Re: [PATCH] watchdog: prevent deferral of watchdogd wakeup on RT

[Guenter Roeck <linux@roeck-us.net>]

On Tue, Nov 05, 2019 at 03:45:06PM +0100, Sebastian Andrzej Siewior wrote:
> From: Julia Cartwright <julia@ni.com>
> 
> 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>

Looks ok for mainline.

Reviewed-by: Guenter Roeck <linux@roeck-us.net>

> ---
>  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,
> +				      HRTIMER_MODE_REL_HARD);
>  	} else {
>  		hrtimer_cancel(&wd_data->timer);
>  	}
> @@ -177,7 +178,7 @@ 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;
>  	}
>  
> @@ -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)
>  		__module_get(wdd->ops->owner);
>  		kref_get(&wd_data->kref);
>  		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);
> -- 
> 2.24.0
>