[PATCH v9 10/13] blk-mq: use hk cpus only when isolcpus=io_queue is enabled

Aaron Tomlin
2026-03-30T22:11:28Z

From: Daniel Wagner <wagi@kernel.org>

Extend the capabilities of the generic CPU to hardware queue (hctx)
mapping code, so it maps houskeeping CPUs and isolated CPUs to the
hardware queues evenly.

A hctx is only operational when there is at least one online
housekeeping CPU assigned (aka active_hctx). Thus, check the final
mapping that there is no hctx which has only offline housekeeing CPU and
online isolated CPUs.

Example mapping result:

16 online CPUs

isolcpus=io_queue,2-3,6-7,12-13

Queue mapping:
hctx0: default 0 2
hctx1: default 1 3
hctx2: default 4 6
hctx3: default 5 7
hctx4: default 8 12
hctx5: default 9 13
hctx6: default 10
hctx7: default 11
hctx8: default 14
hctx9: default 15

IRQ mapping:
irq 42 affinity 0 effective 0 nvme0q0
irq 43 affinity 0 effective 0 nvme0q1
irq 44 affinity 1 effective 1 nvme0q2
irq 45 affinity 4 effective 4 nvme0q3
irq 46 affinity 5 effective 5 nvme0q4
irq 47 affinity 8 effective 8 nvme0q5
irq 48 affinity 9 effective 9 nvme0q6
irq 49 affinity 10 effective 10 nvme0q7
irq 50 affinity 11 effective 11 nvme0q8
irq 51 affinity 14 effective 14 nvme0q9
irq 52 affinity 15 effective 15 nvme0q10

A corner case is when the number of online CPUs and present CPUs
differ and the driver asks for less queues than online CPUs, e.g.

8 online CPUs, 16 possible CPUs

isolcpus=io_queue,2-3,6-7,12-13
virtio_blk.num_request_queues=2

Queue mapping:
hctx0: default 0 1 2 3 4 5 6 7 8 12 13
hctx1: default 9 10 11 14 15

IRQ mapping
irq 27 affinity 0 effective 0 virtio0-config
irq 28 affinity 0-1,4-5,8 effective 5 virtio0-req.0
irq 29 affinity 9-11,14-15 effective 0 virtio0-req.1

Noteworthy is that for the normal/default configuration (!isoclpus) the
mapping will change for systems which have non hyperthreading CPUs. The
main assignment loop will completely rely that group_mask_cpus_evenly to
do the right thing. The old code would distribute the CPUs linearly over
the hardware context:

queue mapping for /dev/nvme0n1
hctx0: default 0 8
hctx1: default 1 9
hctx2: default 2 10
hctx3: default 3 11
hctx4: default 4 12
hctx5: default 5 13
hctx6: default 6 14
hctx7: default 7 15

The assign each hardware context the map generated by the
group_mask_cpus_evenly function:

queue mapping for /dev/nvme0n1
hctx0: default 0 1
hctx1: default 2 3
hctx2: default 4 5
hctx3: default 6 7
hctx4: default 8 9
hctx5: default 10 11
hctx6: default 12 13
hctx7: default 14 15

In case of hyperthreading CPUs, the resulting map stays the same.

Signed-off-by: Daniel Wagner <wagi@kernel.org>
---
block/blk-mq-cpumap.c | 177 +++++++++++++++++++++++++++++++++++++-----
1 file changed, 158 insertions(+), 19 deletions(-)

diff --git a/block/blk-mq-cpumap.c b/block/blk-mq-cpumap.c
index 8244ecf87835..3b4fa3b291c9 100644
--- a/block/blk-mq-cpumap.c
+++ b/block/blk-mq-cpumap.c
@@ -22,7 +22,18 @@ static unsigned int blk_mq_num_queues(const struct cpumask *mask,
{
unsigned int num;

- num = cpumask_weight(mask);
+ if (housekeeping_enabled(HK_TYPE_IO_QUEUE)) {
+ const struct cpumask *hk_mask;
+ struct cpumask avail_mask;
+
+ hk_mask = housekeeping_cpumask(HK_TYPE_IO_QUEUE);
+ cpumask_and(&avail_mask, mask, hk_mask);
+
+ num = cpumask_weight(&avail_mask);
+ } else {
+ num = cpumask_weight(mask);
+ }
+
return min_not_zero(num, max_queues);
}

@@ -31,9 +42,13 @@ static unsigned int blk_mq_num_queues(const struct cpumask *mask,
*
* Returns an affinity mask that represents the queue-to-CPU mapping
* requested by the block layer based on possible CPUs.
+ * This helper takes isolcpus settings into account.
*/
const struct cpumask *blk_mq_possible_queue_affinity(void)
{
+ if (housekeeping_enabled(HK_TYPE_IO_QUEUE))
+ return housekeeping_cpumask(HK_TYPE_IO_QUEUE);
+
return cpu_possible_mask;
}
EXPORT_SYMBOL_GPL(blk_mq_possible_queue_affinity);
@@ -46,6 +61,14 @@ EXPORT_SYMBOL_GPL(blk_mq_possible_queue_affinity);
*/
const struct cpumask *blk_mq_online_queue_affinity(void)
{
+ /*
+ * Return the stable housekeeping mask if enabled. Callers (e.g.,
+ * the IRQ affinity core) are responsible for safely intersecting
+ * this with a local snapshot of the online mask.
+ */
+ if (housekeeping_enabled(HK_TYPE_IO_QUEUE))
+ return housekeeping_cpumask(HK_TYPE_IO_QUEUE);
+
return cpu_online_mask;
}
EXPORT_SYMBOL_GPL(blk_mq_online_queue_affinity);
@@ -57,7 +80,8 @@ EXPORT_SYMBOL_GPL(blk_mq_online_queue_affinity);
* ignored.
*
* Calculates the number of queues to be used for a multiqueue
- * device based on the number of possible CPUs.
+ * device based on the number of possible CPUs. This helper
+ * takes isolcpus settings into account.
*/
unsigned int blk_mq_num_possible_queues(unsigned int max_queues)
{
@@ -72,7 +96,8 @@ EXPORT_SYMBOL_GPL(blk_mq_num_possible_queues);
* ignored.
*
* Calculates the number of queues to be used for a multiqueue
- * device based on the number of online CPUs.
+ * device based on the number of online CPUs. This helper
+ * takes isolcpus settings into account.
*/
unsigned int blk_mq_num_online_queues(unsigned int max_queues)
{
@@ -80,23 +105,104 @@ unsigned int blk_mq_num_online_queues(unsigned int max_queues)
}
EXPORT_SYMBOL_GPL(blk_mq_num_online_queues);

+static bool blk_mq_validate(struct blk_mq_queue_map *qmap,
+ const struct cpumask *active_hctx)
+{
+ /*
+ * Verify if the mapping is usable when housekeeping
+ * configuration is enabled
+ */
+
+ for (int queue = 0; queue < qmap->nr_queues; queue++) {
+ int cpu;
+
+ if (cpumask_test_cpu(queue, active_hctx)) {
+ /*
+ * This htcx has at least one online CPU thus it
+ * is able to serve any assigned isolated CPU.
+ */
+ continue;
+ }
+
+ /*
+ * There is no housekeeping online CPU for this hctx, all
+ * good as long as all non houskeeping CPUs are also
+ * offline.
+ */
+ for_each_online_cpu(cpu) {
+ if (qmap->mq_map[cpu] != queue)
+ continue;
+
+ pr_warn("Unable to create a usable CPU-to-queue mapping with the given constraints\n");
+ return false;
+ }
+ }
+
+ return true;
+}
+
+static void blk_mq_map_fallback(struct blk_mq_queue_map *qmap)
+{
+ unsigned int cpu;
+
+ /*
+ * Map all CPUs to the first hctx to ensure at least one online
+ * CPU is serving it.
+ */
+ for_each_possible_cpu(cpu)
+ qmap->mq_map[cpu] = 0;
+}
+
void blk_mq_map_queues(struct blk_mq_queue_map *qmap)
{
- const struct cpumask *masks;
+ struct cpumask *masks __free(kfree) = NULL;
+ const struct cpumask *constraint;
unsigned int queue, cpu, nr_masks;
+ cpumask_var_t active_hctx;

- masks = group_cpus_evenly(qmap->nr_queues, &nr_masks);
- if (!masks) {
- for_each_possible_cpu(cpu)
- qmap->mq_map[cpu] = qmap->queue_offset;
- return;
- }
+ if (!zalloc_cpumask_var(&active_hctx, GFP_KERNEL))
+ goto fallback;
+
+ if (housekeeping_enabled(HK_TYPE_IO_QUEUE))
+ constraint = housekeeping_cpumask(HK_TYPE_IO_QUEUE);
+ else
+ constraint = cpu_possible_mask;
+
+ /* Map CPUs to the hardware contexts (hctx) */
+ masks = group_mask_cpus_evenly(qmap->nr_queues, constraint, &nr_masks);
+ if (!masks)
+ goto free_fallback;

for (queue = 0; queue < qmap->nr_queues; queue++) {
- for_each_cpu(cpu, &masks[queue % nr_masks])
- qmap->mq_map[cpu] = qmap->queue_offset + queue;
+ unsigned int idx = (qmap->queue_offset + queue) % nr_masks;
+
+ for_each_cpu(cpu, &masks[idx]) {
+ qmap->mq_map[cpu] = idx;
+
+ if (cpu_online(cpu))
+ cpumask_set_cpu(qmap->mq_map[cpu], active_hctx);
+ }
+ }
+
+ /* Map any unassigned CPU evenly to the hardware contexts (hctx) */
+ queue = cpumask_first(active_hctx);
+ for_each_cpu_andnot(cpu, cpu_possible_mask, constraint) {
+ qmap->mq_map[cpu] = (qmap->queue_offset + queue) % nr_masks;
+ queue = cpumask_next_wrap(queue, active_hctx);
}
- kfree(masks);
+
+ if (!blk_mq_validate(qmap, active_hctx))
+ goto free_fallback;
+
+ free_cpumask_var(active_hctx);
+
+ return;
+
+free_fallback:
+ free_cpumask_var(active_hctx);
+
+fallback:
+ blk_mq_map_fallback(qmap);
}
EXPORT_SYMBOL_GPL(blk_mq_map_queues);

@@ -133,24 +239,57 @@ void blk_mq_map_hw_queues(struct blk_mq_queue_map *qmap,
struct device *dev, unsigned int offset)

{
- const struct cpumask *mask;
+ cpumask_var_t active_hctx, mask;
unsigned int queue, cpu;

if (!dev->bus->irq_get_affinity)
goto fallback;

+ if (!zalloc_cpumask_var(&active_hctx, GFP_KERNEL))
+ goto fallback;
+
+ if (!zalloc_cpumask_var(&mask, GFP_KERNEL)) {
+ free_cpumask_var(active_hctx);
+ goto fallback;
+ }
+
+ /* Map CPUs to the hardware contexts (hctx) */
for (queue = 0; queue < qmap->nr_queues; queue++) {
- mask = dev->bus->irq_get_affinity(dev, queue + offset);
- if (!mask)
- goto fallback;
+ const struct cpumask *affinity_mask;
+
+ affinity_mask = dev->bus->irq_get_affinity(dev, offset + queue);
+ if (!affinity_mask)
+ goto free_fallback;

- for_each_cpu(cpu, mask)
+ for_each_cpu(cpu, affinity_mask) {
qmap->mq_map[cpu] = qmap->queue_offset + queue;
+
+ cpumask_set_cpu(cpu, mask);
+ if (cpu_online(cpu))
+ cpumask_set_cpu(qmap->mq_map[cpu], active_hctx);
+ }
}

+ /* Map any unassigned CPU evenly to the hardware contexts (hctx) */
+ queue = cpumask_first(active_hctx);
+ for_each_cpu_andnot(cpu, cpu_possible_mask, mask) {
+ qmap->mq_map[cpu] = qmap->queue_offset + queue;
+ queue = cpumask_next_wrap(queue, active_hctx);
+ }
+
+ if (!blk_mq_validate(qmap, active_hctx))
+ goto free_fallback;
+
+ free_cpumask_var(active_hctx);
+ free_cpumask_var(mask);
+
return;

+free_fallback:
+ free_cpumask_var(active_hctx);
+ free_cpumask_var(mask);
+
fallback:
- blk_mq_map_queues(qmap);
+ blk_mq_map_fallback(qmap);
}
EXPORT_SYMBOL_GPL(blk_mq_map_hw_queues);
--
2.51.0