blk-mq: introduce .get_budget and .put_budget in blk_mq_ops

For SCSI devices, there is often a per-request-queue depth, which needs
to be respected before queuing one request.

Currently blk-mq always dequeues the request first, then calls
.queue_rq() to dispatch the request to lld. One obvious issue with this
approach is that I/O merging may not be successful, because when the
per-request-queue depth can't be respected, .queue_rq() has to return
BLK_STS_RESOURCE, and then this request has to stay in hctx->dispatch
list. This means it never gets a chance to be merged with other IO.

This patch introduces .get_budget and .put_budget callback in blk_mq_ops,
then we can try to get reserved budget first before dequeuing request.
If the budget for queueing I/O can't be satisfied, we don't need to
dequeue request at all. Hence the request can be left in the IO
scheduler queue, for more merging opportunities.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index be29ba84940852503cf78b855b8da0d2adf6bfa0..8e525e66a0d971aaa50d5b57cd27471f7cc96e6e 100644 (file)
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -89,31 +89,57 @@ static bool blk_mq_sched_restart_hctx(struct blk_mq_hw_ctx *hctx)
        return false;
 }
 
-static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
+/* return true if hctx need to run again */
+static bool blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
 {
        struct request_queue *q = hctx->queue;
        struct elevator_queue *e = q->elevator;
        LIST_HEAD(rq_list);
 
        do {
-               struct request *rq = e->type->ops.mq.dispatch_request(hctx);
+               struct request *rq;
+               blk_status_t ret;
 
-               if (!rq)
+               if (e->type->ops.mq.has_work &&
+                               !e->type->ops.mq.has_work(hctx))
                        break;
+
+               ret = blk_mq_get_dispatch_budget(hctx);
+               if (ret == BLK_STS_RESOURCE)
+                       return true;
+
+               rq = e->type->ops.mq.dispatch_request(hctx);
+               if (!rq) {
+                       blk_mq_put_dispatch_budget(hctx);
+                       break;
+               } else if (ret != BLK_STS_OK) {
+                       blk_mq_end_request(rq, ret);
+                       continue;
+               }
+
+               /*
+                * Now this rq owns the budget which has to be released
+                * if this rq won't be queued to driver via .queue_rq()
+                * in blk_mq_dispatch_rq_list().
+                */
                list_add(&rq->queuelist, &rq_list);
-       } while (blk_mq_dispatch_rq_list(q, &rq_list));
+       } while (blk_mq_dispatch_rq_list(q, &rq_list, true));
+
+       return false;
 }
 
-void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
+/* return true if hw queue need to be run again */
+bool blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
 {
        struct request_queue *q = hctx->queue;
        struct elevator_queue *e = q->elevator;
        const bool has_sched_dispatch = e && e->type->ops.mq.dispatch_request;
        LIST_HEAD(rq_list);
+       bool run_queue = false;
 
        /* RCU or SRCU read lock is needed before checking quiesced flag */
        if (unlikely(blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q)))
-               return;
+               return false;
 
        hctx->run++;
 
@@ -143,14 +169,23 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
         */
        if (!list_empty(&rq_list)) {
                blk_mq_sched_mark_restart_hctx(hctx);
-               if (blk_mq_dispatch_rq_list(q, &rq_list) && has_sched_dispatch)
-                       blk_mq_do_dispatch_sched(hctx);
+               if (blk_mq_dispatch_rq_list(q, &rq_list, false) &&
+                               has_sched_dispatch)
+                       run_queue = blk_mq_do_dispatch_sched(hctx);
        } else if (has_sched_dispatch) {
-               blk_mq_do_dispatch_sched(hctx);
+               run_queue = blk_mq_do_dispatch_sched(hctx);
        } else {
                blk_mq_flush_busy_ctxs(hctx, &rq_list);
-               blk_mq_dispatch_rq_list(q, &rq_list);
+               blk_mq_dispatch_rq_list(q, &rq_list, false);
        }
+
+       if (run_queue && !blk_mq_sched_needs_restart(hctx) &&
+                       !test_bit(BLK_MQ_S_TAG_WAITING, &hctx->state)) {
+               blk_mq_sched_mark_restart_hctx(hctx);
+               return true;
+       }
+
+       return false;
 }
 
 bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio,

diff --git a/block/blk-mq-sched.h b/block/blk-mq-sched.h
index 9267d0b7c1978ae1a01dc218f07c1e16cb6643cc..2434061cc5b75dabf61a4eda7b56483137a840af 100644 (file)
--- a/block/blk-mq-sched.h
+++ b/block/blk-mq-sched.h
@@ -22,7 +22,7 @@ void blk_mq_sched_insert_requests(struct request_queue *q,
                                  struct blk_mq_ctx *ctx,
                                  struct list_head *list, bool run_queue_async);
 
-void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx);
+bool blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx);
 
 int blk_mq_init_sched(struct request_queue *q, struct elevator_type *e);
 void blk_mq_exit_sched(struct request_queue *q, struct elevator_queue *e);

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 40cba1b1978f24fe8e22af5dd1640768f3bf1fd3..dcb467369999e3fbb4cb192e7b853bbe18b04b3f 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1048,7 +1048,8 @@ static bool blk_mq_dispatch_wait_add(struct blk_mq_hw_ctx *hctx)
        return true;
 }
 
-bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list)
+bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
+               bool got_budget)
 {
        struct blk_mq_hw_ctx *hctx;
        struct request *rq;
@@ -1057,6 +1058,8 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list)
        if (list_empty(list))
                return false;
 
+       WARN_ON(!list_is_singular(list) && got_budget);
+
        /*
         * Now process all the entries, sending them to the driver.
         */
@@ -1074,16 +1077,30 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list)
                         * The initial allocation attempt failed, so we need to
                         * rerun the hardware queue when a tag is freed.
                         */
-                       if (!blk_mq_dispatch_wait_add(hctx))
+                       if (!blk_mq_dispatch_wait_add(hctx)) {
+                               if (got_budget)
+                                       blk_mq_put_dispatch_budget(hctx);
                                break;
+                       }
 
                        /*
                         * 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))
+                       if (!blk_mq_get_driver_tag(rq, &hctx, false)) {
+                               if (got_budget)
+                                       blk_mq_put_dispatch_budget(hctx);
+                               break;
+                       }
+               }
+
+               if (!got_budget) {
+                       ret = blk_mq_get_dispatch_budget(hctx);
+                       if (ret == BLK_STS_RESOURCE)
                                break;
+                       if (ret != BLK_STS_OK)
+                               goto fail_rq;
                }
 
                list_del_init(&rq->queuelist);
@@ -1111,6 +1128,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list)
                        break;
                }
 
+ fail_rq:
                if (unlikely(ret != BLK_STS_OK)) {
                        errors++;
                        blk_mq_end_request(rq, BLK_STS_IOERR);
@@ -1169,6 +1187,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list)
 static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
 {
        int srcu_idx;
+       bool run_queue;
 
        /*
         * We should be running this queue from one of the CPUs that
@@ -1185,15 +1204,18 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
 
        if (!(hctx->flags & BLK_MQ_F_BLOCKING)) {
                rcu_read_lock();
-               blk_mq_sched_dispatch_requests(hctx);
+               run_queue = blk_mq_sched_dispatch_requests(hctx);
                rcu_read_unlock();
        } else {
                might_sleep();
 
                srcu_idx = srcu_read_lock(hctx->queue_rq_srcu);
-               blk_mq_sched_dispatch_requests(hctx);
+               run_queue = blk_mq_sched_dispatch_requests(hctx);
                srcu_read_unlock(hctx->queue_rq_srcu, srcu_idx);
        }
+
+       if (run_queue)
+               blk_mq_run_hw_queue(hctx, true);
 }
 
 /*
@@ -1582,6 +1604,13 @@ static void __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
        if (!blk_mq_get_driver_tag(rq, NULL, false))
                goto insert;
 
+       ret = blk_mq_get_dispatch_budget(hctx);
+       if (ret == BLK_STS_RESOURCE) {
+               blk_mq_put_driver_tag(rq);
+               goto insert;
+       } else if (ret != BLK_STS_OK)
+               goto fail_rq;
+
        new_cookie = request_to_qc_t(hctx, rq);
 
        /*
@@ -1598,6 +1627,7 @@ static void __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
                __blk_mq_requeue_request(rq);
                goto insert;
        default:
+ fail_rq:
                *cookie = BLK_QC_T_NONE;
                blk_mq_end_request(rq, ret);
                return;
@@ -2582,6 +2612,9 @@ int blk_mq_alloc_tag_set(struct blk_mq_tag_set *set)
        if (!set->ops->queue_rq)
                return -EINVAL;
 
+       if (!set->ops->get_budget ^ !set->ops->put_budget)
+               return -EINVAL;
+
        if (set->queue_depth > BLK_MQ_MAX_DEPTH) {
                pr_info("blk-mq: reduced tag depth to %u\n",
                        BLK_MQ_MAX_DEPTH);

diff --git a/block/blk-mq.h b/block/blk-mq.h
index ef15b3414da548f4b5c8d2a438d8b52a54cf8e36..e413b732374e6cd045051818e410d17d68eaf922 100644 (file)
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -30,7 +30,7 @@ void blk_mq_freeze_queue(struct request_queue *q);
 void blk_mq_free_queue(struct request_queue *q);
 int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr);
 void blk_mq_wake_waiters(struct request_queue *q);
-bool blk_mq_dispatch_rq_list(struct request_queue *, struct list_head *);
+bool blk_mq_dispatch_rq_list(struct request_queue *, struct list_head *, bool);
 void blk_mq_flush_busy_ctxs(struct blk_mq_hw_ctx *hctx, struct list_head *list);
 bool blk_mq_hctx_has_pending(struct blk_mq_hw_ctx *hctx);
 bool blk_mq_get_driver_tag(struct request *rq, struct blk_mq_hw_ctx **hctx,
@@ -137,4 +137,22 @@ static inline bool blk_mq_hw_queue_mapped(struct blk_mq_hw_ctx *hctx)
 void blk_mq_in_flight(struct request_queue *q, struct hd_struct *part,
                        unsigned int inflight[2]);
 
+static inline void blk_mq_put_dispatch_budget(struct blk_mq_hw_ctx *hctx)
+{
+       struct request_queue *q = hctx->queue;
+
+       if (q->mq_ops->put_budget)
+               q->mq_ops->put_budget(hctx);
+}
+
+static inline blk_status_t blk_mq_get_dispatch_budget(
+               struct blk_mq_hw_ctx *hctx)
+{
+       struct request_queue *q = hctx->queue;
+
+       if (q->mq_ops->get_budget)
+               return q->mq_ops->get_budget(hctx);
+       return BLK_STS_OK;
+}
+
 #endif

diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 50c6485cb04f5d4f306ef794758e280113b5756a..901457df3d64dd74bea45088c5035d502d461f08 100644 (file)
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -90,6 +90,8 @@ struct blk_mq_queue_data {
 
 typedef blk_status_t (queue_rq_fn)(struct blk_mq_hw_ctx *,
                const struct blk_mq_queue_data *);
+typedef blk_status_t (get_budget_fn)(struct blk_mq_hw_ctx *);
+typedef void (put_budget_fn)(struct blk_mq_hw_ctx *);
 typedef enum blk_eh_timer_return (timeout_fn)(struct request *, bool);
 typedef int (init_hctx_fn)(struct blk_mq_hw_ctx *, void *, unsigned int);
 typedef void (exit_hctx_fn)(struct blk_mq_hw_ctx *, unsigned int);
@@ -111,6 +113,15 @@ struct blk_mq_ops {
         */
        queue_rq_fn             *queue_rq;
 
+       /*
+        * Reserve budget before queue request, once .queue_rq is
+        * run, it is driver's responsibility to release the
+        * reserved budget. Also we have to handle failure case
+        * of .get_budget for avoiding I/O deadlock.
+        */
+       get_budget_fn           *get_budget;
+       put_budget_fn           *put_budget;
+
        /*
         * Called on request timeout
         */