blk-mq: use sbq wait queues instead of restart for driver tags

Commit 50e1dab86aa2 ("blk-mq-sched: fix starvation for multiple hardware
queues and shared tags") fixed one starvation issue for shared tags.
However, we can still get into a situation where we fail to allocate a
tag because all tags are allocated but we don't have any pending
requests on any hardware queue.

One solution for this would be to restart all queues that share a tag
map, but that really sucks. Ideally, we could just block and wait for a
tag, but that isn't always possible from blk_mq_dispatch_rq_list().

However, we can still use the struct sbitmap_queue wait queues with a
custom callback instead of blocking. This has a few benefits:

1. It avoids iterating over all hardware queues when completing an I/O,
   which the current restart code has to do.
2. It benefits from the existing rolling wakeup code.
3. It avoids punting to another thread just to have it block.

Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index b29e7dc7b309e4cf939cd2c011b26b38dfd4df73..9e6b064e533979446a936c45c18f500c6f87725b 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -904,6 +904,44 @@ static bool reorder_tags_to_front(struct list_head *list)
        return first != NULL;
 }
 
+static int blk_mq_dispatch_wake(wait_queue_t *wait, unsigned mode, int flags,
+                               void *key)
+{
+       struct blk_mq_hw_ctx *hctx;
+
+       hctx = container_of(wait, struct blk_mq_hw_ctx, dispatch_wait);
+
+       list_del(&wait->task_list);
+       clear_bit_unlock(BLK_MQ_S_TAG_WAITING, &hctx->state);
+       blk_mq_run_hw_queue(hctx, true);
+       return 1;
+}
+
+static bool blk_mq_dispatch_wait_add(struct blk_mq_hw_ctx *hctx)
+{
+       struct sbq_wait_state *ws;
+
+       /*
+        * The TAG_WAITING bit serves as a lock protecting hctx->dispatch_wait.
+        * The thread which wins the race to grab this bit adds the hardware
+        * queue to the wait queue.
+        */
+       if (test_bit(BLK_MQ_S_TAG_WAITING, &hctx->state) ||
+           test_and_set_bit_lock(BLK_MQ_S_TAG_WAITING, &hctx->state))
+               return false;
+
+       init_waitqueue_func_entry(&hctx->dispatch_wait, blk_mq_dispatch_wake);
+       ws = bt_wait_ptr(&hctx->tags->bitmap_tags, hctx);
+
+       /*
+        * As soon as this returns, it's no longer safe to fiddle with
+        * hctx->dispatch_wait, since a completion can wake up the wait queue
+        * and unlock the bit.
+        */
+       add_wait_queue(&ws->wait, &hctx->dispatch_wait);
+       return true;
+}
+
 bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *hctx, struct list_head *list)
 {
        struct request_queue *q = hctx->queue;
@@ -931,15 +969,22 @@ bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *hctx, struct list_head *list)
                                continue;
 
                        /*
-                        * We failed getting a driver tag. Mark the queue(s)
-                        * as needing a restart. Retry getting a tag again,
-                        * in case the needed IO completed right before we
-                        * marked the queue as needing a restart.
+                        * The initial allocation attempt failed, so we need to
+                        * rerun the hardware queue when a tag is freed.
                         */
-                       blk_mq_sched_mark_restart(hctx);
-                       if (!blk_mq_get_driver_tag(rq, &hctx, false))
+                       if (blk_mq_dispatch_wait_add(hctx)) {
+                               /*
+                                * It's possible that a tag was freed in the
+                                * window between the allocation failure and
+                                * adding the hardware queue to the wait queue.
+                                */
+                               if (!blk_mq_get_driver_tag(rq, &hctx, false))
+                                       break;
+                       } else {
                                break;
+                       }
                }
+
                list_del_init(&rq->queuelist);
 
                bd.rq = rq;
@@ -995,10 +1040,11 @@ bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *hctx, struct list_head *list)
                 *
                 * blk_mq_run_hw_queue() already checks the STOPPED bit
                 *
-                * If RESTART is set, then let completion restart the queue
-                * instead of potentially looping here.
+                * If RESTART or TAG_WAITING is set, then let completion restart
+                * the queue instead of potentially looping here.
                 */
-               if (!blk_mq_sched_needs_restart(hctx))
+               if (!blk_mq_sched_needs_restart(hctx) &&
+                   !test_bit(BLK_MQ_S_TAG_WAITING, &hctx->state))
                        blk_mq_run_hw_queue(hctx, true);
        }
 

diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 8e4df3d6c8cd9dbd1ddd96e25e1abae9a4aa4b6f..001d30d727c56c4d46e5e572ded575ca50cc85ff 100644 (file)
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -33,6 +33,7 @@ struct blk_mq_hw_ctx {
        struct blk_mq_ctx       **ctxs;
        unsigned int            nr_ctx;
 
+       wait_queue_t            dispatch_wait;
        atomic_t                wait_index;
 
        struct blk_mq_tags      *tags;
@@ -160,6 +161,7 @@ enum {
        BLK_MQ_S_STOPPED        = 0,
        BLK_MQ_S_TAG_ACTIVE     = 1,
        BLK_MQ_S_SCHED_RESTART  = 2,
+       BLK_MQ_S_TAG_WAITING    = 3,
 
        BLK_MQ_MAX_DEPTH        = 10240,