Re: Slow I/O on USB media after commit f664a3cc17b7d0a2bc3b3ab96181e1029b0ec0e6

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

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On Sat, Nov 23, 2019 at 04:44:55PM +0100, Andrea Vai wrote:
> Il giorno sab, 23/11/2019 alle 15.28 +0800, Ming Lei ha scritto:
> > 
> > Please post the log of 'lsusb -v', and I will try to make a patch
> > for
> > addressing the issue.
> 
> attached,

Please apply the attached patch, and re-build & install & reboot kernel.

This time, please don't switch io scheduler.

Thanks,
Ming

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diff --git a/block/blk-mq.c b/block/blk-mq.c
index 5c9adcaa27ac..eecb46020bfb 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1436,7 +1436,13 @@ static void __blk_mq_delay_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async,
 	if (unlikely(blk_mq_hctx_stopped(hctx)))
 		return;
 
-	if (!async && !(hctx->flags & BLK_MQ_F_BLOCKING)) {
+	/*
+	 * Some single-queue devices may need to dispatch IO in order
+	 * which was guaranteed for the legacy queue via the big queue
+	 * lock. Now we reply on single hctx->run_work for that.
+	 */
+	if (!async && !(hctx->flags & (BLK_MQ_F_BLOCKING |
+					BLK_MQ_F_STRICT_DISPATCH_ORDER))) {
 		int cpu = get_cpu();
 		if (cpumask_test_cpu(cpu, hctx->cpumask)) {
 			__blk_mq_run_hw_queue(hctx);
@@ -3042,6 +3048,10 @@ int blk_mq_alloc_tag_set(struct blk_mq_tag_set *set)
 	if (!set->ops->get_budget ^ !set->ops->put_budget)
 		return -EINVAL;
 
+	if (set->queue_depth > 1 && (set->flags &
+				BLK_MQ_F_STRICT_DISPATCH_ORDER))
+		return -EINVAL;
+
 	if (set->queue_depth > BLK_MQ_MAX_DEPTH) {
 		pr_info("blk-mq: reduced tag depth to %u\n",
 			BLK_MQ_MAX_DEPTH);
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index d3d237a09a78..563188844143 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -1939,6 +1939,9 @@ int scsi_mq_setup_tags(struct Scsi_Host *shost)
 	shost->tag_set.flags = BLK_MQ_F_SHOULD_MERGE;
 	shost->tag_set.flags |=
 		BLK_ALLOC_POLICY_TO_MQ_FLAG(shost->hostt->tag_alloc_policy);
+	if (shost->hostt->strict_dispatch_order)
+		shost->tag_set.flags |= BLK_MQ_F_STRICT_DISPATCH_ORDER;
+
 	shost->tag_set.driver_data = shost;
 
 	return blk_mq_alloc_tag_set(&shost->tag_set);
diff --git a/drivers/usb/storage/scsiglue.c b/drivers/usb/storage/scsiglue.c
index 6737fab94959..77795edad8e8 100644
--- a/drivers/usb/storage/scsiglue.c
+++ b/drivers/usb/storage/scsiglue.c
@@ -661,6 +661,18 @@ static const struct scsi_host_template usb_stor_host_template = {
 	/* we do our own delay after a device or bus reset */
 	.skip_settle_delay =		1,
 
+
+	/*
+	 * Some USB storage, such as Kingston Technology DataTraveler 100
+	 * G3/G4/SE9 G2(ID 0951:1666), requires IO dispatched in the
+	 * sequential order, otherwise IO performance may drop drastically.
+	 *
+	 * can_queue is always 1, so we set .strict_dispatch_order for
+	 * USB mass storage HBA. Another reason is that there can be such
+	 * kind of devices too.
+	 */
+	.strict_dispatch_order =	1,
+
 	/* sysfs device attributes */
 	.sdev_attrs =			sysfs_device_attr_list,
 
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index dc03e059fdff..844539690a27 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -388,6 +388,7 @@ struct blk_mq_ops {
 enum {
 	BLK_MQ_F_SHOULD_MERGE	= 1 << 0,
 	BLK_MQ_F_TAG_SHARED	= 1 << 1,
+	BLK_MQ_F_STRICT_DISPATCH_ORDER	= 1 << 2,
 	BLK_MQ_F_BLOCKING	= 1 << 5,
 	BLK_MQ_F_NO_SCHED	= 1 << 6,
 	BLK_MQ_F_ALLOC_POLICY_START_BIT = 8,
diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
index d4452d0ea3c7..f932d6fa1a4c 100644
--- a/include/scsi/scsi_host.h
+++ b/include/scsi/scsi_host.h
@@ -442,6 +442,13 @@ struct scsi_host_template {
 	/* True if the low-level driver supports blk-mq only */
 	unsigned force_blk_mq:1;
 
+	/*
+	 * True if the low-level driver needs IO to be dispatched in
+	 * the order provided by legacy IO path. The flag is only
+	 * valid for single queue device.
+	 */
+	unsigned strict_dispatch_order:1;
+
 	/*
 	 * Countdown for host blocking with no commands outstanding.
 	 */