[PATCH] watchdog: core: make sure the watchdog_worker always works

[Christophe Leroy]

When running a command like 'chrt -f 99 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 patchs replaces the delayed work logic by hrtimer, in order to
ensure that the watchdog_worker will already have priority.

Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxx>
---
 drivers/watchdog/watchdog_dev.c | 87 +++++++++++++++++++----------------------
 1 file changed, 41 insertions(+), 46 deletions(-)

diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
index 1e971a50d7fb..e9b234c4ff67 100644
--- a/drivers/watchdog/watchdog_dev.c
+++ b/drivers/watchdog/watchdog_dev.c
@@ -36,7 +36,7 @@
 #include <linux/errno.h>	/* For the -ENODEV/... values */
 #include <linux/fs.h>		/* For file operations */
 #include <linux/init.h>		/* For __init/__exit/... */
-#include <linux/jiffies.h>	/* For timeout functions */
+#include <linux/hrtimer.h>
 #include <linux/kernel.h>	/* For printk/panic/... */
 #include <linux/kref.h>		/* For data references */
 #include <linux/miscdevice.h>	/* For handling misc devices */
@@ -46,7 +46,6 @@
 #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 "watchdog_core.h"
@@ -65,9 +64,9 @@ struct watchdog_core_data {
 	struct cdev cdev;
 	struct watchdog_device *wdd;
 	struct mutex lock;
-	unsigned long last_keepalive;
-	unsigned long last_hw_keepalive;
-	struct delayed_work work;
+	ktime_t last_keepalive;
+	ktime_t last_hw_keepalive;
+	struct hrtimer timer;
 	unsigned long status;		/* Internal status bits */
 #define _WDOG_DEV_OPEN		0	/* Opened ? */
 #define _WDOG_ALLOW_RELEASE	1	/* Did we receive the magic char ? */
@@ -79,8 +78,6 @@ 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 bool handle_boot_enabled =
 	IS_ENABLED(CONFIG_WATCHDOG_HANDLE_BOOT_ENABLED);
 
@@ -107,18 +104,19 @@ static inline bool watchdog_need_worker(struct watchdog_device *wdd)
 		(t && !watchdog_active(wdd) && watchdog_hw_running(wdd));
 }
 
-static long watchdog_next_keepalive(struct watchdog_device *wdd)
+static ktime_t watchdog_next_keepalive(struct watchdog_device *wdd)
 {
 	struct watchdog_core_data *wd_data = wdd->wd_data;
 	unsigned int timeout_ms = wdd->timeout * 1000;
-	unsigned long keepalive_interval;
-	unsigned long last_heartbeat;
-	unsigned long virt_timeout;
+	ktime_t keepalive_interval;
+	ktime_t last_heartbeat, latest_heartbeat;
+	ktime_t virt_timeout;
 	unsigned int hw_heartbeat_ms;
 
-	virt_timeout = wd_data->last_keepalive + msecs_to_jiffies(timeout_ms);
+	virt_timeout = ktime_add(wd_data->last_keepalive,
+				 ms_to_ktime(timeout_ms));
 	hw_heartbeat_ms = min_not_zero(timeout_ms, wdd->max_hw_heartbeat_ms);
-	keepalive_interval = msecs_to_jiffies(hw_heartbeat_ms / 2);
+	keepalive_interval = ms_to_ktime(hw_heartbeat_ms / 2);
 
 	if (!watchdog_active(wdd))
 		return keepalive_interval;
@@ -128,8 +126,11 @@ static long watchdog_next_keepalive(struct watchdog_device *wdd)
 	 * after the most recent ping from userspace, the last
 	 * worker ping has to come in hw_heartbeat_ms before this timeout.
 	 */
-	last_heartbeat = virt_timeout - msecs_to_jiffies(hw_heartbeat_ms);
-	return min_t(long, last_heartbeat - jiffies, keepalive_interval);
+	last_heartbeat = ktime_sub(virt_timeout, ms_to_ktime(hw_heartbeat_ms));
+	latest_heartbeat = ktime_sub(last_heartbeat, ktime_get());
+	if (ktime_before(latest_heartbeat, keepalive_interval))
+		return latest_heartbeat;
+	return keepalive_interval;
 }
 
 static inline void watchdog_update_worker(struct watchdog_device *wdd)
@@ -137,29 +138,33 @@ static inline void watchdog_update_worker(struct watchdog_device *wdd)
 	struct watchdog_core_data *wd_data = wdd->wd_data;
 
 	if (watchdog_need_worker(wdd)) {
-		long t = watchdog_next_keepalive(wdd);
+		ktime_t t = watchdog_next_keepalive(wdd);
 
 		if (t > 0)
-			mod_delayed_work(watchdog_wq, &wd_data->work, t);
+			hrtimer_start(&wd_data->timer, t, HRTIMER_MODE_REL);
 	} else {
-		cancel_delayed_work(&wd_data->work);
+		hrtimer_cancel(&wd_data->timer);
 	}
 }
 
 static int __watchdog_ping(struct watchdog_device *wdd)
 {
 	struct watchdog_core_data *wd_data = wdd->wd_data;
-	unsigned long earliest_keepalive = wd_data->last_hw_keepalive +
-				msecs_to_jiffies(wdd->min_hw_heartbeat_ms);
+	ktime_t earliest_keepalive, now;
 	int err;
 
-	if (time_is_after_jiffies(earliest_keepalive)) {
-		mod_delayed_work(watchdog_wq, &wd_data->work,
-				 earliest_keepalive - jiffies);
+	earliest_keepalive = ktime_add(wd_data->last_hw_keepalive,
+				       ms_to_ktime(wdd->min_hw_heartbeat_ms));
+	now = ktime_get();
+
+	if (ktime_after(earliest_keepalive, now)) {
+		hrtimer_start(&wd_data->timer,
+			      ktime_sub(earliest_keepalive, now),
+			      HRTIMER_MODE_REL);
 		return 0;
 	}
 
-	wd_data->last_hw_keepalive = jiffies;
+	wd_data->last_hw_keepalive = now;
 
 	if (wdd->ops->ping)
 		err = wdd->ops->ping(wdd);  /* ping the watchdog */
@@ -192,7 +197,7 @@ static int watchdog_ping(struct watchdog_device *wdd)
 
 	set_bit(_WDOG_KEEPALIVE, &wd_data->status);
 
-	wd_data->last_keepalive = jiffies;
+	wd_data->last_keepalive = ktime_get();
 	return __watchdog_ping(wdd);
 }
 
@@ -203,17 +208,18 @@ 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)
+enum hrtimer_restart watchdog_ping_work(struct hrtimer *timer)
 {
 	struct watchdog_core_data *wd_data;
 
-	wd_data = container_of(to_delayed_work(work), struct watchdog_core_data,
-			       work);
+	wd_data = container_of(timer, struct watchdog_core_data, timer);
 
 	mutex_lock(&wd_data->lock);
 	if (watchdog_worker_should_ping(wd_data))
 		__watchdog_ping(wd_data->wdd);
 	mutex_unlock(&wd_data->lock);
+
+	return HRTIMER_NORESTART;
 }
 
 /*
@@ -230,7 +236,7 @@ static void watchdog_ping_work(struct work_struct *work)
 static int watchdog_start(struct watchdog_device *wdd)
 {
 	struct watchdog_core_data *wd_data = wdd->wd_data;
-	unsigned long started_at;
+	ktime_t started_at;
 	int err;
 
 	if (watchdog_active(wdd))
@@ -238,7 +244,7 @@ static int watchdog_start(struct watchdog_device *wdd)
 
 	set_bit(_WDOG_KEEPALIVE, &wd_data->status);
 
-	started_at = jiffies;
+	started_at = ktime_get();
 	if (watchdog_hw_running(wdd) && wdd->ops->ping)
 		err = wdd->ops->ping(wdd);
 	else
@@ -919,10 +925,8 @@ static int watchdog_cdev_register(struct watchdog_device *wdd, dev_t devno)
 	wd_data->wdd = wdd;
 	wdd->wd_data = wd_data;
 
-	if (!watchdog_wq)
-		return -ENODEV;
-
-	INIT_DELAYED_WORK(&wd_data->work, watchdog_ping_work);
+	hrtimer_init(&wd_data->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+	wd_data->timer.function = watchdog_ping_work;
 
 	if (wdd->id == 0) {
 		old_wd_data = wd_data;
@@ -958,7 +962,7 @@ static int watchdog_cdev_register(struct watchdog_device *wdd, dev_t devno)
 	}
 
 	/* Record time of most recent heartbeat as 'just before now'. */
-	wd_data->last_hw_keepalive = jiffies - 1;
+	wd_data->last_hw_keepalive = ktime_get();
 
 	/*
 	 * If the watchdog is running, prevent its driver from being unloaded,
@@ -968,7 +972,7 @@ static int watchdog_cdev_register(struct watchdog_device *wdd, dev_t devno)
 		if (handle_boot_enabled) {
 			__module_get(wdd->ops->owner);
 			kref_get(&wd_data->kref);
-			queue_delayed_work(watchdog_wq, &wd_data->work, 0);
+			hrtimer_start(&wd_data->timer, 0, HRTIMER_MODE_REL);
 		} else {
 			pr_info("watchdog%d running and kernel based pre-userspace handler disabled\n",
 					wdd->id);
@@ -1006,7 +1010,7 @@ static void watchdog_cdev_unregister(struct watchdog_device *wdd)
 		watchdog_stop(wdd);
 	}
 
-	cancel_delayed_work_sync(&wd_data->work);
+	hrtimer_cancel(&wd_data->timer);
 
 	kref_put(&wd_data->kref, watchdog_core_data_release);
 }
@@ -1111,13 +1115,6 @@ int __init watchdog_dev_init(void)
 {
 	int err;
 
-	watchdog_wq = alloc_workqueue("watchdogd",
-				      WQ_HIGHPRI | WQ_MEM_RECLAIM, 0);
-	if (!watchdog_wq) {
-		pr_err("Failed to create watchdog workqueue\n");
-		return -ENOMEM;
-	}
-
 	err = class_register(&watchdog_class);
 	if (err < 0) {
 		pr_err("couldn't register class\n");
@@ -1135,7 +1132,6 @@ int __init watchdog_dev_init(void)
 err_alloc:
 	class_unregister(&watchdog_class);
 err_register:
-	destroy_workqueue(watchdog_wq);
 	return err;
 }
 
@@ -1149,7 +1145,6 @@ void __exit watchdog_dev_exit(void)
 {
 	unregister_chrdev_region(watchdog_devt, MAX_DOGS);
 	class_unregister(&watchdog_class);
-	destroy_workqueue(watchdog_wq);
 }
 
 module_param(handle_boot_enabled, bool, 0444);
-- 
2.13.3