Re: 4.14: WARNING: CPU: 4 PID: 2895 at block/blk-mq.c:1144 with virtio-blk (also 4.12 stable)

[Christoph Hellwig <hch@lst.de>]

FYI, the patch below changes both the irq and block mappings to
always use the cpu possible map (should be split in two in due time).

I think this is the right way forward.  For every normal machine
those two are the same, but for VMs with maxcpus above their normal
count or some big iron that can grow more cpus it means we waster
a few more resources for the not present but reserved cpus.  It
fixes the reported issue for me:

diff --git a/block/blk-mq-cpumap.c b/block/blk-mq-cpumap.c
index 9f8cffc8a701..3eb169f15842 100644
--- a/block/blk-mq-cpumap.c
+++ b/block/blk-mq-cpumap.c
@@ -16,11 +16,6 @@
 
 static int cpu_to_queue_index(unsigned int nr_queues, const int cpu)
 {
-	/*
-	 * Non present CPU will be mapped to queue index 0.
-	 */
-	if (!cpu_present(cpu))
-		return 0;
 	return cpu % nr_queues;
 }
 
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 11097477eeab..612ce1fb7c4e 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2114,16 +2114,11 @@ static void blk_mq_init_cpu_queues(struct request_queue *q,
 		INIT_LIST_HEAD(&__ctx->rq_list);
 		__ctx->queue = q;
 
-		/* If the cpu isn't present, the cpu is mapped to first hctx */
-		if (!cpu_present(i))
-			continue;
-
-		hctx = blk_mq_map_queue(q, i);
-
 		/*
 		 * Set local node, IFF we have more than one hw queue. If
 		 * not, we remain on the home node of the device
 		 */
+		hctx = blk_mq_map_queue(q, i);
 		if (nr_hw_queues > 1 && hctx->numa_node == NUMA_NO_NODE)
 			hctx->numa_node = local_memory_node(cpu_to_node(i));
 	}
@@ -2180,7 +2175,7 @@ static void blk_mq_map_swqueue(struct request_queue *q)
 	 *
 	 * If the cpu isn't present, the cpu is mapped to first hctx.
 	 */
-	for_each_present_cpu(i) {
+	for_each_possible_cpu(i) {
 		hctx_idx = q->mq_map[i];
 		/* unmapped hw queue can be remapped after CPU topo changed */
 		if (!set->tags[hctx_idx] &&
diff --git a/kernel/irq/affinity.c b/kernel/irq/affinity.c
index e12d35108225..a37a3b4b6342 100644
--- a/kernel/irq/affinity.c
+++ b/kernel/irq/affinity.c
@@ -39,7 +39,7 @@ static void irq_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk,
 	}
 }
 
-static cpumask_var_t *alloc_node_to_present_cpumask(void)
+static cpumask_var_t *alloc_node_to_possible_cpumask(void)
 {
 	cpumask_var_t *masks;
 	int node;
@@ -62,7 +62,7 @@ static cpumask_var_t *alloc_node_to_present_cpumask(void)
 	return NULL;
 }
 
-static void free_node_to_present_cpumask(cpumask_var_t *masks)
+static void free_node_to_possible_cpumask(cpumask_var_t *masks)
 {
 	int node;
 
@@ -71,22 +71,22 @@ static void free_node_to_present_cpumask(cpumask_var_t *masks)
 	kfree(masks);
 }
 
-static void build_node_to_present_cpumask(cpumask_var_t *masks)
+static void build_node_to_possible_cpumask(cpumask_var_t *masks)
 {
 	int cpu;
 
-	for_each_present_cpu(cpu)
+	for_each_possible_cpu(cpu)
 		cpumask_set_cpu(cpu, masks[cpu_to_node(cpu)]);
 }
 
-static int get_nodes_in_cpumask(cpumask_var_t *node_to_present_cpumask,
+static int get_nodes_in_cpumask(cpumask_var_t *node_to_possible_cpumask,
 				const struct cpumask *mask, nodemask_t *nodemsk)
 {
 	int n, nodes = 0;
 
 	/* Calculate the number of nodes in the supplied affinity mask */
 	for_each_node(n) {
-		if (cpumask_intersects(mask, node_to_present_cpumask[n])) {
+		if (cpumask_intersects(mask, node_to_possible_cpumask[n])) {
 			node_set(n, *nodemsk);
 			nodes++;
 		}
@@ -109,7 +109,7 @@ irq_create_affinity_masks(int nvecs, const struct irq_affinity *affd)
 	int last_affv = affv + affd->pre_vectors;
 	nodemask_t nodemsk = NODE_MASK_NONE;
 	struct cpumask *masks;
-	cpumask_var_t nmsk, *node_to_present_cpumask;
+	cpumask_var_t nmsk, *node_to_possible_cpumask;
 
 	/*
 	 * If there aren't any vectors left after applying the pre/post
@@ -125,8 +125,8 @@ irq_create_affinity_masks(int nvecs, const struct irq_affinity *affd)
 	if (!masks)
 		goto out;
 
-	node_to_present_cpumask = alloc_node_to_present_cpumask();
-	if (!node_to_present_cpumask)
+	node_to_possible_cpumask = alloc_node_to_possible_cpumask();
+	if (!node_to_possible_cpumask)
 		goto out;
 
 	/* Fill out vectors at the beginning that don't need affinity */
@@ -135,8 +135,8 @@ irq_create_affinity_masks(int nvecs, const struct irq_affinity *affd)
 
 	/* Stabilize the cpumasks */
 	get_online_cpus();
-	build_node_to_present_cpumask(node_to_present_cpumask);
-	nodes = get_nodes_in_cpumask(node_to_present_cpumask, cpu_present_mask,
+	build_node_to_possible_cpumask(node_to_possible_cpumask);
+	nodes = get_nodes_in_cpumask(node_to_possible_cpumask, cpu_possible_mask,
 				     &nodemsk);
 
 	/*
@@ -146,7 +146,7 @@ irq_create_affinity_masks(int nvecs, const struct irq_affinity *affd)
 	if (affv <= nodes) {
 		for_each_node_mask(n, nodemsk) {
 			cpumask_copy(masks + curvec,
-				     node_to_present_cpumask[n]);
+				     node_to_possible_cpumask[n]);
 			if (++curvec == last_affv)
 				break;
 		}
@@ -160,7 +160,7 @@ irq_create_affinity_masks(int nvecs, const struct irq_affinity *affd)
 		vecs_per_node = (affv - (curvec - affd->pre_vectors)) / nodes;
 
 		/* Get the cpus on this node which are in the mask */
-		cpumask_and(nmsk, cpu_present_mask, node_to_present_cpumask[n]);
+		cpumask_and(nmsk, cpu_possible_mask, node_to_possible_cpumask[n]);
 
 		/* Calculate the number of cpus per vector */
 		ncpus = cpumask_weight(nmsk);
@@ -192,7 +192,7 @@ irq_create_affinity_masks(int nvecs, const struct irq_affinity *affd)
 	/* Fill out vectors at the end that don't need affinity */
 	for (; curvec < nvecs; curvec++)
 		cpumask_copy(masks + curvec, irq_default_affinity);
-	free_node_to_present_cpumask(node_to_present_cpumask);
+	free_node_to_possible_cpumask(node_to_possible_cpumask);
 out:
 	free_cpumask_var(nmsk);
 	return masks;
@@ -214,7 +214,7 @@ int irq_calc_affinity_vectors(int minvec, int maxvec, const struct irq_affinity
 		return 0;
 
 	get_online_cpus();
-	ret = min_t(int, cpumask_weight(cpu_present_mask), vecs) + resv;
+	ret = min_t(int, cpumask_weight(cpu_possible_mask), vecs) + resv;
 	put_online_cpus();
 	return ret;
 }