Re: [PATCH v5] tty: tty_port: add workqueue to flip tty buffer

From: Jiri Slaby
Date: Mon Dec 08 2025 - 02:28:39 EST

Hi,

On 05. 12. 25, 4:08, Xin Zhao wrote:
On the embedded platform, certain critical data, such as IMU data, is
transmitted through UART. The tty_flip_buffer_push interface in the TTY

In commit logs, we tend to add () after function names.

layer uses system_unbound_wq to handle the flipping of the TTY buffer.
Although the unbound workqueue can create new threads on demand and wake
up the kworker thread on an idle CPU, it may be preeempted by real-time

Too many 'e's in preeempted :).

tasks or other high-prio tasks.
In flush_to_ldisc, when executing n_tty_receive_buf_common,
it wakes up

What is this "it" supposed to refer to? In English, this "it" here actually refers to n_tty_receive_buf_common().

other tasks. __wake_up_common_lock calls spin_lock_irqsave, which does
not disable preemption but disable migration in RT-Linux. This prevents

"disables"

the kworker thread from being migrated to other cores by CPU's balancing
logic, resulting in long delays.

Here should be another \n to separate paragraphs.

In our system, the processing interval for each frame of IMU data
transmitted via UART can experience significant jitter due to this issue.
Instead of the expected 10 to 15 ms frame processing interval, we see
spikes up to 30 to 35 ms. Moreover, in just one or two hours, there can
be 2 to 3 occurrences of such high jitter, which is quite frequent. This
jitter exceeds the software's tolerable limit of 20 ms.

One more \n here too.

Introduce flip_wq in tty_port which can be set by tty_port_link_wq or as
default linked to workqueue allocated when tty_register_driver using flag
WQ_SYSFS, so that cpumask and nice can be set dynamically.

This is a heavy sentence. Split it.

Introduce TTY_DRIVER_CUSTOM_WORKQUEUE flag meaning not to create the
default single tty_driver workqueue.
We set the cpumask to the same cpu where the IMU data is handled and has
less long-time high-prio jobs, and then set nice to -20, the frame

Period after 20, not comma.

processing interval remains between 10 and 15ms, no jitter occurs.

You can perhaps use some LLM to rephrase the text?

---
Change in v5:
- Do not allocate workqueue twice when CONFIG_UNIX98_PTYS and
CONFIG_LEGACY_PTYS are all enabled.

Change in v4:
- Simplify the logic for creating and releasing the workqueue,
as suggested by Tejun Heo.
- Allocate single workqueue of one tty_driver as default, link it to
port when tty_port register device or tty_driver.
- Introduce tty_port_link_wq to link specific workqueue to port.
- Add driver flag TTY_DRIVER_CUSTOM_WORKQUEUE meaning not to create the
default single tty_driver workqueue.
- Link to v4: https://lore.kernel.org/all/202512041303.7192024b-lkp@xxxxxxxxx/T/#t

Change in v3:
- Add tty flip workqueue for all tty ports, as suggested by Greg KH.
Every tty port use an individual flip workqueue, while all pty ports
share the same workqueue created in pty_flip_wq_init.
- Modify the commit log to describe the reason for latency spikes in
RT-Linux.
- Link to v3: https://lore.kernel.org/all/20251027060929.394053-1-jackzxcui1989@xxxxxxx/

Change in v2:
- Do not add new module parameters
as suggested by Greg KH
- Set WQ_SYSFS to allow properties changes from userspace
as suggested by Tejun Heo
- Link to v2: https://lore.kernel.org/all/20251024155534.2302590-1-jackzxcui1989@xxxxxxx

Signed-off-by: Xin Zhao <jackzxcui1989@xxxxxxx>

This S-O-B is too late -- it would be dropped. You have to add it before the first "---".

--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -44,6 +44,8 @@ static struct tty_driver *pts_driver;
static DEFINE_MUTEX(devpts_mutex);
#endif
+static struct workqueue_struct *pty_flip_wq;

It's not clear to me, why ptys need a separate wq. IOW: you should describe this in the commit log.

--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -3452,6 +3452,20 @@ int tty_register_driver(struct tty_driver *driver)
goto err_unreg_char;
}
+ if (!(driver->flags & TTY_DRIVER_CUSTOM_WORKQUEUE)) {
+ driver->flip_wq = alloc_workqueue("%s-flip-wq",
+ WQ_UNBOUND | WQ_SYSFS,
+ 0, driver->name);

Do you have to wrap the line here?

+ if (!driver->flip_wq) {
+ error = -ENOMEM;
+ goto err_unreg_char;

Who is going to free cdevs in this fail path?

+ }
+ for (i = 0; i < driver->num; i++) {
+ if (driver->ports[i] && !driver->ports[i]->buf.flip_wq)

You test it here and again in tty_port_link_driver_wq().

+ tty_port_link_driver_wq(driver->ports[i], driver);

There are not many drivers having tty ports set at this point. Why are you doing this here, actually? Given you do this later again in register_device().

+ }
+ }
+
scoped_guard(mutex, &tty_mutex)
list_add(&driver->tty_drivers, &tty_drivers);
@@ -3475,6 +3489,9 @@ int tty_register_driver(struct tty_driver *driver)
scoped_guard(mutex, &tty_mutex)
list_del(&driver->tty_drivers);
+ if (!(driver->flags & TTY_DRIVER_CUSTOM_WORKQUEUE))
+ destroy_workqueue(driver->flip_wq);
+
err_unreg_char:
unregister_chrdev_region(dev, driver->num);
err:
...

--- a/drivers/tty/tty_port.c
+++ b/drivers/tty/tty_port.c
@@ -103,6 +103,22 @@ void tty_port_init(struct tty_port *port)
}
EXPORT_SYMBOL(tty_port_init);
+/**
+ * tty_port_link_wq - link tty_port and flip workqueue
+ * @port: tty_port of the device
+ * @flip_wq: workqueue to queue flip buffer work on
+ *
+ * Assign a specific workqueue to a certain port, instead of using the
+ * workqueue allocated in tty_register_driver when TTY_DRIVER_CUSTOM_WORKQUEUE

The same as for commit logs: functions end with (). Furthermore, constants start with % IIRC. A period is missing too. It should sound like:
"when TTY_DRIVER_CUSTOM_WORKQUEUE is used."

+ *
+ * Note tty port api will not destroy the workqueue in tty_port_destroy.

the TTY port API

+ */
+void tty_port_link_wq(struct tty_port *port, struct workqueue_struct *flip_wq)
+{
+ port->buf.flip_wq = flip_wq;
+}
+EXPORT_SYMBOL(tty_port_link_wq);

_GPL likely?

+
/**
* tty_port_link_device - link tty and tty_port
* @port: tty_port of the device
@@ -161,6 +177,7 @@ struct device *tty_port_register_device_attr(struct tty_port *port,
const struct attribute_group **attr_grp)
{
tty_port_link_device(port, driver, index);
+ tty_port_link_driver_wq(port, driver);
return tty_register_device_attr(driver, index, device, drvdata,
attr_grp);
}
@@ -187,6 +204,7 @@ struct device *tty_port_register_device_attr_serdev(struct tty_port *port,
struct device *dev;
tty_port_link_device(port, driver, index);
+ tty_port_link_driver_wq(port, driver);
dev = serdev_tty_port_register(port, host, parent, driver, index);
if (PTR_ERR(dev) != -ENODEV) {
@@ -718,6 +736,7 @@ int tty_port_install(struct tty_port *port, struct tty_driver *driver,
struct tty_struct *tty)
{
tty->port = port;
+ tty_port_link_driver_wq(port, driver);
return tty_standard_install(driver, tty);
}
EXPORT_SYMBOL_GPL(tty_port_install);
diff --git a/include/linux/tty_buffer.h b/include/linux/tty_buffer.h
index 31125e3be..48adcb0e8 100644
--- a/include/linux/tty_buffer.h
+++ b/include/linux/tty_buffer.h
@@ -34,6 +34,7 @@ static inline u8 *flag_buf_ptr(struct tty_buffer *b, unsigned int ofs)
struct tty_bufhead {
struct tty_buffer *head; /* Queue head */
+ struct workqueue_struct *flip_wq;
struct work_struct work;
struct mutex lock;
atomic_t priority;
diff --git a/include/linux/tty_driver.h b/include/linux/tty_driver.h
index 188ee9b76..cd93345bd 100644
--- a/include/linux/tty_driver.h
+++ b/include/linux/tty_driver.h
@@ -69,6 +69,10 @@ struct serial_struct;
* Do not create numbered ``/dev`` nodes. For example, create
* ``/dev/ttyprintk`` and not ``/dev/ttyprintk0``. Applicable only when a
* driver for a single tty device is being allocated.
+ *
+ * @TTY_DRIVER_CUSTOM_WORKQUEUE:
+ * Do not create workqueue when tty_register_driver. Set flip buffer
+ * workqueue by tty_port_link_wq every port.

Sorry, parser error.

*/
enum tty_driver_flag {
TTY_DRIVER_INSTALLED = BIT(0),
@@ -79,6 +83,7 @@ enum tty_driver_flag {
TTY_DRIVER_HARDWARE_BREAK = BIT(5),
TTY_DRIVER_DYNAMIC_ALLOC = BIT(6),
TTY_DRIVER_UNNUMBERED_NODE = BIT(7),
+ TTY_DRIVER_CUSTOM_WORKQUEUE = BIT(8),
};
enum tty_driver_type {
@@ -506,6 +511,7 @@ struct tty_operations {
* @flags: tty driver flags (%TTY_DRIVER_)
* @proc_entry: proc fs entry, used internally
* @other: driver of the linked tty; only used for the PTY driver
+ * @flip_wq: workqueue to queue flip buffer work on
* @ttys: array of active &struct tty_struct, set by tty_standard_install()
* @ports: array of &struct tty_port; can be set during initialization by
* tty_port_link_device() and similar
@@ -539,6 +545,7 @@ struct tty_driver {
unsigned long flags;
struct proc_dir_entry *proc_entry;
struct tty_driver *other;
+ struct workqueue_struct *flip_wq;
/*
* Pointer to the tty data structures
diff --git a/include/linux/tty_port.h b/include/linux/tty_port.h
index 332ddb936..86e01bd51 100644
--- a/include/linux/tty_port.h
+++ b/include/linux/tty_port.h
@@ -138,6 +138,7 @@ struct tty_port {
kernel */
void tty_port_init(struct tty_port *port);
+void tty_port_link_wq(struct tty_port *port, struct workqueue_struct *flip_wq);
void tty_port_link_device(struct tty_port *port, struct tty_driver *driver,
unsigned index);
struct device *tty_port_register_device(struct tty_port *port,
@@ -165,6 +166,14 @@ static inline struct tty_port *tty_port_get(struct tty_port *port)
return NULL;
}
+/* No effect when TTY_DRIVER_CUSTOM_WORKQUEUE, as driver->flip_wq is NULL */
+static inline void tty_port_link_driver_wq(struct tty_port *port,
+ struct tty_driver *driver)

I am not sure why you introduce two interfaces: tty_port_link_driver_wq() and tty_port_link_wq(). Can't you add the if to the latter and drop the former? To me at least, the latter is confusing.

+{
+ if (!port->buf.flip_wq)
+ port->buf.flip_wq = driver->flip_wq;
+}
+
/* If the cts flow control is enabled, return true. */
static inline bool tty_port_cts_enabled(const struct tty_port *port)
{

thanks,
--
js
suse labs