Use a dedicated kthread for handling ports marked as low_latency. Since
this thread is RT_FIFO, it is not subject to the same types of
starvation as the normal priority kworker threads.
Some UART applications are very latency sensitive. In at least one case
(motor controller card controlled over RS-232), a reply needs to be sent
within about 1ms after receiving a byte. Without this patch, normal
priority kworker threads can be delayed substantially by userspace
real-time threads. This has been observed to cause latencies up to
100ms. With this patch, no occurences of communications failure were
observed, meaning the 1ms latency is reliably achieved.
Signed-off-by: Steven Walter <stevenrwalter@xxxxxxxxx>
---
drivers/tty/tty_buffer.c | 39 +++++++++++++++++++++++++++++++++++----
drivers/tty/tty_io.c | 1 +
include/linux/tty.h | 3 +++
3 files changed, 39 insertions(+), 4 deletions(-)
diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index ed7b5c8..0560ca7 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -3,6 +3,7 @@
*/
#include <linux/types.h>
+#include <linux/kthread.h>
#include <linux/errno.h>
#include <linux/tty.h>
#include <linux/tty_driver.h>
@@ -11,6 +12,7 @@
#include <linux/string.h>
#include <linux/slab.h>
#include <linux/sched.h>
+#include <linux/sched/prio.h>
#include <linux/wait.h>
#include <linux/bitops.h>
#include <linux/delay.h>
@@ -431,9 +433,8 @@ receive_buf(struct tty_struct *tty, struct tty_buffer *head, int count)
* 'consumer'
*/
-static void flush_to_ldisc(struct work_struct *work)
+static void flush_to_ldisc(struct tty_port *port)
{
- struct tty_port *port = container_of(work, struct tty_port, buf.work);
struct tty_bufhead *buf = &port->buf;
struct tty_struct *tty;
struct tty_ldisc *disc;
@@ -482,6 +483,18 @@ static void flush_to_ldisc(struct work_struct *work)
tty_ldisc_deref(disc);
}
+static void flush_to_ldisc_kthread(struct kthread_work *work)
+{
+ struct tty_port *port = container_of(work, struct tty_port, buf.kwork);
+ flush_to_ldisc(port);
+}
+
+static void flush_to_ldisc_workq(struct work_struct *work)
+{
+ struct tty_port *port = container_of(work, struct tty_port, buf.work);
+ flush_to_ldisc(port);
+}
+
/**
* tty_flush_to_ldisc
* @tty: tty to push
@@ -520,19 +533,36 @@ EXPORT_SYMBOL(tty_flip_buffer_push);
* Must be called before the other tty buffer functions are used.
*/
+static DEFINE_KTHREAD_WORKER(tty_buffer_worker);
void tty_buffer_queue_work(struct tty_port *port)
{
struct tty_bufhead *buf = &port->buf;
- schedule_work(&buf->work);
+ if (port->low_latency)
+ queue_kthread_work(&tty_buffer_worker, &buf->kwork);
+ else
+ schedule_work(&buf->work);
}
void tty_buffer_flush_work(struct tty_port *port)
{
struct tty_bufhead *buf = &port->buf;
+ /* Flush both so we don't have to worry about racing with
+ * changes to port->low_latency */
+ flush_kthread_work(&buf->kwork);
flush_work(&buf->work);
}
+void tty_buffer_init_kthread()
+{
+ struct task_struct *task;
+ /* Use same default priority as threaded irq handlers */
+ struct sched_param param = { .sched_priority = MAX_USER_RT_PRIO/2 };
+
+ task = kthread_run(kthread_worker_fn, &tty_buffer_worker, "tty");
+ sched_setscheduler(task, SCHED_FIFO, ¶m);
+}
+
void tty_buffer_init(struct tty_port *port)
{
struct tty_bufhead *buf = &port->buf;
@@ -544,7 +574,8 @@ void tty_buffer_init(struct tty_port *port)
init_llist_head(&buf->free);
atomic_set(&buf->mem_used, 0);
atomic_set(&buf->priority, 0);
- INIT_WORK(&buf->work, flush_to_ldisc);
+ INIT_WORK(&buf->work, flush_to_ldisc_workq);
+ init_kthread_work(&buf->kwork, flush_to_ldisc_kthread);
buf->mem_limit = TTYB_DEFAULT_MEM_LIMIT;
}
diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index c288473..abcd9a5 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -3602,6 +3602,7 @@ void console_sysfs_notify(void)
*/
int __init tty_init(void)
{
+ tty_buffer_init_kthread();
cdev_init(&tty_cdev, &tty_fops);
if (cdev_add(&tty_cdev, MKDEV(TTYAUX_MAJOR, 0), 1) ||
register_chrdev_region(MKDEV(TTYAUX_MAJOR, 0), 1, "/dev/tty") < 0)
diff --git a/include/linux/tty.h b/include/linux/tty.h
index 7bad787..4dc584d 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -5,6 +5,7 @@
#include <linux/major.h>
#include <linux/termios.h>
#include <linux/workqueue.h>
+#include <linux/kthread.h>
#include <linux/tty_driver.h>
#include <linux/tty_ldisc.h>
#include <linux/mutex.h>
@@ -60,6 +61,7 @@ static inline char *flag_buf_ptr(struct tty_buffer *b, int ofs)
struct tty_bufhead {
struct tty_buffer *head; /* Queue head */
struct work_struct work;
+ struct kthread_work kwork;
struct mutex lock;
atomic_t priority;
struct tty_buffer sentinel;
@@ -448,6 +450,7 @@ extern void tty_buffer_flush(struct tty_struct *tty);
extern void tty_buffer_init(struct tty_port *port);
extern void tty_buffer_queue_work(struct tty_port *port);
extern void tty_buffer_flush_work(struct tty_port *port);
+extern void tty_buffer_init_kthread(void);
extern speed_t tty_termios_baud_rate(struct ktermios *termios);
extern speed_t tty_termios_input_baud_rate(struct ktermios *termios);
extern void tty_termios_encode_baud_rate(struct ktermios *termios,
--
1.9.1
--
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