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Subject[PATCH V10 5/8] blk-mq: introduce .get_budget and .put_budget in blk_mq_ops
Date
For SCSI devices, there is often per-request-queue depth, which need
to be respected before queuing one request.

The current blk-mq always dequeues request first, then calls .queue_rq()
to dispatch the request to lld. One obvious issue of this way is that I/O
merge may not be good, because when the per-request-queue depth can't be
respected, .queue_rq() has to return BLK_STS_RESOURCE, then this request
has to stay in hctx->dispatch list, and never got chance to participate
into I/O merge.

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.
Once the budget for queueing I/O can't be satisfied, we don't need to
dequeue request at all, then I/O merge can get improved a lot.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
block/blk-mq-sched.c | 55 +++++++++++++++++++++++++++++++++++++++++---------
block/blk-mq-sched.h | 2 +-
block/blk-mq.c | 43 ++++++++++++++++++++++++++++++++++-----
block/blk-mq.h | 20 +++++++++++++++++-
include/linux/blk-mq.h | 11 ++++++++++
5 files changed, 114 insertions(+), 17 deletions(-)

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index be29ba849408..8e525e66a0d9 100644
--- 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 9267d0b7c197..2434061cc5b7 100644
--- 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 40cba1b1978f..dcb467369999 100644
--- 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 ef15b3414da5..e413b732374e 100644
--- 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 50c6485cb04f..901457df3d64 100644
--- 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);
@@ -112,6 +114,15 @@ 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
*/
timeout_fn *timeout;
--
2.9.5
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