[PATCH] blk-mq: remap queues when adding/removing hardware queues

[Omar Sandoval]

From: Omar Sandoval <osan...@fb.com>

blk_mq_update_nr_hw_queues() used to remap hardware queues, which is the
behavior that drivers expect. However, commit 4e68a011428a changed
blk_mq_queue_reinit() to not remap queues for the case of CPU
hotplugging, inadvertently making blk_mq_update_nr_hw_queues() not remap
queues as well. This breaks, for example, NBD's multi-connection mode,
leaving the added hardware queues unused. Fix it by making
blk_mq_queue_reinit() optionally remap queues, which we do when updating
the number of hardware queues but not when hotplugging.

Fixes: 4e68a011428a ("blk-mq: don't redistribute hardware queues on a CPU 
hotplug event")
Signed-off-by: Omar Sandoval <osan...@fb.com>
---
The only callers of blk_mq_update_nr_hw_queues() are nbd and nbd. I *think*
nbd_dev_add() also wants this remap behavior.

 block/blk-mq.c | 25 ++++++++++++++++++-------
 1 file changed, 18 insertions(+), 7 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 061fc2cc88d3..1abbf7c83193 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2343,14 +2343,27 @@ void blk_mq_free_queue(struct request_queue *q)
        blk_mq_exit_hw_queues(q, set, set->nr_hw_queues);
 }
 
+static int blk_mq_update_queue_map(struct blk_mq_tag_set *set)
+{
+       if (set->ops->map_queues)
+               return set->ops->map_queues(set);
+       else
+               return blk_mq_map_queues(set);
+}
+
+
 /* Basically redo blk_mq_init_queue with queue frozen */
 static void blk_mq_queue_reinit(struct request_queue *q,
-                               const struct cpumask *online_mask)
+                               const struct cpumask *online_mask,
+                               bool remap_queues)
 {
        WARN_ON_ONCE(!atomic_read(&q->mq_freeze_depth));
 
        blk_mq_sysfs_unregister(q);
 
+       if (remap_queues)
+               blk_mq_update_queue_map(q->tag_set);
+
        /*
         * redo blk_mq_init_cpu_queues and blk_mq_init_hw_queues. FIXME: maybe
         * we should change hctx numa_node according to new topology (this
@@ -2387,7 +2400,7 @@ static void blk_mq_queue_reinit_work(void)
                blk_mq_freeze_queue_wait(q);
 
        list_for_each_entry(q, &all_q_list, all_q_node)
-               blk_mq_queue_reinit(q, &cpuhp_online_new);
+               blk_mq_queue_reinit(q, &cpuhp_online_new, false);
 
        list_for_each_entry(q, &all_q_list, all_q_node)
                blk_mq_unfreeze_queue(q);
@@ -2532,10 +2545,7 @@ int blk_mq_alloc_tag_set(struct blk_mq_tag_set *set)
        if (!set->mq_map)
                goto out_free_tags;
 
-       if (set->ops->map_queues)
-               ret = set->ops->map_queues(set);
-       else
-               ret = blk_mq_map_queues(set);
+       ret = blk_mq_update_queue_map(set);
        if (ret)
                goto out_free_mq_map;
 
@@ -2629,11 +2639,12 @@ void blk_mq_update_nr_hw_queues(struct blk_mq_tag_set 
*set, int nr_hw_queues)
        set->nr_hw_queues = nr_hw_queues;
        list_for_each_entry(q, &set->tag_list, tag_set_list) {
                blk_mq_realloc_hw_ctxs(set, q);
-               blk_mq_queue_reinit(q, cpu_online_mask);
+               blk_mq_queue_reinit(q, cpu_online_mask, true);
        }
 
        list_for_each_entry(q, &set->tag_list, tag_set_list)
                blk_mq_unfreeze_queue(q);
+
 }
 EXPORT_SYMBOL_GPL(blk_mq_update_nr_hw_queues);
 
-- 
2.12.1