watchdog: core: make sure the watchdog worker always works

When running a command like 'chrt -f 50 dd if=/dev/zero of=/dev/null',
the watchdog_worker fails to service the HW watchdog and the
HW watchdog fires long before the watchdog soft timeout.

At the moment, the watchdog_worker is invoked as a delayed work.
Delayed works are handled by non realtime kernel threads. The
WQ_HIGHPRI flag only increases the niceness of that threads.

This patch replaces the delayed work logic by kthread delayed work,
and sets the associated kernel task to SCHED_FIFO with the highest
priority, in order to ensure that the watchdog worker will run as
soon as possible.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

drivers/watchdog/watchdog_dev.c

@@ -39,6 +39,7 @@
 #include <linux/jiffies.h>	/* For timeout functions */
 #include <linux/kernel.h>	/* For printk/panic/... */
 #include <linux/kref.h>		/* For data references */
+#include <linux/kthread.h>	/* For kthread_delayed_work */
 #include <linux/miscdevice.h>	/* For handling misc devices */
 #include <linux/module.h>	/* For module stuff/... */
 #include <linux/mutex.h>	/* For mutexes */
@@ -46,9 +47,10 @@
 #include <linux/slab.h>		/* For memory functions */
 #include <linux/types.h>	/* For standard types (like size_t) */
 #include <linux/watchdog.h>	/* For watchdog specific items */
-#include <linux/workqueue.h>	/* For workqueue */
 #include <linux/uaccess.h>	/* For copy_to_user/put_user/... */
 
+#include <uapi/linux/sched/types.h>	/* For struct sched_param */
+
 #include "watchdog_core.h"
 #include "watchdog_pretimeout.h"
 
@@ -67,7 +69,7 @@ struct watchdog_core_data {
 	struct mutex lock;
 	unsigned long last_keepalive;
 	unsigned long last_hw_keepalive;
-	struct delayed_work work;
+	struct kthread_delayed_work work;
 	unsigned long status;		/* Internal status bits */
 #define _WDOG_DEV_OPEN		0	/* Opened ? */
 #define _WDOG_ALLOW_RELEASE	1	/* Did we receive the magic char ? */
@@ -79,7 +81,7 @@ static dev_t watchdog_devt;
 /* Reference to watchdog device behind /dev/watchdog */
 static struct watchdog_core_data *old_wd_data;
 
-static struct workqueue_struct *watchdog_wq;
+static struct kthread_worker *watchdog_kworker;
 
 static bool handle_boot_enabled =
 	IS_ENABLED(CONFIG_WATCHDOG_HANDLE_BOOT_ENABLED);
@@ -140,9 +142,10 @@ static inline void watchdog_update_worker(struct watchdog_device *wdd)
 		long t = watchdog_next_keepalive(wdd);
 
 		if (t > 0)
-			mod_delayed_work(watchdog_wq, &wd_data->work, t);
+			kthread_mod_delayed_work(watchdog_kworker,
+						 &wd_data->work, t);
 	} else {
-		cancel_delayed_work(&wd_data->work);
+		kthread_cancel_delayed_work_sync(&wd_data->work);
 	}
 }
 
@@ -154,8 +157,8 @@ static int __watchdog_ping(struct watchdog_device *wdd)
 	int err;
 
 	if (time_is_after_jiffies(earliest_keepalive)) {
-		mod_delayed_work(watchdog_wq, &wd_data->work,
-				 earliest_keepalive - jiffies);
+		kthread_mod_delayed_work(watchdog_kworker, &wd_data->work,
+					 earliest_keepalive - jiffies);
 		return 0;
 	}
 
@@ -203,12 +206,13 @@ static bool watchdog_worker_should_ping(struct watchdog_core_data *wd_data)
 	return wdd && (watchdog_active(wdd) || watchdog_hw_running(wdd));
 }
 
-static void watchdog_ping_work(struct work_struct *work)
+static void watchdog_ping_work(struct kthread_work *work)
 {
 	struct watchdog_core_data *wd_data;
 
-	wd_data = container_of(to_delayed_work(work), struct watchdog_core_data,
-			       work);
+	wd_data = container_of(container_of(work, struct kthread_delayed_work,
+					    work),
+			       struct watchdog_core_data, work);
 
 	mutex_lock(&wd_data->lock);
 	if (watchdog_worker_should_ping(wd_data))
@@ -921,10 +925,10 @@ static int watchdog_cdev_register(struct watchdog_device *wdd, dev_t devno)
 	wd_data->wdd = wdd;
 	wdd->wd_data = wd_data;
 
-	if (!watchdog_wq)
+	if (IS_ERR_OR_NULL(watchdog_kworker))
 		return -ENODEV;
 
-	INIT_DELAYED_WORK(&wd_data->work, watchdog_ping_work);
+	kthread_init_delayed_work(&wd_data->work, watchdog_ping_work);
 
 	if (wdd->id == 0) {
 		old_wd_data = wd_data;
@@ -970,7 +974,8 @@ static int watchdog_cdev_register(struct watchdog_device *wdd, dev_t devno)
 		__module_get(wdd->ops->owner);
 		kref_get(&wd_data->kref);
 		if (handle_boot_enabled)
-			queue_delayed_work(watchdog_wq, &wd_data->work, 0);
+			kthread_queue_delayed_work(watchdog_kworker,
+						   &wd_data->work, 0);
 		else
 			pr_info("watchdog%d running and kernel based pre-userspace handler disabled\n",
 				wdd->id);
@@ -1007,7 +1012,7 @@ static void watchdog_cdev_unregister(struct watchdog_device *wdd)
 		watchdog_stop(wdd);
 	}
 
-	cancel_delayed_work_sync(&wd_data->work);
+	kthread_cancel_delayed_work_sync(&wd_data->work);
 
 	kref_put(&wd_data->kref, watchdog_core_data_release);
 }
@@ -1111,13 +1116,14 @@ void watchdog_dev_unregister(struct watchdog_device *wdd)
 int __init watchdog_dev_init(void)
 {
 	int err;
+	struct sched_param param = {.sched_priority = MAX_RT_PRIO - 1,};
 
-	watchdog_wq = alloc_workqueue("watchdogd",
-				      WQ_HIGHPRI | WQ_MEM_RECLAIM, 0);
-	if (!watchdog_wq) {
-		pr_err("Failed to create watchdog workqueue\n");
-		return -ENOMEM;
+	watchdog_kworker = kthread_create_worker(0, "watchdogd");
+	if (IS_ERR(watchdog_kworker)) {
+		pr_err("Failed to create watchdog kworker\n");
+		return PTR_ERR(watchdog_kworker);
 	}
+	sched_setscheduler(watchdog_kworker->task, SCHED_FIFO, &param);
 
 	err = class_register(&watchdog_class);
 	if (err < 0) {
@@ -1136,7 +1142,7 @@ int __init watchdog_dev_init(void)
 err_alloc:
 	class_unregister(&watchdog_class);
 err_register:
-	destroy_workqueue(watchdog_wq);
+	kthread_destroy_worker(watchdog_kworker);
 	return err;
 }
 
@@ -1150,7 +1156,7 @@ void __exit watchdog_dev_exit(void)
 {
 	unregister_chrdev_region(watchdog_devt, MAX_DOGS);
 	class_unregister(&watchdog_class);
-	destroy_workqueue(watchdog_wq);
+	kthread_destroy_worker(watchdog_kworker);
 }
 
 module_param(handle_boot_enabled, bool, 0444);