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

Andrea Vai
2019-11-26T07:46:16Z

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Il giorno mar, 26/11/2019 alle 10.32 +0800, Ming Lei ha scritto:
> On Mon, Nov 25, 2019 at 07:51:33PM +0100, Andrea Vai wrote:
> > Il giorno lun, 25/11/2019 alle 23.15 +0800, Ming Lei ha scritto:
> > > On Mon, Nov 25, 2019 at 03:58:34PM +0100, Andrea Vai wrote:
> > >
> > > [...]
> > >
> > > > What to try next?
> > >
> > > 1) cat /sys/kernel/debug/block/$DISK/hctx0/flags
> > result:
> >
> > alloc_policy=FIFO SHOULD_MERGE|2
> >
> > >
> > >
> > > 2) echo 128 > /sys/block/$DISK/queue/nr_requests and run your
> copy
> > > 1GB
> > > test again.
> >
> > done, and still fails. What to try next?
>
> I just run 256M cp test

I would like to point out that 256MB is a filesize that usually don't
trigger the issue (don't know if it matters, sorry).

Another info I would provide is about another strange behavior I
noticed: yesterday I ran the test two times (as usual with 1GB
filesize) and took 2370s, 1786s, and a third test was going on when I
stopped it. Then I started another set of 100 trials and let them run
tonight, and the first 10 trials were around 1000s, then gradually
decreased to ~300s, and finally settled around 200s with some trials
below 70-80s. This to say, times are extremely variable and for the
first time I noticed a sort of "performance increase" with time.

> to one USB storage device on patched kernel,
> and WRITE data IO is really in ascending order. The filesystem is
> ext4,
> and mount without '-o sync'. From previous discussion, looks that is
> exactly your test setting. The order can be observed via the
> following script:
>
> #!/bin/sh
> MAJ=$1
> MIN=$2
> MAJ=$(( $MAJ << 20 ))
> DEV=$(( $MAJ | $MIN ))
> /usr/share/bcc/tools/trace -t -C \
> 't:block:block_rq_issue (args->dev == '$DEV') "%s %d %d", args-
> >rwbs, args->sector, args->nr_sector'
>
> $MAJ & $MIN can be retrieved via lsblk for your USB storage disk.
>
> So I think we need to check if the patch is applied correctly first.
>
> If your kernel tree is managed via git,
yes it is,

> please post 'git diff'.
attached. Is it correctly patched? thanks.

> Otherwise, share us your kernel version,
btw, is 5.4.0+

> and I will send you one
> backported patch on the kernel version.
>
> Meantime, you can collect IO order log via the above script as you
> did last
> time, then send us the log.

ok, will try; is it just required to run it for a short period of time
(say, some seconds) during the copy, or should I run it before the
beginning (or before the mount?), and terminate it after the end of
the copy? (Please note that in the latter case a large amount of time
(and data, I suppose) would be involved, because, as said, to be sure
the problem triggers I have to use a large file... but we can try to
better understand and tune this. If it can help, you can get an ods
file with the complete statistic at [1] (look at the "prove_nov19"
sheet)).

Thanks,
Andrea

[1]: http://fisica.unipv.it/transfer/kernelstats.zip

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# git diff
diff --git a/block/blk-mq.c b/block/blk-mq.c
index ec791156e9cc..92d60a5e1d15 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1465,7 +1465,13 @@ 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);
@@ -3055,6 +3061,10 @@ 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 91c007d26c1e..f013630275c9 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -1902,6 +1902,9 @@ 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 54a3c8195c96..df1674d7f0fc 100644
--- a/drivers/usb/storage/scsiglue.c
+++ b/drivers/usb/storage/scsiglue.c
@@ -651,6 +651,18 @@ 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,
diff --git a/block/blk-mq.c b/block/blk-mq.c
index ec791156e9cc..92d60a5e1d15 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1465,7 +1465,13 @@ 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);
@@ -3055,6 +3061,10 @@ 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 91c007d26c1e..f013630275c9 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -1902,6 +1902,9 @@ 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 54a3c8195c96..df1674d7f0fc 100644
--- a/drivers/usb/storage/scsiglue.c
+++ b/drivers/usb/storage/scsiglue.c
@@ -651,6 +651,18 @@ 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 0bf056de5cc3..89b1c28da36a 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -226,6 +226,7 @@ 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 31e0d6ca1eba..dbcbc9ef6993 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.
*/