blk-mq: add hctx->restart to blk-mq debugfs

This info is useful to diagnose either IO hang or performance issue
related with blk-mq restart mechanism.

Signed-off-by: Ming Lei <ming.lei@redhat.com>

block/blk-mq-debugfs.c

@@ -593,6 +593,23 @@ static ssize_t hctx_run_write(void *data, const char __user *buf, size_t count,
 	return count;
 }
 
+static int hctx_restart_show(void *data, struct seq_file *m)
+{
+	struct blk_mq_hw_ctx *hctx = data;
+
+	seq_printf(m, "%lu\n", hctx->restart);
+	return 0;
+}
+
+static ssize_t hctx_restart_write(void *data, const char __user *buf,
+				  size_t count, loff_t *ppos)
+{
+	struct blk_mq_hw_ctx *hctx = data;
+
+	hctx->restart = 0;
+	return count;
+}
+
 static int hctx_active_show(void *data, struct seq_file *m)
 {
 	struct blk_mq_hw_ctx *hctx = data;
@@ -765,6 +782,7 @@ static const struct blk_mq_debugfs_attr blk_mq_debugfs_hctx_attrs[] = {
 	{"dispatched", 0600, hctx_dispatched_show, hctx_dispatched_write},
 	{"queued", 0600, hctx_queued_show, hctx_queued_write},
 	{"run", 0600, hctx_run_show, hctx_run_write},
+	{"restart", 0600, hctx_restart_show, hctx_restart_write},
 	{"active", 0400, hctx_active_show},
 	{},
 };

block/blk-mq.c

@@ -1257,6 +1257,8 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
 	 * that is where we will continue on next queue run.
 	 */
 	if (!list_empty(list)) {
+		bool needs_restart;
+
 		spin_lock(&hctx->lock);
 		list_splice_init(list, &hctx->dispatch);
 		spin_unlock(&hctx->lock);
@@ -1281,9 +1283,12 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
 		 *   returning BLK_STS_RESOURCE. Two exceptions are scsi-mq
 		 *   and dm-rq.
 		 */
-		if (!blk_mq_sched_needs_restart(hctx) ||
+		needs_restart = blk_mq_sched_needs_restart(hctx);
+		if (!needs_restart ||
 		    (no_tag && list_empty_careful(&hctx->dispatch_wait.entry)))
 			blk_mq_run_hw_queue(hctx, true);
+		else if (needs_restart)
+			hctx->restart++;
 	}
 
 	return (queued + errors) != 0;

include/linux/blk-mq.h

@@ -44,6 +44,7 @@ struct blk_mq_hw_ctx {
 
 	unsigned long		queued;
 	unsigned long		run;
+	unsigned long		restart;
 #define BLK_MQ_MAX_DISPATCH_ORDER	7
 	unsigned long		dispatched[BLK_MQ_MAX_DISPATCH_ORDER];