Re: [PATCH v5 2/5] arm/irq: Migrate IRQs during CPU up/down operations

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

Hi Mykyta.

> On 13 Jan 2026, at 09:45, Mykyta Poturai <Mykyta_Poturai@xxxxxxxx> wrote:
> 
> Move IRQs from dying CPU to the online ones when a CPU is getting
> offlined. When onlining, rebalance all IRQs in a round-robin fashion.
> Guest-bound IRQs are already handled by scheduler in the process of
> moving vCPUs to active pCPUs, so we only need to handle IRQs used by Xen
> itself.
> 
> Signed-off-by: Mykyta Poturai <mykyta_poturai@xxxxxxxx>
> ---
> v4->v5:
> * handle CPU onlining as well
> * more comments
> * fix crash when ESPI is disabled
> * don't assume CPU 0 is a boot CPU
> * use insigned int for irq number
> * remove assumption that all irqs a bound to CPU 0 by default from the
>  commit message
> 
> v3->v4:
> * patch introduced
> ---
> xen/arch/arm/include/asm/irq.h |  2 ++
> xen/arch/arm/irq.c             | 54 ++++++++++++++++++++++++++++++++++
> xen/arch/arm/smpboot.c         |  6 ++++
> 3 files changed, 62 insertions(+)
> 
> diff --git a/xen/arch/arm/include/asm/irq.h b/xen/arch/arm/include/asm/irq.h
> index 09788dbfeb..a0250bac85 100644
> --- a/xen/arch/arm/include/asm/irq.h
> +++ b/xen/arch/arm/include/asm/irq.h
> @@ -126,6 +126,8 @@ bool irq_type_set_by_domain(const struct domain *d);
> void irq_end_none(struct irq_desc *irq);
> #define irq_end_none irq_end_none
> 
> +void rebalance_irqs(unsigned int from, bool up);
> +
> #endif /* _ASM_HW_IRQ_H */
> /*
>  * Local variables:
> diff --git a/xen/arch/arm/irq.c b/xen/arch/arm/irq.c
> index 7204bc2b68..a32dc729f8 100644
> --- a/xen/arch/arm/irq.c
> +++ b/xen/arch/arm/irq.c
> @@ -158,6 +158,58 @@ static int init_local_irq_data(unsigned int cpu)
>     return 0;
> }
> 
> +static int cpu_next;
> +
> +static void balance_irq(int irq, unsigned int from, bool up)
> +{
> +    struct irq_desc *desc = irq_to_desc(irq);
> +    unsigned long flags;
> +
> +    ASSERT(!cpumask_empty(&cpu_online_map));
> +
> +    spin_lock_irqsave(&desc->lock, flags);
> +    if ( likely(!desc->action) )
> +        goto out;
> +
> +    if ( likely(test_bit(_IRQ_GUEST, &desc->status) ||
> +                test_bit(_IRQ_MOVE_PENDING, &desc->status)) )
> +        goto out;
> +
> +    /*
> +     * Setting affinity to a mask of multiple CPUs causes the GIC drivers to
> +     * select one CPU from that mask. If the dying CPU was included in the 
> IRQ's
> +     * affinity mask, we cannot determine exactly which CPU the interrupt is
> +     * currently routed to, as GIC drivers lack a concrete get_affinity API. 
> So
> +     * to be safe we must reroute it to a new, definitely online, CPU. In the
> +     * case of CPU going down, we move only the interrupt that could reside 
> on
> +     * it. Otherwise, we rearrange all interrupts in a round-robin fashion.
> +     */
> +    if ( !up && !cpumask_test_cpu(from, desc->affinity) )
> +        goto out;

I am a bit lost here on what you are trying to do in the case where
a cpu is coming up here, it feels like you are trying to change the
affinity of all interrupts in this case to cycle everything.
Is it really what is expected ?
If affinity was set by a VM on its interrupts, I would not expect
Xen to round-robin everything each time a cpu comes up.

> +
> +    cpu_next = cpumask_cycle(cpu_next, &cpu_online_map);
> +    irq_set_affinity(desc, cpumask_of(cpu_next));
> +
> +out:
> +    spin_unlock_irqrestore(&desc->lock, flags);
> +}
> +
> +void rebalance_irqs(unsigned int from, bool up)
> +{
> +    int irq;
> +
> +    if ( cpumask_empty(&cpu_online_map) )
> +        return;
> +
> +    for ( irq = NR_LOCAL_IRQS; irq < NR_IRQS; irq++ )
> +        balance_irq(irq, from, up);
> +
> +#ifdef CONFIG_GICV3_ESPI
> +    for ( irq = ESPI_BASE_INTID; irq < ESPI_MAX_INTID; irq++ )
> +        balance_irq(irq, from, up);
> +#endif
> +}
> +
> static int cpu_callback(struct notifier_block *nfb, unsigned long action,
>                         void *hcpu)
> {
> @@ -172,6 +224,8 @@ static int cpu_callback(struct notifier_block *nfb, 
> unsigned long action,
>             printk(XENLOG_ERR "Unable to allocate local IRQ for CPU%u\n",
>                    cpu);
>         break;
> +    case CPU_ONLINE:
> +        rebalance_irqs(cpu, true);
>     }
> 
>     return notifier_from_errno(rc);
> diff --git a/xen/arch/arm/smpboot.c b/xen/arch/arm/smpboot.c
> index 7f3cfa812e..e1b9f94458 100644
> --- a/xen/arch/arm/smpboot.c
> +++ b/xen/arch/arm/smpboot.c
> @@ -425,6 +425,12 @@ void __cpu_disable(void)
> 
>     smp_mb();
> 
> +    /*
> +     * Now that the interrupts are cleared and the CPU marked as offline,
> +     * move interrupts out of it
> +     */
> +    rebalance_irqs(cpu, false);
> +

I would expect this to only be useful when HOTPLUG is enabled, maybe
we could have a static inline doing nothing when HOTPLUG is not on
and only do something if HOTPLUG is enabled here ?

Cheers
Bertrand

>     /* Return to caller; eventually the IPI mechanism will unwind and the 
>      * scheduler will drop to the idle loop, which will call stop_cpu(). */
> }
> -- 
> 2.51.2