watchdog: prevent deferral of watchdogd wakeup on RT

When PREEMPT_RT is enabled, all hrtimer expiry functions are
deferred for execution into the context of ksoftirqd unless otherwise
annotated.

Deferring the expiry of the hrtimer used by the watchdog core, however,
is a waste, as the callback does nothing but queue a kthread work item
and wakeup watchdogd.

It's worst then that, too: the deferral through ksoftirqd also means
that for correct behavior a user must adjust the scheduling parameters
of both watchdogd _and_ ksoftirqd, which is unnecessary and has other
side effects (like causing unrelated expiry functions to execute at
potentially elevated priority).

Instead, mark the hrtimer used by the watchdog core as being _HARD to
allow it's execution directly from hardirq context.  The work done in
this expiry function is well-bounded and minimal.

A user still must adjust the scheduling parameters of the watchdogd
to be correct w.r.t. their application needs.

Link: https://lkml.kernel.org/r/0e02d8327aeca344096c246713033887bc490dd7.1538089180.git.julia@ni.com
Cc: Guenter Roeck <[email protected]>
Reported-and-tested-by: Steffen Trumtrar <[email protected]>
Reported-by: Tim Sander <[email protected]>
Signed-off-by: Julia Cartwright <[email protected]>
Acked-by: Guenter Roeck <[email protected]>
[bigeasy: use only HRTIMER_MODE_REL_HARD]
Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Reviewed-by: Guenter Roeck <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Guenter Roeck <[email protected]>
Signed-off-by: Wim Van Sebroeck <[email protected]>

Author: Julia Cartwright

drivers/watchdog/watchdog_dev.c

@@ -157,7 +157,8 @@ static inline void watchdog_update_worker(struct watchdog_device *wdd)
 		ktime_t t = watchdog_next_keepalive(wdd);
 
 		if (t > 0)
-			hrtimer_start(&wd_data->timer, t, HRTIMER_MODE_REL);
+			hrtimer_start(&wd_data->timer, t,
+				      HRTIMER_MODE_REL_HARD);
 	} else {
 		hrtimer_cancel(&wd_data->timer);
 	}
@@ -176,7 +177,7 @@ static int __watchdog_ping(struct watchdog_device *wdd)
 	if (ktime_after(earliest_keepalive, now)) {
 		hrtimer_start(&wd_data->timer,
 			      ktime_sub(earliest_keepalive, now),
-			      HRTIMER_MODE_REL);
+			      HRTIMER_MODE_REL_HARD);
 		return 0;
 	}
 
@@ -974,7 +975,7 @@ static int watchdog_cdev_register(struct watchdog_device *wdd)
 		return -ENODEV;
 
 	kthread_init_work(&wd_data->work, watchdog_ping_work);
-	hrtimer_init(&wd_data->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
+	hrtimer_init(&wd_data->timer, CLOCK_MONOTONIC, HRTIMER_MODE_REL_HARD);
 	wd_data->timer.function = watchdog_timer_expired;
 
 	if (wdd->id == 0) {
@@ -1032,7 +1033,8 @@ static int watchdog_cdev_register(struct watchdog_device *wdd)
 		__module_get(wdd->ops->owner);
 		get_device(&wd_data->dev);
 		if (handle_boot_enabled)
-			hrtimer_start(&wd_data->timer, 0, HRTIMER_MODE_REL);
+			hrtimer_start(&wd_data->timer, 0,
+				      HRTIMER_MODE_REL_HARD);
 		else
 			pr_info("watchdog%d running and kernel based pre-userspace handler disabled\n",
 				wdd->id);