On Tue, Jun 13, 2017 at 2:29 PM, Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> wrote: > On 2017-06-12 15:48:49 [+0200], Niels Kolthoff wrote: >> Hi, >> >> I'm currently testing a RT application that communicates with sensor >> hardware using a tty serial port. The application polls the tty port >> (non-blocking) in a user space thread (priority 80) for incoming data. >> On a raspberry PI3 (ttyAMA0), with kernel 4.4.9-rt17, response times < >> 650usec are achieved, which is fine. >> >> Any version equal or above 4.4.12-rt20 will increase response peaks to >> (about 0.1%) > 2msec, with some peaks > 5msec. >> >> cyclictest doesn't reproduce this, so I suspected a change in the tty >> driver, or related to raspberry pi. >> >> I found that, by reverting the attached patch, response times return >> to < 650usec. I suspect it has something to do with a change in the >> read poll. >> >> I'd like to help solving this issue, can anyone give me some pointers >> on how to continue? > > My guess is that the newly introduced tty_buffer_flush_work() waits > until all in-kernel data is processed before returning -EAGAIN (if the > buffer is empty) for the non-blocking case which unbreaks ssh. So this > wait could blocking you for longer than required. > If that is really it, then this is what could be optimized :) > Ok, thanks, that's valuable input. I'll investigate this and keep u posted. >> Regards, >> Niels Kolthoff > > Sebastian -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
