blk-mq: Restart a single queue if tag sets are shared

To improve scalability, if hardware queues are shared, restart
a single hardware queue in round-robin fashion. Rename
blk_mq_sched_restart_queues() to reflect the new semantics.
Remove blk_mq_sched_mark_restart_queue() because this function
has no callers. Remove flag QUEUE_FLAG_RESTART because this
patch removes the code that uses this flag.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Hannes Reinecke <hare@suse.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

4 files changed, 55 insertions, 27 deletions

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index e8c2ed654ef0..c974a1bbf4cb 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -318,25 +318,68 @@ static bool blk_mq_sched_bypass_insert(struct blk_mq_hw_ctx *hctx,
         return true;
 }
 
-static void blk_mq_sched_restart_hctx(struct blk_mq_hw_ctx *hctx)
+static bool blk_mq_sched_restart_hctx(struct blk_mq_hw_ctx *hctx)
 {
         if (test_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state)) {
                 clear_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);
-                if (blk_mq_hctx_has_pending(hctx))
+                if (blk_mq_hctx_has_pending(hctx)) {
                         blk_mq_run_hw_queue(hctx, true);
+                        return true;
+                }
         }
+        return false;
 }
 
-void blk_mq_sched_restart_queues(struct blk_mq_hw_ctx *hctx)
-{
-        struct request_queue *q = hctx->queue;
-        unsigned int i;
+/**
+ * list_for_each_entry_rcu_rr - iterate in a round-robin fashion over rcu list
+ * @pos:    loop cursor.
+ * @skip:   the list element that will not be examined. Iteration starts at
+ *          @skip->next.
+ * @head:   head of the list to examine. This list must have at least one
+ *          element, namely @skip.
+ * @member: name of the list_head structure within typeof(*pos).
+ */
+#define list_for_each_entry_rcu_rr(pos, skip, head, member)             \
+        for ((pos) = (skip);                                            \
+             (pos = (pos)->member.next != (head) ? list_entry_rcu(      \
+                        (pos)->member.next, typeof(*pos), member) :     \
+              list_entry_rcu((pos)->member.next->next, typeof(*pos), member)), \
+             (pos) != (skip); )
 
-        if (test_bit(QUEUE_FLAG_RESTART, &q->queue_flags)) {
-                if (test_and_clear_bit(QUEUE_FLAG_RESTART, &q->queue_flags)) {
-                        queue_for_each_hw_ctx(q, hctx, i)
-                                blk_mq_sched_restart_hctx(hctx);
+/*
+ * Called after a driver tag has been freed to check whether a hctx needs to
+ * be restarted. Restarts @hctx if its tag set is not shared. Restarts hardware
+ * queues in a round-robin fashion if the tag set of @hctx is shared with other
+ * hardware queues.
+ */
+void blk_mq_sched_restart(struct blk_mq_hw_ctx *const hctx)
+{
+        struct blk_mq_tags *const tags = hctx->tags;
+        struct blk_mq_tag_set *const set = hctx->queue->tag_set;
+        struct request_queue *const queue = hctx->queue, *q;
+        struct blk_mq_hw_ctx *hctx2;
+        unsigned int i, j;
+
+        if (set->flags & BLK_MQ_F_TAG_SHARED) {
+                rcu_read_lock();
+                list_for_each_entry_rcu_rr(q, queue, &set->tag_list,
+                                           tag_set_list) {
+                        queue_for_each_hw_ctx(q, hctx2, i)
+                                if (hctx2->tags == tags &&
+                                    blk_mq_sched_restart_hctx(hctx2))
+                                        goto done;
+                }
+                j = hctx->queue_num + 1;
+                for (i = 0; i < queue->nr_hw_queues; i++, j++) {
+                        if (j == queue->nr_hw_queues)
+                                j = 0;
+                        hctx2 = queue->queue_hw_ctx[j];
+                        if (hctx2->tags == tags &&
+                            blk_mq_sched_restart_hctx(hctx2))
+                                break;
                 }
+done:
+                rcu_read_unlock();
         } else {
                 blk_mq_sched_restart_hctx(hctx);
         }
diff --git a/block/blk-mq-sched.h b/block/blk-mq-sched.h
index e704956e0862..3a9e6e40558b 100644
--- a/block/blk-mq-sched.h
+++ b/block/blk-mq-sched.h
@@ -19,7 +19,7 @@ bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio,
                                 struct request **merged_request);
 bool __blk_mq_sched_bio_merge(struct request_queue *q, struct bio *bio);
 bool blk_mq_sched_try_insert_merge(struct request_queue *q, struct request *rq);
-void blk_mq_sched_restart_queues(struct blk_mq_hw_ctx *hctx);
+void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx);
 
 void blk_mq_sched_insert_request(struct request *rq, bool at_head,
                                  bool run_queue, bool async, bool can_block);
@@ -136,20 +136,6 @@ static inline void blk_mq_sched_mark_restart_hctx(struct blk_mq_hw_ctx *hctx)
                 set_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);
 }
 
-/*
- * Mark a hardware queue and the request queue it belongs to as needing a
- * restart.
- */
-static inline void blk_mq_sched_mark_restart_queue(struct blk_mq_hw_ctx *hctx)
-{
-        struct request_queue *q = hctx->queue;
-
-        if (!test_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))
-                set_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);
-        if (!test_bit(QUEUE_FLAG_RESTART, &q->queue_flags))
-                set_bit(QUEUE_FLAG_RESTART, &q->queue_flags);
-}
-
 static inline bool blk_mq_sched_needs_restart(struct blk_mq_hw_ctx *hctx)
 {
         return test_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);
diff --git a/block/blk-mq.c b/block/blk-mq.c
index ad45ae743186..572966f49596 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -348,7 +348,7 @@ void __blk_mq_finish_request(struct blk_mq_hw_ctx *hctx, struct blk_mq_ctx *ctx,
                 blk_mq_put_tag(hctx, hctx->tags, ctx, rq->tag);
         if (sched_tag != -1)
                 blk_mq_sched_completed_request(hctx, rq);
-        blk_mq_sched_restart_queues(hctx);
+        blk_mq_sched_restart(hctx);
         blk_queue_exit(q);
 }
 
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 5a7da607ca04..7548f332121a 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -610,7 +610,6 @@ struct request_queue {
 #define QUEUE_FLAG_FLUSH_NQ    25       /* flush not queueuable */
 #define QUEUE_FLAG_DAX         26       /* device supports DAX */
 #define QUEUE_FLAG_STATS       27       /* track rq completion times */
-#define QUEUE_FLAG_RESTART     28       /* queue needs restart at completion */
 
 #define QUEUE_FLAG_DEFAULT      ((1 << QUEUE_FLAG_IO_STAT) |            \
                                  (1 << QUEUE_FLAG_STACKABLE)    |       \