Linux-Watchdog Archive on
 help / color / Atom feed
From: Sebastian Andrzej Siewior <>
Cc: Wim Van Sebroeck <>,
	Guenter Roeck <>,
	Steffen Trumtrar <>,
	Tim Sander <>, Julia Cartwright <>,
	Sebastian Andrzej Siewior <>,
	Thomas Gleixner <>
Subject: [PATCH] watchdog: prevent deferral of watchdogd wakeup on RT
Date: Tue, 5 Nov 2019 15:45:06 +0100
Message-ID: <> (raw)

From: Julia Cartwright <>

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

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.

Cc: Guenter Roeck <>
Reported-and-tested-by: Steffen Trumtrar <>
Reported-by: Tim Sander <>
Signed-off-by: Julia Cartwright <>
Acked-by: Guenter Roeck <>
[bigeasy: use only HRTIMER_MODE_REL_HARD]
Signed-off-by: Sebastian Andrzej Siewior <>
 drivers/watchdog/watchdog_dev.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
index dbd2ad4c92948..d3acc0a7256ca 100644
--- a/drivers/watchdog/watchdog_dev.c
+++ b/drivers/watchdog/watchdog_dev.c
@@ -158,7 +158,8 @@ 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,
 	} else {
@@ -177,7 +178,7 @@ static int __watchdog_ping(struct watchdog_device *wdd)
 	if (ktime_after(earliest_keepalive, now)) {
 			      ktime_sub(earliest_keepalive, now),
-			      HRTIMER_MODE_REL);
 		return 0;
@@ -971,7 +972,7 @@ static int watchdog_cdev_register(struct watchdog_device *wdd, dev_t devno)
 		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) {
@@ -1019,7 +1020,8 @@ static int watchdog_cdev_register(struct watchdog_device *wdd, dev_t devno)
 		if (handle_boot_enabled)
-			hrtimer_start(&wd_data->timer, 0, HRTIMER_MODE_REL);
+			hrtimer_start(&wd_data->timer, 0,
 			pr_info("watchdog%d running and kernel based pre-userspace handler disabled\n",

             reply index

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-05 14:45 Sebastian Andrzej Siewior [this message]
2019-11-05 16:02 ` Guenter Roeck
2019-11-05 16:19   ` Sebastian Andrzej Siewior

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-Watchdog Archive on

Archives are clonable:
	git clone --mirror linux-watchdog/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-watchdog linux-watchdog/ \
	public-inbox-index linux-watchdog

Example config snippet for mirrors

Newsgroup available over NNTP:

AGPL code for this site: git clone