CONFIG_SCSI_MQ_DEFAULT cause slow direct write speed to usb stick

Hi,

i had found a problem with CONFIG_SCSI_MQ_DEFAULT config option. If it activated, then the write speeds to the /dev/sd* of an usb stick drops
dramatically: it's only about 250 kb/CONFIG_SCSI_MQ_DEFAULTs, but should be about 7 Mb/s. Git bisect also points to commit 24c20f10583647e30afe87b6f6d5e14bc7b1cbc6 'scsi: add a CONFIG_SCSI_MQ_DEFAULT option' (i always set CONFIG_SCSI_MQ_DEFAULT to y, because it sounded interesting). Same problem is also in 3.19 present. Write speeds to a filesystem on that USB-Stick were not so bad, but as i tried to dd in Live-DVD Ubuntu image it was painfully slow. After i disabled CONFIG_SCSI_MQ_DEFAULT write speeds are back to normal.

I created bug report in bugzilla:
https://bugzilla.kernel.org/show_bug.cgi?id=92571

Regards,
Andrej

Perhaps a related datapoint: when reading from a uSD card via a USB dongle in lk 3.19 (SCSI_MQ set), blk_get_request() sometimes returns EAGAIN to the sg driver. I'm not sure that I have seen the SG_IO ioctl return EAGAIN via this route before (i.e. before it can even set up the SCSI command). Arguably doing that breaks the long standing interface of ioctl(sg, SG_IO).

Faster storage devices do not seem to have this problem. The laptop involved has plenty of ram and was lightly loaded. This was a synchronous copy of a slow device onto local storage (a SSD) so it is hard to see why there might be a resource problem. By adjusting my user space code (ddpt and sg_dd) to repeat the ioctl, the copy continues normally.

Here is an example:

# ddpt if=/dev/sg1 bs=512 of=x.bin
1953792+0 records in
1953792+0 records out
1281 retries after EAGAIN error(s) during IO
time to transfer data: 59.678788 secs at 16.76 MB/sec

That is not a bad transfer time but other utilities or drivers might dwell longer on those (nuisance) EAGAINs.

Doug Gilbert

Hi,

my problem was, that write speed to the device is got really slow from 7 Mb/s to 250 Kb/s, so 16.76 Mb/sec is probably the speed limitation of the uSD Card or is close to that. Do you activated blk-mq for scsi in kernel config or in boot parameters?

Regards,
Andrej