[PATCH v3 3/7] block: Requeue requests if a CPU is unplugged

[Bart Van Assche <bvanassche@xxxxxxx>]

Requeue requests instead of sending these to the dispatch list if a CPU
is unplugged. This gives the I/O scheduler the chance to preserve the
order of zoned write requests.

Cc: Christoph Hellwig <hch@xxxxxx>
Cc: Damien Le Moal <dlemoal@xxxxxxxxxx>
Cc: Ming Lei <ming.lei@xxxxxxxxxx>
Cc: Mike Snitzer <snitzer@xxxxxxxxxx>
Signed-off-by: Bart Van Assche <bvanassche@xxxxxxx>
---
 block/blk-mq.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 632aee9af60f..bc52a57641e2 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -3497,13 +3497,13 @@ static int blk_mq_hctx_notify_online(unsigned int cpu, struct hlist_node *node)
 }
 
 /*
- * 'cpu' is going away. splice any existing rq_list entries from this
- * software queue to the hw queue dispatch list, and ensure that it
- * gets run.
+ * @cpu is going away. Requeue any existing rq_list entries from its
+ * software queue and run the hw queue associated with @cpu.
  */
 static int blk_mq_hctx_notify_dead(unsigned int cpu, struct hlist_node *node)
 {
 	struct blk_mq_hw_ctx *hctx;
+	struct request *rq, *next;
 	struct blk_mq_ctx *ctx;
 	LIST_HEAD(tmp);
 	enum hctx_type type;
@@ -3525,9 +3525,9 @@ static int blk_mq_hctx_notify_dead(unsigned int cpu, struct hlist_node *node)
 	if (list_empty(&tmp))
 		return 0;
 
-	spin_lock(&hctx->lock);
-	list_splice_tail_init(&tmp, &hctx->dispatch);
-	spin_unlock(&hctx->lock);
+	list_for_each_entry_safe(rq, next, &tmp, queuelist)
+		blk_mq_requeue_request(rq, false);
+	blk_mq_kick_requeue_list(hctx->queue);
 
 	blk_mq_run_hw_queue(hctx, true);
 	return 0;