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

[Aaron Tomlin <atomlin@atomlin.com>]

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