John Ogness <john.ogness@xxxxxxxxxxxxx> writes: > Hi, > > On 2018-07-06, André Pribil <a.pribil@xxxxxxxxxxxx> wrote: >> are there any plans to improve the latency with serial ttys and RT? >> >> I'm seeing very high latencies (up to 100 ms) when a high priority >> RT thread accesses a serial tty port. I'm working with 4.14.52-rt34 >> on a ARM system. > > As you later point out, this is a problem with the tty subsystem > (kworker) and is not RT-specific. This problem needs to be fixed in > mainline. > >> The reason for this issue seems to be that a kworker thread with a >> normal priority is involved here. When the high priority RT >> thread needs to wait for this kworker thread, it can easily be blocked >> by other lower priority RT threads running in the system. >> >> I have seen that Steven Walter proposed some change in 2015, where >> this kworker thread in replaced by a kthread with a RT priority if >> the low_latency mode is selected. >> See: https://www.spinics.net/lists/linux-serial/msg17782.html >> >> The follow-up discussion in this thread also proposes a solution that >> allows low_latency ports to directly execute the worker. >> As interrupts are already threaded in PREEMPT_RT, wouldn't that be a >> solution? > > The patches and discussions in that thread provide some good ground work > for a real solution. Someone needs to take more time to work with the > maintainers to get it mainline. I am using the following patch. Not sure if it is worth proposing it for mainline inclusion, though. RFC: commit ca633caa8a18e8043f7f59cf9b3b9cbe2097a5ee Author: Esben Haabendal <eha@xxxxxxxx> Date: Fri Jun 1 13:03:41 2018 +0200 tty: Re-introduce annoying historical pain in the butt Since a9c3f68f3cd8d55f809fbdb0c138ed061ea1bd25, use of UART for real-time applications has been problematic. When all CPU cores are running SCHED_FIFO tasks, tty buffers will be stalled, causing unbounded latency of UART RX. For applications with real-time requirements to reaction UART RX, this is of-course unacceptable. This change reuses the old low_latency flag, but only respects it for drivers using threaded interrupt handler. Applications caring about real-time requirements should do that anyway. Signed-off-by: Esben Haabendal <eha@xxxxxxxx> diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c index f8eba1c5412f..eb9ad23341c1 100644 --- a/drivers/tty/tty_buffer.c +++ b/drivers/tty/tty_buffer.c @@ -399,6 +399,7 @@ EXPORT_SYMBOL(__tty_insert_flip_char); void tty_schedule_flip(struct tty_port *port) { struct tty_bufhead *buf = &port->buf; + WARN_ON(port->low_latency); /* paired w/ acquire in flush_to_ldisc(); ensures * flush_to_ldisc() sees buffer data. @@ -408,6 +409,16 @@ void tty_schedule_flip(struct tty_port *port) } EXPORT_SYMBOL(tty_schedule_flip); +static void flush_to_ldisc(struct work_struct *work); + +void tty_do_flip(struct tty_port *port) +{ + struct tty_bufhead *buf = &port->buf; + smp_store_release(&buf->tail->commit, buf->tail->used); + flush_to_ldisc(&buf->work); +} +EXPORT_SYMBOL(tty_do_flip); + /** * tty_prepare_flip_string - make room for characters * @port: tty port @@ -543,7 +554,18 @@ static void flush_to_ldisc(struct work_struct *work) void tty_flip_buffer_push(struct tty_port *port) { - tty_schedule_flip(port); + if (in_interrupt() || !port->low_latency) + /* Schedule buffer flip on workqueue if running in hardirq + context, or if low_latency mode is not enabled. + */ + tty_schedule_flip(port); + else + /* Flip buffer immediately when when low_latency flag is set + and running in threaded interrupt handler. Without this, + latency can increase dramatically when one or more + SCHED_FIFO tasks are hogging the CPU(s). + */ + tty_do_flip(port); } EXPORT_SYMBOL(tty_flip_buffer_push); diff --git a/include/linux/tty_flip.h b/include/linux/tty_flip.h index 767f62086bd9..005443e83b2d 100644 --- a/include/linux/tty_flip.h +++ b/include/linux/tty_flip.h @@ -13,6 +13,7 @@ extern int tty_prepare_flip_string(struct tty_port *port, unsigned char **chars, size_t size); extern void tty_flip_buffer_push(struct tty_port *port); void tty_schedule_flip(struct tty_port *port); +void tty_do_flip(struct tty_port *port); int __tty_insert_flip_char(struct tty_port *port, unsigned char ch, char flag); static inline int tty_insert_flip_char(struct tty_port *port, -- 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
