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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 <linux@roeck-us.net>
Reported-and-tested-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Reported-by: Tim Sander <tim@krieglstein.org>
Signed-off-by: Julia Cartwright <julia@ni.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
[bigeasy: use only HRTIMER_MODE_REL_HARD]
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20191105144506.clyadjbvnn7b7b2m@linutronix.de
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>
This commit is contained in:
Julia Cartwright 2019-11-05 15:45:06 +01:00 committed by Wim Van Sebroeck
parent eccb7fe5e7
commit a19f89335f
1 changed files with 6 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

10
drivers/watchdog/watchdog_dev.c
View File

@ -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);