Re: [PATCH 2/3 v3] genirq: Fix interrupt threads affinity vs. cpuset isolated partitions

[Waiman Long <llong@redhat.com>]

On 11/21/25 9:34 AM, Frederic Weisbecker wrote:
> When a cpuset isolated partition is created / updated or destroyed, the
> interrupt threads are affine blindly to all the non-isolated CPUs. And this
> happens without taking into account the interrupt threads initial affinity
> that becomes ignored.
>
> For example in a system with 8 CPUs, if an interrupt and its kthread are
> initially affine to CPU 5, creating an isolated partition with only CPU 2
> inside will eventually end up affining the interrupt kthread to all CPUs
> but CPU 2 (that is CPUs 0,1,3-7), losing the kthread preference for CPU 5.
>
> Besides the blind re-affinity, this doesn't take care of the actual low
> level interrupt which isn't migrated. As of today the only way to isolate
> non managed interrupts, along with their kthreads, is to overwrite their
> affinity separately, for example through /proc/irq/
>
> To avoid doing that manually, future development should focus on updating
> the interrupt's affinity whenever cpuset isolated partitions are updated.
>
> In the meantime, cpuset shouldn't fiddle with interrupt threads directly.
> To prevent from that, set the PF_NO_SETAFFINITY flag to them.
>
> Suggested-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Link: https://patch.msgid.link/20251118143052.68778-2-frederic@kernel.org
> ---
>   kernel/irq/manage.c | 23 +++++++++++++++--------
>   1 file changed, 15 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
> index c1ce30c9c3ab..98b9b8b4de27 100644
> --- a/kernel/irq/manage.c
> +++ b/kernel/irq/manage.c
> @@ -1408,16 +1408,23 @@ setup_irq_thread(struct irqaction *new, unsigned int irq, bool secondary)
>   	 * references an already freed task_struct.
>   	 */
>   	new->thread = get_task_struct(t);
> +
>   	/*
> -	 * Tell the thread to set its affinity. This is
> -	 * important for shared interrupt handlers as we do
> -	 * not invoke setup_affinity() for the secondary
> -	 * handlers as everything is already set up. Even for
> -	 * interrupts marked with IRQF_NO_BALANCE this is
> -	 * correct as we want the thread to move to the cpu(s)
> -	 * on which the requesting code placed the interrupt.
> +	 * The affinity may not yet be available, but it will be once
> +	 * the IRQ will be enabled. Delay and defer the actual setting
> +	 * to the thread itself once it is ready to run. In the meantime,
> +	 * prevent it from ever being reaffined directly by cpuset or
> +	 * housekeeping. The proper way to do it is to reaffine the whole
> +	 * vector.
>   	 */
> -	set_bit(IRQTF_AFFINITY, &new->thread_flags);
> +	kthread_bind_mask(t, cpu_possible_mask);
> +
> +	/*
> +	 * Ensure the thread adjusts the affinity once it reaches the
> +	 * thread function.
> +	 */
> +	new->thread_flags = BIT(IRQTF_AFFINITY);
> +
>   	return 0;
>   }
>   

LGTM

Acked-by: Waiman Long <longman@redhat.com>