From 801afdfbfcd90ff62a4b2469bbda1d958f7a5353 Mon Sep 17 00:00:00 2001 From: Frederic Weisbecker Date: Fri, 21 Nov 2025 15:34:59 +0100 Subject: [PATCH] genirq: Fix interrupt threads affinity vs. cpuset isolated partitions When a cpuset isolated partition is created / updated or destroyed, the interrupt threads are affined blindly to all the non-isolated CPUs. 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-affining, 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. This is done through kthread_bind_mask() by affining them initially to all possible CPUs as at that point the interrupt is not started up which means the affinity of the hard interrupt is not known. The thread will adjust that once it reaches the handler, which is guaranteed to happen after the initial affinity of the hard interrupt is established. Suggested-by: Thomas Gleixner Signed-off-by: Frederic Weisbecker Signed-off-by: Thomas Gleixner Signed-off-by: Ingo Molnar Link: https://patch.msgid.link/20251121143500.42111-3-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 c1ce30c9c3ab6..61da1c68ff82a 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 can not be established yet, but it will be once the + * interrupt is 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 re-affined directly by cpuset or + * housekeeping. The proper way to do it is to re-affine 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; } -- 2.47.3