linux-kernel - [tip: irq/core] genirq: Fix interrupt threads affinity vs. cpuset isolated partitions

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Message-ID: <176347960014.498.6283912824750064104.tip-bot2@tip-bot2>
Date: Tue, 18 Nov 2025 15:26:40 -0000
From: "tip-bot2 for Frederic Weisbecker" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Frederic Weisbecker <frederic@...nel.org>,
 Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
 linux-kernel@...r.kernel.org, maz@...nel.org
Subject: [tip: irq/core] genirq: Fix interrupt threads affinity vs. cpuset
 isolated partitions

The following commit has been merged into the irq/core branch of tip:

Commit-ID:     844dcacab287a46862a244ed413df26ef77400a6
Gitweb:        https://git.kernel.org/tip/844dcacab287a46862a244ed413df26ef77400a6
Author:        Frederic Weisbecker <frederic@...nel.org>
AuthorDate:    Tue, 18 Nov 2025 15:30:51 +01:00
Committer:     Thomas Gleixner <tglx@...utronix.de>
CommitterDate: Tue, 18 Nov 2025 16:19:40 +01:00

genirq: Fix interrupt threads affinity vs. cpuset isolated partitions

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.

Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
Link: https://patch.msgid.link/20251118143052.68778-2-frederic@kernel.org
---
 kernel/irq/manage.c | 26 +++++++++++++-------------
 1 file changed, 13 insertions(+), 13 deletions(-)

diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index c1ce30c..3c55805 100644
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -176,7 +176,7 @@ bool irq_can_set_affinity_usr(unsigned int irq)
 }

 /**
- * irq_set_thread_affinity - Notify irq threads to adjust affinity
+ * irq_thread_update_affinity - Notify irq threads to adjust affinity
  * @desc:	irq descriptor which has affinity changed
  *
  * Just set IRQTF_AFFINITY and delegate the affinity setting to the
@@ -184,7 +184,7 @@ bool irq_can_set_affinity_usr(unsigned int irq)
  * we hold desc->lock and this code can be called from hard interrupt
  * context.
  */
-static void irq_set_thread_affinity(struct irq_desc *desc)
+static void irq_thread_update_affinity(struct irq_desc *desc)
 {
 	struct irqaction *action;

@@ -283,7 +283,7 @@ int irq_do_set_affinity(struct irq_data *data, const struct cpumask *mask,
 		fallthrough;
 	case IRQ_SET_MASK_OK_NOCOPY:
 		irq_validate_effective_affinity(data);
-		irq_set_thread_affinity(desc);
+		irq_thread_update_affinity(desc);
 		ret = 0;
 	}

@@ -1035,8 +1035,16 @@ static void irq_thread_check_affinity(struct irq_desc *desc, struct irqaction *a
 		set_cpus_allowed_ptr(current, mask);
 	free_cpumask_var(mask);
 }
+
+static inline void irq_thread_set_affinity(struct task_struct *t,
+					   struct irq_desc *desc)
+{
+	kthread_bind_mask(t, irq_data_get_effective_affinity_mask(&desc->irq_data));
+}
 #else
 static inline void irq_thread_check_affinity(struct irq_desc *desc, struct irqaction *action) { }
+static inline void irq_thread_set_affinity(struct task_struct *t,
+					   struct irq_desc *desc) { }
 #endif

 static int irq_wait_for_interrupt(struct irq_desc *desc,
@@ -1221,6 +1229,7 @@ static void wake_up_and_wait_for_irq_thread_ready(struct irq_desc *desc,
 	if (!action || !action->thread)
 		return;

+	irq_thread_set_affinity(action->thread, desc);
 	wake_up_process(action->thread);
 	wait_event(desc->wait_for_threads,
 		   test_bit(IRQTF_READY, &action->thread_flags));
@@ -1408,16 +1417,7 @@ 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.
-	 */
-	set_bit(IRQTF_AFFINITY, &new->thread_flags);
+
 	return 0;
 }