Il giorno lun, 08/07/2019 alle 11.38 -0400, Alan Stern ha scritto:
> 
> [...]
>  Andrea, another thing you could try is to collect a usbmon trace
> under 
> > > one of the "slow" kernels.  Follow the instructions in 
> > > Documentation/usb/usbmon.txt.  I think you could kill the file-
> copy 
> > > operation after just a couple of seconds; that should provide
> enough 
> > > trace information to help see what causes the slowdown.
> > > 
> > > (If you want, do the same test with a "fast" kernel and then
> we'll 
> > > compare the results.)
> > > 
> > > Alan Stern
> > > 
> > 
> > Thanks Alan,
> >   so I attach two ouputs, one for a "good" and one for a "bad"
> kernel.
> 
> [...]
> I don't know what the results mean, but I _can_ tell you what's
> happening.  Every so often (at intervals of about a second) the pen
> drive completely stops communicating with the "bad" kernel for about
> one second and then starts up again.
> 
> Here's a short example from the "bad" trace:
> 
> ffff9169f0d399c0 513072808 S Bo:6:008:2 -115 122880 = 00000000
> 00000000 00000000 00000000 00000000 00000000 00000000 00000000
> ffff9169f0d399c0 514262176 C Bo:6:008:2 0 122880 >
> 
> The second column is a timestamp (in microseconds).  This little
> extract shows a 120-KB write starting at time 513.072 and ending at
> 514.262, more than a second later.  Normally such a write would
> complete in about 0.06 s.
> 
> The cumulative effect of all these delays is to slow the transfer 
> drastically.  The "good" kernel trace shows a few delays like this,
> but 
> only one or two.
> 
> I have no idea how commit f664a3cc17b7, getting rid of the legacy
> IO 
> path, could have caused these delays.  It seems more likely that
> the 
> pen drive itself is the cause, perhaps because it is flushing
> buffers 
> more often under the "bad" kernel.
> 
> I'd like you to try doing another pair of usbmon tests.  This time,
> start collecting the usbmon trace _before_ you plug in the pen
> drive,
> and stop the trace shortly after the pen drive has been mounted.  
> Don't try to transfer any data.  Perhaps the two kernels are
> initializing the pen drive with different settings and that accounts
> for the different behaviors.

Thanks Alan, sorry for the delay (I haven't had physical access to the
device for a while). I attach the two new usbmon logs.

Meanwhile, I can try with other pendrives, although I already tried
two more and both behave "bad" (but one is identical to the first, and
the other one is similar).

Thanks, and bye
Andrea