[RFC/PATCH v2 2/2] block: Add API for urgent request handling

[tlinder]

From: Tatyana Brokhman <tlinder@xxxxxxxxxxxxxx>

This patch add support in block & elevator layers for handling
urgent requests.
Urgent request notification passed to underlying driver (eMMC for
example) and causes interruption of low priority current request in
order to execute the urgent one.

Signed-off-by: Tatyana Brokhman <tlinder@xxxxxxxxxxxxxx>

diff --git a/block/blk-core.c b/block/blk-core.c
index 8881e46..ba11425 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -306,7 +306,23 @@ void __blk_run_queue(struct request_queue *q)
 	if (unlikely(blk_queue_stopped(q)))
 		return;
 
-	q->request_fn(q);
+	/*
+	 * Notify the driver of urgent request pending under the folowing
+	 * conditions:
+	 * 1. There isn't already an urgent request in flight, meaning
+	 * previously notified urgent request completed (!q->notified_urgent)
+	 * 2. The driver and the current scheduler support urgent request
+	 * handling
+	 * 3. There is an urgent request pending in the scheduler
+	 */
+	if (!q->notified_urgent &&
+		q->elevator->type->ops.elevator_is_urgent_fn &&
+	    q->urgent_request_fn &&
+	    q->elevator->type->ops.elevator_is_urgent_fn(q)) {
+		q->urgent_request_fn(q);
+		q->notified_urgent = true;
+	} else
+		q->request_fn(q);
 }
 EXPORT_SYMBOL(__blk_run_queue);
 
@@ -2227,8 +2243,17 @@ struct request *blk_fetch_request(struct request_queue *q)
 	struct request *rq;
 
 	rq = blk_peek_request(q);
-	if (rq)
+	if (rq) {
+		/*
+		 * Assumption: the next request fetched from scheduler after we
+		 * notified "urgent request pending" - will be the urgent one
+		 */
+		if (q->notified_urgent && !q->urgent_req) {
+			q->urgent_req = rq;
+			(void)blk_mark_rq_urgent(rq);
+		}
 		blk_start_request(rq);
+	}
 	return rq;
 }
 EXPORT_SYMBOL(blk_fetch_request);
diff --git a/block/blk-settings.c b/block/blk-settings.c
index 779bb76..8d07e06 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -100,6 +100,18 @@ void blk_queue_lld_busy(struct request_queue *q, lld_busy_fn *fn)
 EXPORT_SYMBOL_GPL(blk_queue_lld_busy);
 
 /**
+ * blk_urgent_request - Set an urgent_request handler function for queue
+ * @q:		queue
+ * @fn:		handler for urgent requests
+ *
+ */
+void blk_urgent_request(struct request_queue *q, request_fn_proc *fn)
+{
+	q->urgent_request_fn = fn;
+}
+EXPORT_SYMBOL(blk_urgent_request);
+
+/**
  * blk_set_default_limits - reset limits to default values
  * @lim:  the queue_limits structure to reset
  *
diff --git a/block/blk.h b/block/blk.h
index ca51543..5fba856 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -42,6 +42,7 @@ void blk_add_timer(struct request *);
  */
 enum rq_atomic_flags {
 	REQ_ATOM_COMPLETE = 0,
+	REQ_ATOM_URGENT = 1,
 };
 
 /*
@@ -58,6 +59,16 @@ static inline void blk_clear_rq_complete(struct request *rq)
 	clear_bit(REQ_ATOM_COMPLETE, &rq->atomic_flags);
 }
 
+static inline int blk_mark_rq_urgent(struct request *rq)
+{
+	return test_and_set_bit(REQ_ATOM_URGENT, &rq->atomic_flags);
+}
+
+static inline void blk_clear_rq_urgent(struct request *rq)
+{
+	clear_bit(REQ_ATOM_URGENT, &rq->atomic_flags);
+}
+
 /*
  * Internal elevator interface
  */
diff --git a/block/elevator.c b/block/elevator.c
index 121a351..dc2bc75 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -743,6 +743,11 @@ void elv_completed_request(struct request_queue *q, struct request *rq)
 {
 	struct elevator_queue *e = q->elevator;
 
+	if (blk_mark_rq_urgent(rq)) {
+		q->notified_urgent = false;
+		q->urgent_req = NULL;
+	}
+	blk_clear_rq_urgent(rq);
 	/*
 	 * request is released from the driver, io must be done
 	 */
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index e725303..962ee54 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -303,6 +303,7 @@ struct request_queue {
 	struct request_list	root_rl;
 
 	request_fn_proc		*request_fn;
+	request_fn_proc		*urgent_request_fn;
 	make_request_fn		*make_request_fn;
 	prep_rq_fn		*prep_rq_fn;
 	unprep_rq_fn		*unprep_rq_fn;
@@ -391,6 +392,8 @@ struct request_queue {
 #endif
 
 	struct queue_limits	limits;
+	bool			notified_urgent;
+	struct request		*urgent_req;
 
 	/*
 	 * sg stuff
@@ -894,6 +897,7 @@ extern struct request_queue *blk_init_queue_node(request_fn_proc *rfn,
 extern struct request_queue *blk_init_queue(request_fn_proc *, spinlock_t *);
 extern struct request_queue *blk_init_allocated_queue(struct request_queue *,
 						      request_fn_proc *, spinlock_t *);
+extern void blk_urgent_request(struct request_queue *q, request_fn_proc *fn);
 extern void blk_cleanup_queue(struct request_queue *);
 extern void blk_queue_make_request(struct request_queue *, make_request_fn *);
 extern void blk_queue_bounce_limit(struct request_queue *, u64);
diff --git a/include/linux/elevator.h b/include/linux/elevator.h
index f70d05d..910f878 100644
--- a/include/linux/elevator.h
+++ b/include/linux/elevator.h
@@ -24,6 +24,8 @@ typedef int (elevator_dispatch_fn) (struct request_queue *, int);
 typedef void (elevator_add_req_fn) (struct request_queue *, struct request *);
 typedef int (elevator_reinsert_req_fn) (struct request_queue *,
 					struct request *);
+typedef bool (elevator_is_urgent_fn) (struct request_queue *);
+
 typedef struct request *(elevator_request_list_fn) (struct request_queue *, struct request *);
 typedef void (elevator_completed_req_fn) (struct request_queue *, struct request *);
 typedef int (elevator_may_queue_fn) (struct request_queue *, int);
@@ -50,6 +52,7 @@ struct elevator_ops
 	elevator_dispatch_fn *elevator_dispatch_fn;
 	elevator_add_req_fn *elevator_add_req_fn;
 	elevator_reinsert_req_fn *elevator_reinsert_req_fn;
+	elevator_is_urgent_fn *elevator_is_urgent_fn;
 
 	elevator_activate_req_fn *elevator_activate_req_fn;
 	elevator_deactivate_req_fn *elevator_deactivate_req_fn;
-- 
1.7.6
--
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