Re: [PATCH v3 03/16] arm64: Allow IPIs to be handled as normal interrupts

[Vincent Guittot]

Hi Marc,

On Tue, 1 Sep 2020 at 16:44, Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> In order to deal with IPIs as normal interrupts, let's add
> a new way to register them with the architecture code.
>
> set_smp_ipi_range() takes a range of interrupts, and allows
> the arch code to request them as if the were normal interrupts.
> A standard handler is then called by the core IRQ code to deal
> with the IPI.
>
> This means that we don't need to call irq_enter/irq_exit, and
> that we don't need to deal with set_irq_regs either. So let's
> move the dispatcher into its own function, and leave handle_IPI()
> as a compatibility function.
>
> On the sending side, let's make use of ipi_send_mask, which
> already exists for this purpose.
>
> One of the major difference is that we end up, in some cases
> (such as when performing IRQ time accounting on the scheduler
> IPI), end up with nested irq_enter()/irq_exit() pairs.
> Other than the (relatively small) overhead, there should be
> no consequences to it (these pairs are designed to nest
> correctly, and the accounting shouldn't be off).

While rebasing on mainline, I have faced a performance regression for
the benchmark:
perf bench sched pipe
on my arm64 dual quad core (hikey) and my 2 nodes x 112 CPUS (thx2)

The regression comes from:
commit: d3afc7f12987 ("arm64: Allow IPIs to be handled as normal interrupts")

          v5.9              + this patch
hikey :   48818(+/- 0.31)   37503(+/- 0.15%)  -23.2%
thx2  :  132410(+/- 1.72)  122646(+/- 1.92%)   -7.4%

By + this patch,  I mean merging branch from this patch. Whereas
merging the previous:
commit: 83cfac95c018 ("genirq: Allow interrupts to be excluded from
/proc/interrupts")
 It doesn't show any regression

Vincent

>
> Reviewed-by: Valentin Schneider <valentin.schneider@xxxxxxx>
> Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
> ---
>  arch/arm64/Kconfig           |  1 +
>  arch/arm64/include/asm/smp.h |  5 ++
>  arch/arm64/kernel/smp.c      | 93 +++++++++++++++++++++++++++++++-----
>  3 files changed, 87 insertions(+), 12 deletions(-)
>
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index 6d232837cbee..d0fdbe5fb32f 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -106,6 +106,7 @@ config ARM64
>         select GENERIC_CPU_VULNERABILITIES
>         select GENERIC_EARLY_IOREMAP
>         select GENERIC_IDLE_POLL_SETUP
> +       select GENERIC_IRQ_IPI
>         select GENERIC_IRQ_MULTI_HANDLER
>         select GENERIC_IRQ_PROBE
>         select GENERIC_IRQ_SHOW
> diff --git a/arch/arm64/include/asm/smp.h b/arch/arm64/include/asm/smp.h
> index 0eadbf933e35..57c5db15f6b7 100644
> --- a/arch/arm64/include/asm/smp.h
> +++ b/arch/arm64/include/asm/smp.h
> @@ -78,6 +78,11 @@ extern void set_smp_cross_call(void (*)(const struct cpumask *, unsigned int));
>
>  extern void (*__smp_cross_call)(const struct cpumask *, unsigned int);
>
> +/*
> + * Register IPI interrupts with the arch SMP code
> + */
> +extern void set_smp_ipi_range(int ipi_base, int nr_ipi);
> +
>  /*
>   * Called from the secondary holding pen, this is the secondary CPU entry point.
>   */
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index 355ee9eed4dd..00c9db1b61b5 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -75,6 +75,13 @@ enum ipi_msg_type {
>         IPI_WAKEUP
>  };
>
> +static int ipi_irq_base __read_mostly;
> +static int nr_ipi __read_mostly = NR_IPI;
> +static struct irq_desc *ipi_desc[NR_IPI] __read_mostly;
> +
> +static void ipi_setup(int cpu);
> +static void ipi_teardown(int cpu);
> +
>  #ifdef CONFIG_HOTPLUG_CPU
>  static int op_cpu_kill(unsigned int cpu);
>  #else
> @@ -237,6 +244,8 @@ asmlinkage notrace void secondary_start_kernel(void)
>          */
>         notify_cpu_starting(cpu);
>
> +       ipi_setup(cpu);
> +
>         store_cpu_topology(cpu);
>         numa_add_cpu(cpu);
>
> @@ -302,6 +311,7 @@ int __cpu_disable(void)
>          * and we must not schedule until we're ready to give up the cpu.
>          */
>         set_cpu_online(cpu, false);
> +       ipi_teardown(cpu);
>
>         /*
>          * OK - migrate IRQs away from this CPU
> @@ -890,10 +900,9 @@ static void ipi_cpu_crash_stop(unsigned int cpu, struct pt_regs *regs)
>  /*
>   * Main handler for inter-processor interrupts
>   */
> -void handle_IPI(int ipinr, struct pt_regs *regs)
> +static void do_handle_IPI(int ipinr)
>  {
>         unsigned int cpu = smp_processor_id();
> -       struct pt_regs *old_regs = set_irq_regs(regs);
>
>         if ((unsigned)ipinr < NR_IPI) {
>                 trace_ipi_entry_rcuidle(ipi_types[ipinr]);
> @@ -906,21 +915,16 @@ void handle_IPI(int ipinr, struct pt_regs *regs)
>                 break;
>
>         case IPI_CALL_FUNC:
> -               irq_enter();
>                 generic_smp_call_function_interrupt();
> -               irq_exit();
>                 break;
>
>         case IPI_CPU_STOP:
> -               irq_enter();
>                 local_cpu_stop();
> -               irq_exit();
>                 break;
>
>         case IPI_CPU_CRASH_STOP:
>                 if (IS_ENABLED(CONFIG_KEXEC_CORE)) {
> -                       irq_enter();
> -                       ipi_cpu_crash_stop(cpu, regs);
> +                       ipi_cpu_crash_stop(cpu, get_irq_regs());
>
>                         unreachable();
>                 }
> @@ -928,17 +932,13 @@ void handle_IPI(int ipinr, struct pt_regs *regs)
>
>  #ifdef CONFIG_GENERIC_CLOCKEVENTS_BROADCAST
>         case IPI_TIMER:
> -               irq_enter();
>                 tick_receive_broadcast();
> -               irq_exit();
>                 break;
>  #endif
>
>  #ifdef CONFIG_IRQ_WORK
>         case IPI_IRQ_WORK:
> -               irq_enter();
>                 irq_work_run();
> -               irq_exit();
>                 break;
>  #endif
>
> @@ -957,9 +957,78 @@ void handle_IPI(int ipinr, struct pt_regs *regs)
>
>         if ((unsigned)ipinr < NR_IPI)
>                 trace_ipi_exit_rcuidle(ipi_types[ipinr]);
> +}
> +
> +/* Legacy version, should go away once all irqchips have been converted */
> +void handle_IPI(int ipinr, struct pt_regs *regs)
> +{
> +       struct pt_regs *old_regs = set_irq_regs(regs);
> +
> +       irq_enter();
> +       do_handle_IPI(ipinr);
> +       irq_exit();
> +
>         set_irq_regs(old_regs);
>  }
>
> +static irqreturn_t ipi_handler(int irq, void *data)
> +{
> +       do_handle_IPI(irq - ipi_irq_base);
> +       return IRQ_HANDLED;
> +}
> +
> +static void ipi_send(const struct cpumask *target, unsigned int ipi)
> +{
> +       __ipi_send_mask(ipi_desc[ipi], target);
> +}
> +
> +static void ipi_setup(int cpu)
> +{
> +       int i;
> +
> +       if (!ipi_irq_base)
> +               return;
> +
> +       for (i = 0; i < nr_ipi; i++)
> +               enable_percpu_irq(ipi_irq_base + i, 0);
> +}
> +
> +static void ipi_teardown(int cpu)
> +{
> +       int i;
> +
> +       if (!ipi_irq_base)
> +               return;
> +
> +       for (i = 0; i < nr_ipi; i++)
> +               disable_percpu_irq(ipi_irq_base + i);
> +}
> +
> +void __init set_smp_ipi_range(int ipi_base, int n)
> +{
> +       int i;
> +
> +       WARN_ON(n < NR_IPI);
> +       nr_ipi = min(n, NR_IPI);
> +
> +       for (i = 0; i < nr_ipi; i++) {
> +               int err;
> +
> +               err = request_percpu_irq(ipi_base + i, ipi_handler,
> +                                        "IPI", &irq_stat);
> +               WARN_ON(err);
> +
> +               ipi_desc[i] = irq_to_desc(ipi_base + i);
> +               irq_set_status_flags(ipi_base + i, IRQ_HIDDEN);
> +       }
> +
> +       ipi_irq_base = ipi_base;
> +       __smp_cross_call = ipi_send;
> +
> +       /* Setup the boot CPU immediately */
> +       ipi_setup(smp_processor_id());
> +}
> +
>  void smp_send_reschedule(int cpu)
>  {
>         smp_cross_call(cpumask_of(cpu), IPI_RESCHEDULE);
> --
> 2.27.0
>