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

[Ming Lei]

On Fri, Nov 08, 2019 at 08:42:53AM +0000, Damien Le Moal wrote:
> On 2019/11/08 4:00, Andrea Vai wrote:
> > [Sorry for the duplicate message, it didn't reach the lists due to
> > html formatting]
> > Il giorno gio 7 nov 2019 alle ore 08:54 Damien Le Moal
> > <Damien.LeMoal@xxxxxxx> ha scritto:
> >>
> >> On 2019/11/07 16:04, Andrea Vai wrote:
> >>> Il giorno mer, 06/11/2019 alle 22.13 +0000, Damien Le Moal ha scritto:
> >>>>
> >>>>
> >>>> Please simply try your write tests after doing this:
> >>>>
> >>>> echo mq-deadline > /sys/block/<name of your USB
> >>>> disk>/queue/scheduler
> >>>>
> >>>> And confirm that mq-deadline is selected with:
> >>>>
> >>>> cat /sys/block/<name of your USB disk>/queue/scheduler
> >>>> [mq-deadline] kyber bfq none
> >>>
> >>> ok, which kernel should I test with this: the fresh git cloned, or the
> >>> one just patched with Alan's patch, or doesn't matter which one?
> >>
> >> Probably all of them to see if there are any differences.
> > 
> > with both kernels, the output of
> > cat /sys/block/sdh/queue/schedule
> > 
> > already contains [mq-deadline]: is it correct to assume that the echo
> > command and the subsequent testing is useless? What to do now?
> 
> Probably, yes. Have you obtained a blktrace of the workload during these
> tests ? Any significant difference in the IO pattern (IO size and
> randomness) and IO timing (any device idle time where the device has no
> command to process) ? Asking because the problem may be above the block
> layer, with the file system for instance.

You may get the IO pattern via the previous trace 

https://lore.kernel.org/linux-usb/20190710024439.GA2621@xxxxxxxxxx/

IMO, if it is related write order, one possibility could be that
the queue lock is killed in .make_request_fn().


Thanks,
Ming