On 06/08/2015 05:48 PM, Steven Walter wrote: > On Mon, Jun 8, 2015 at 4:15 PM, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote: >> On Mon, Jun 08, 2015 at 03:28:21PM -0400, Steven Walter wrote: >>> Ports that are marked low-latency get their own kthread that is >>> high-priority. >>> >>> Signed-off-by: Steven Walter <stevenrwalter@xxxxxxxxx> >>> --- >>> drivers/tty/tty_buffer.c | 14 +++++++++++++- >>> 1 file changed, 13 insertions(+), 1 deletion(-) >> >> What kind of measurements have you done to show that these changes >> actually matter? > > The application that benefits from this is driving a motor controller > card over an RS-232 link. The userspace application that communicates > over the TTY needs to respond to received bytes within about 1ms. > Without this patch, I have observed (via ftrace) the kworker threads > get delayed by over 100ms, Which is what happens when a high-prio RT thread starves the system (which is effectively a priority inversion with the i/o workers). On a vanilla preemptive kernel, <10us worker latency is typical, and rarely over 250us (though not bounded). Regards, Peter Hurley > meaning the real-time thread of the > userspace application isn't even woken up until 100ms after the byte > is received. With this patch, I have not observed a single instance > of communications failure (i.e., we reliably meet our real-time > deadline). > > Hope that helps > -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
