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

[Sumit Garg]

On Thu, 24 Aug 2023 at 21:03, Douglas Anderson <dianders@xxxxxxxxxxxx> wrote:
>
> From: Mark Rutland <mark.rutland@xxxxxxx>
>
> 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@xxxxxxx>
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Marc Zyngier <maz@xxxxxxxxxx>
> Cc: Sumit Garg <sumit.garg@xxxxxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>

Reviewed-by: Sumit Garg <sumit.garg@xxxxxxxxxx>

-Sumit

> ---
> I have no idea how to test this. I just took Mark's patch and jammed
> it into my series. Logicially the patch seems reasonable to me.
>
> Changes in v11:
> - arch_send_wakeup_ipi() now takes an unsigned int.
>
> Changes in v10:
> - ("arm64: smp: Remove dedicated wakeup IPI") new for v10.
>
>  arch/arm64/include/asm/smp.h              |  4 ++--
>  arch/arm64/kernel/acpi_parking_protocol.c |  2 +-
>  arch/arm64/kernel/smp.c                   | 28 +++++++++--------------
>  3 files changed, 14 insertions(+), 20 deletions(-)
>
> diff --git a/arch/arm64/include/asm/smp.h b/arch/arm64/include/asm/smp.h
> index 9b31e6d0da17..efb13112b408 100644
> --- a/arch/arm64/include/asm/smp.h
> +++ b/arch/arm64/include/asm/smp.h
> @@ -89,9 +89,9 @@ extern void arch_send_call_function_single_ipi(int cpu);
>  extern void arch_send_call_function_ipi_mask(const struct cpumask *mask);
>
>  #ifdef CONFIG_ARM64_ACPI_PARKING_PROTOCOL
> -extern void arch_send_wakeup_ipi_mask(const struct cpumask *mask);
> +extern void arch_send_wakeup_ipi(unsigned int cpu);
>  #else
> -static inline void arch_send_wakeup_ipi_mask(const struct cpumask *mask)
> +static inline void arch_send_wakeup_ipi(unsigned int cpu)
>  {
>         BUILD_BUG();
>  }
> diff --git a/arch/arm64/kernel/acpi_parking_protocol.c b/arch/arm64/kernel/acpi_parking_protocol.c
> index b1990e38aed0..e1be29e608b7 100644
> --- a/arch/arm64/kernel/acpi_parking_protocol.c
> +++ b/arch/arm64/kernel/acpi_parking_protocol.c
> @@ -103,7 +103,7 @@ static int acpi_parking_protocol_cpu_boot(unsigned int cpu)
>                        &mailbox->entry_point);
>         writel_relaxed(cpu_entry->gic_cpu_id, &mailbox->cpu_id);
>
> -       arch_send_wakeup_ipi_mask(cpumask_of(cpu));
> +       arch_send_wakeup_ipi(cpu);
>
>         return 0;
>  }
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 960b98b43506..a5848f1ef817 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -72,7 +72,6 @@ enum ipi_msg_type {
>         IPI_CPU_CRASH_STOP,
>         IPI_TIMER,
>         IPI_IRQ_WORK,
> -       IPI_WAKEUP,
>         NR_IPI
>  };
>
> @@ -764,7 +763,6 @@ static const char *ipi_types[NR_IPI] __tracepoint_string = {
>         [IPI_CPU_CRASH_STOP]    = "CPU stop (for crash dump) interrupts",
>         [IPI_TIMER]             = "Timer broadcast interrupts",
>         [IPI_IRQ_WORK]          = "IRQ work interrupts",
> -       [IPI_WAKEUP]            = "CPU wake-up interrupts",
>  };
>
>  static void smp_cross_call(const struct cpumask *target, unsigned int ipinr);
> @@ -797,13 +795,6 @@ void arch_send_call_function_single_ipi(int cpu)
>         smp_cross_call(cpumask_of(cpu), IPI_CALL_FUNC);
>  }
>
> -#ifdef CONFIG_ARM64_ACPI_PARKING_PROTOCOL
> -void arch_send_wakeup_ipi_mask(const struct cpumask *mask)
> -{
> -       smp_cross_call(mask, IPI_WAKEUP);
> -}
> -#endif
> -
>  #ifdef CONFIG_IRQ_WORK
>  void arch_irq_work_raise(void)
>  {
> @@ -897,14 +888,6 @@ static void do_handle_IPI(int ipinr)
>                 break;
>  #endif
>
> -#ifdef CONFIG_ARM64_ACPI_PARKING_PROTOCOL
> -       case IPI_WAKEUP:
> -               WARN_ONCE(!acpi_parking_protocol_valid(cpu),
> -                         "CPU%u: Wake-up IPI outside the ACPI parking protocol\n",
> -                         cpu);
> -               break;
> -#endif
> -
>         default:
>                 pr_crit("CPU%u: Unknown IPI message 0x%x\n", cpu, ipinr);
>                 break;
> @@ -979,6 +962,17 @@ void arch_smp_send_reschedule(int cpu)
>         smp_cross_call(cpumask_of(cpu), IPI_RESCHEDULE);
>  }
>
> +#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);
> +}
> +#endif
> +
>  #ifdef CONFIG_GENERIC_CLOCKEVENTS_BROADCAST
>  void tick_broadcast(const struct cpumask *mask)
>  {
> --
> 2.42.0.rc1.204.g551eb34607-goog
>