[PATCH v14 7/8] genirq/affinity: Restrict managed IRQ affinity to housekeeping CPUs

[Aaron Tomlin]

At present, the managed interrupt spreading algorithm distributes vectors
across all available CPUs within a given node or system. On systems
employing CPU isolation (e.g., "isolcpus=io_queue"), this behaviour
defeats the primary purpose of isolation by routing hardware interrupts
(such as NVMe completion queues) directly to isolated cores.

Update irq_create_affinity_masks() to respect the housekeeping CPU mask.
By passing the HK_TYPE_IO_QUEUE mask directly to the topological
distribution function (group_mask_cpus_evenly()), we ensure that managed
interrupts are kept strictly off isolated CPUs.

This patch additionally addresses the architectural constraints of
restricted vector distribution:

    1.  Vector Limits and Overrides: Updated irq_calc_affinity_vectors()
        to strictly bound the maximum number of allocated vectors to the
        weight of the housekeeping mask. This correctly overrides
        drivers providing a calc_sets() callback, preventing them from
        wasting memory on dead hardware queues that cannot be routed to
        isolated CPUs.

    2.  Multi-set Alignment and Leak Prevention: When isolation
        constraints result in fewer available masks than requested
        vectors for a given set, the remaining vector slots are padded
        with the housekeeping mask. This replaces the historical
        irq_default_affinity padding, ensuring excess managed queues do
        not leak interrupts onto isolated CPUs.

    3.  Minimum Vector Safety Net: To prevent fatal -ENOSPC device probe
        aborts on heavily isolated systems (where the housekeeping CPU
        count might be lower than a device's structural minimum), the
        final vector calculation is safeguarded to never drop below
        minvec. Queues will safely share the available housekeeping CPUs
        instead of failing the probe.

    4.  Zero Overhead: The housekeeping mask is conditionally assigned
        via a direct pointer, completely avoiding temporary mask
        allocations (e.g., alloc_cpumask_var) and bitwise operations
        when CPU isolation is disabled. This guarantees zero performance
        or memory overhead for standard configurations.

Signed-off-by: Aaron Tomlin <atomlin@xxxxxxxxxxx>
---
 kernel/irq/affinity.c | 31 +++++++++++++++++++++++--------
 1 file changed, 23 insertions(+), 8 deletions(-)

diff --git a/kernel/irq/affinity.c b/kernel/irq/affinity.c
index 78f2418a8925..dade92f8b4b3 100644
--- a/kernel/irq/affinity.c
+++ b/kernel/irq/affinity.c
@@ -8,6 +8,7 @@
 #include <linux/slab.h>
 #include <linux/cpu.h>
 #include <linux/group_cpus.h>
+#include <linux/sched/isolation.h>
 
 static void default_calc_sets(struct irq_affinity *affd, unsigned int affvecs)
 {
@@ -25,8 +26,10 @@ static void default_calc_sets(struct irq_affinity *affd, unsigned int affvecs)
 struct irq_affinity_desc *
 irq_create_affinity_masks(unsigned int nvecs, struct irq_affinity *affd)
 {
-	unsigned int affvecs, curvec, usedvecs, i;
+	unsigned int affvecs, curvec, usedvecs, i, j;
 	struct irq_affinity_desc *masks = NULL;
+	const struct cpumask *hk_mask = housekeeping_cpumask(HK_TYPE_IO_QUEUE);
+	bool hk_enabled = housekeeping_enabled(HK_TYPE_IO_QUEUE);
 
 	/*
 	 * Determine the number of vectors which need interrupt affinities
@@ -70,19 +73,29 @@ irq_create_affinity_masks(unsigned int nvecs, struct irq_affinity *affd)
 	 */
 	for (i = 0, usedvecs = 0; i < affd->nr_sets; i++) {
 		unsigned int nr_masks, this_vecs = affd->set_size[i];
-		struct cpumask *result = group_cpus_evenly(this_vecs, &nr_masks);
+		struct cpumask *result;
+		const struct cpumask *mask;
 
+		if (hk_enabled)
+			mask = hk_mask;
+		else
+			mask = cpu_possible_mask;
+
+		result = group_mask_cpus_evenly(this_vecs, mask,
+						&nr_masks);
 		if (!result) {
 			kfree(masks);
 			return NULL;
 		}
-
-		for (int j = 0; j < nr_masks; j++)
+		for (j = 0; j < nr_masks; j++)
 			cpumask_copy(&masks[curvec + j].mask, &result[j]);
+		for (j = nr_masks; j < this_vecs; j++)
+			cpumask_copy(&masks[curvec + j].mask, mask);
+
 		kfree(result);
 
-		curvec += nr_masks;
-		usedvecs += nr_masks;
+		curvec += this_vecs;
+		usedvecs += this_vecs;
 	}
 
 	/* Fill out vectors at the end that don't need affinity */
@@ -115,10 +128,12 @@ unsigned int irq_calc_affinity_vectors(unsigned int minvec, unsigned int maxvec,
 	if (resv > minvec)
 		return 0;
 
-	if (affd->calc_sets)
+	if (housekeeping_enabled(HK_TYPE_IO_QUEUE))
+		set_vecs = cpumask_weight(housekeeping_cpumask(HK_TYPE_IO_QUEUE));
+	else if (affd->calc_sets)
 		set_vecs = maxvec - resv;
 	else
 		set_vecs = cpumask_weight(cpu_possible_mask);
 
-	return resv + min(set_vecs, maxvec - resv);
+	return max(minvec, resv + min(set_vecs, maxvec - resv));
 }
-- 
2.51.0