Boyd <swboyd@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arm-kernel@...ts.infradead.org,
        kgdb-bugreport@...ts.sourceforge.net,
        Masayoshi Mizuma <msys.mizuma@...il.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Lecopzer Chen <lecopzer.chen@...iatek.com>,
        Ben Dooks <ben-linux@...ff.org>,
        Ingo Molnar <mingo@...nel.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Valentin Schneider <vschneid@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v9 4/7] arm64: smp: Assign and setup the debug IPI

On Thu, Jun 01, 2023 at 02:31:48PM -0700, Douglas Anderson wrote:
> From: Sumit Garg <sumit.garg@...aro.org>
> 
> All current arm64 interrupt controllers have at least 8
> IPIs. Currently we are only using 7 of them on arm64. Let's use the
> 8th one as a debug IPI. This uses the new "debug IPI" infrastructure
> which will try to allocate this IPI as an NMI/pseudo NMI if possible.
> 
> Signed-off-by: Sumit Garg <sumit.garg@...aro.org>
> Signed-off-by: Douglas Anderson <dianders@...omium.org>

I think with my suggestion on the prior patch, we don't need the additional
logic here.

> ---
> I could imagine that people object to using up the last free IPI on
> interrupt controllers with only 8 IPIs. However, it shouldn't be a big
> deal. If we later need an extra IPI, it shouldn't be too hard to
> combine some of the existing ones. Presumably we could just get rid of
> the "crash stop" IPI and have the normal "stop" IPI do the crash if
> "waiting_for_crash_ipi" is non-zero

TBH, I'd love to unify the logic for IPI_CPU_STOP and IPI_CPU_CRASH_STOP as
they have a bunch of pointless divergence.

We could also remove IPI_WAKEUP and replace that with something else (e.g.
IPI_CALL_FUNC with an empty function) in order to free up an IPI.

I'd *also* prefer to have separate IPI_CPU_BACKTRACE and IPI_CPU_DEBUG as the
backtrace logic is used for a bunch of things other than KGDB (e.g. RCU stalls,
panic), and that would clearly separate the logic for the two cases.

Thanks,
Mark.

> 
> Changes in v9:
> - Add a warning if we don't have enough IPIs for the NMI IPI
> - Renamed "NMI IPI" to "debug IPI" since it might not be backed by NMI.
> - Update commit description
> 
> Changes in v8:
> - debug_ipi_setup() and debug_ipi_teardown() no longer take cpu param
> 
>  arch/arm64/kernel/smp.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index edd63894d61e..db019b49d3bd 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -53,6 +53,8 @@
>  
>  #include <trace/events/ipi.h>
>  
> +#include "ipi_debug.h"
> +
>  DEFINE_PER_CPU_READ_MOSTLY(int, cpu_number);
>  EXPORT_PER_CPU_SYMBOL(cpu_number);
>  
> @@ -935,6 +937,8 @@ static void ipi_setup(int cpu)
>  
>  	for (i = 0; i < nr_ipi; i++)
>  		enable_percpu_irq(ipi_irq_base + i, 0);
> +
> +	debug_ipi_setup();
>  }
>  
>  #ifdef CONFIG_HOTPLUG_CPU
> @@ -947,6 +951,8 @@ static void ipi_teardown(int cpu)
>  
>  	for (i = 0; i < nr_ipi; i++)
>  		disable_percpu_irq(ipi_irq_base + i);
> +
> +	debug_ipi_teardown();
>  }
>  #endif
>  
> @@ -968,6 +974,11 @@ void __init set_smp_ipi_range(int ipi_base, int n)
>  		irq_set_status_flags(ipi_base + i, IRQ_HIDDEN);
>  	}
>  
> +	if (n > nr_ipi)
> +		set_smp_debug_ipi(ipi_base + nr_ipi);
> +	else
> +		WARN(1, "Not enough IPIs for NMI IPI\n");
> +
>  	ipi_irq_base = ipi_base;
>  
>  	/* Setup the boot CPU immediately */
> -- 
> 2.41.0.rc2.161.g9c6817b8e7-goog
>