On Sun, 7 Jul 2019, Andrea Vai wrote: > On 03/07/19 10:23:13, Alan Stern wrote: > > > > [...] > > 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. > > Both killed after roughly 20 seconds, using the command > > cat /sys/kernel/debug/usb/usbmon/6u > file > > (my pendrive turns out to be attached to bus #6) > > Hope it helps. 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. Alan Stern
