block: simplify I/O stat accounting

This simplifies I/O stat accounting switching code and separates it
completely from I/O scheduler switch code.

Requests are accounted according to the state of their request queue
at the time of the request allocation. There is no need anymore to
flush the request queue when switching I/O accounting state.

Signed-off-by: Jerome Marchand <[e-mail]>
Signed-off-by: Jens Axboe <[e-mail]>

block/blk-core.c

···
 }
 
 static struct request *
-blk_alloc_request(struct request_queue *q, int rw, int priv, gfp_t gfp_mask)
+blk_alloc_request(struct request_queue *q, int flags, int priv, gfp_t gfp_mask)
 {
 	struct request *rq = mempool_alloc(q->rq.rq_pool, gfp_mask);
 
···
 
 	blk_rq_init(q, rq);
 
-	rq->cmd_flags = rw | REQ_ALLOCED;
+	rq->cmd_flags = flags | REQ_ALLOCED;
 
 	if (priv) {
 		if (unlikely(elv_set_request(q, rq, gfp_mask))) {
···
 	if (priv)
 		rl->elvpriv++;
 
+	if (blk_queue_io_stat(q))
+		rw_flags |= REQ_IO_STAT;
 	spin_unlock_irq(q->queue_lock);
 
 	rq = blk_alloc_request(q, rw_flags, priv, gfp_mask);

block/blk-merge.c

···
 
 	elv_merge_requests(q, req, next);
 
-	blk_account_io_merge(req);
+	/*
+	 * 'next' is going away, so update stats accordingly
+	 */
+	blk_account_io_merge(next);
 
 	req->ioprio = ioprio_best(req->ioprio, next->ioprio);
 	if (blk_rq_cpu_valid(next))

block/blk-sysfs.c

···
 	ssize_t ret = queue_var_store(&stats, page, count);
 
 	spin_lock_irq(q->queue_lock);
-	elv_quiesce_start(q);
-
 	if (stats)
 		queue_flag_set(QUEUE_FLAG_IO_STAT, q);
 	else
 		queue_flag_clear(QUEUE_FLAG_IO_STAT, q);
-
-	elv_quiesce_end(q);
 	spin_unlock_irq(q->queue_lock);
 
 	return ret;

block/blk.h

···
 
 static inline int blk_do_io_stat(struct request *rq)
 {
-	struct gendisk *disk = rq->rq_disk;
-
-	if (!disk || !disk->queue)
-		return 0;
-
-	return blk_queue_io_stat(disk->queue) && (rq->cmd_flags & REQ_ELVPRIV);
+	return rq->rq_disk && blk_rq_io_stat(rq);
 }
 
 #endif

include/linux/blkdev.h

···
 	__REQ_COPY_USER,	/* contains copies of user pages */
 	__REQ_INTEGRITY,	/* integrity metadata has been remapped */
 	__REQ_NOIDLE,		/* Don't anticipate more IO after this one */
+	__REQ_IO_STAT,		/* account I/O stat */
 	__REQ_NR_BITS,		/* stops here */
 };
 
···
 #define REQ_COPY_USER	(1 << __REQ_COPY_USER)
 #define REQ_INTEGRITY	(1 << __REQ_INTEGRITY)
 #define REQ_NOIDLE	(1 << __REQ_NOIDLE)
+#define REQ_IO_STAT	(1 << __REQ_IO_STAT)
 
 #define BLK_MAX_CDB	16
 
···
 				 blk_failfast_transport(rq) ||	\
 				 blk_failfast_driver(rq))
 #define blk_rq_started(rq)	((rq)->cmd_flags & REQ_STARTED)
+#define blk_rq_io_stat(rq)	((rq)->cmd_flags & REQ_IO_STAT)
 
 #define blk_account_rq(rq)	(blk_rq_started(rq) && (blk_fs_request(rq) || blk_discard_rq(rq)))