Hi Mykyta,
> On 5 Feb 2026, at 14:23, Mykyta Poturai <[email protected]> wrote:
>
> On 04.02.26 16:20, Bertrand Marquis wrote:
>> Hi Mykyta.
>>
>>> On 13 Jan 2026, at 09:45, Mykyta Poturai <[email protected]> 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 <[email protected]>
>>> ---
>>> 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.
>>
>
> The idea is to evenly spread interrupts between CPUs when the new ones
> are being brought online. This is needed to prevent Xen-bound IRQs from
> piling up on CPU 0 when other cores are being offlined and then onlined
> back. It shouldn’t mess with guest affinities, as the code skips
> everything that is assigned to guests and leaves it to be handled by the
> scheduler/vgic. Performance-wise, it should also be okay, as from what
> I’ve seen, there are not many interrupts used by Xen, and I expect CPU
> hotplug operations to be fairly infrequent.
My fear here is a bit that by removing and adding a cpu we will completely
change irq affinities. I am not so sure that those kind of random assignments
are compatible with embedded or safety use cases and here there is no way
to configure this.
@Julien, Stefano and Michal: What do you think here ?
Cheers
Bertrand
>
>>> +
>>> + 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 ?
>>
>
> Yes I will add this in the next version.
>
>> 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
>>
>
> --
> Mykyta
