Re: [PATCH v2 2/3] loop: Select I/O scheduler 'none' from inside add_disk()

[Ming Lei <ming.lei@redhat.com>]

On Tue, Aug 03, 2021 at 11:23:03AM -0700, Bart Van Assche wrote:
> We noticed that the user interface of Android devices becomes very slow
> under memory pressure. This is because Android uses the zram driver on top
> of the loop driver for swapping, because under memory pressure the swap
> code alternates reads and writes quickly, because mq-deadline is the
> default scheduler for loop devices and because mq-deadline delays writes by

Maybe we can bypass io scheduler always for request with REQ_SWAP, such as:

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index 0f006cabfd91..d86709ac9d1f 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -420,7 +420,8 @@ static bool blk_mq_sched_bypass_insert(struct blk_mq_hw_ctx *hctx,
 	 * passthrough request is added to scheduler queue, there isn't any
 	 * chance to dispatch it given we prioritize requests in hctx->dispatch.
 	 */
-	if ((rq->rq_flags & RQF_FLUSH_SEQ) || blk_rq_is_passthrough(rq))
+	if ((rq->rq_flags & RQF_FLUSH_SEQ) || blk_rq_is_passthrough(rq) ||
+			blk_rq_is_swap(rq))
 		return true;
 
 	return false;
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index d3afea47ade6..71aaa99614ab 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -251,6 +251,11 @@ static inline bool blk_rq_is_passthrough(struct request *rq)
 	return blk_op_is_passthrough(req_op(rq));
 }
 
+static inline bool blk_rq_is_swap(struct request *rq)
+{
+	return rq->cmd_flags & REQ_SWAP;
+}
+
 static inline unsigned short req_get_ioprio(struct request *req)
 {
 	return req->ioprio;


Thanks, 
Ming