Re: [PATCH v4] Extend watchdog timeout during kernel panic.

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

On 2/3/21 7:36 AM, JP Ertola wrote:
> If the watchdog timeout is set such that the crash kernel does not
> have time to collect a coredump and the crash kernel is not equipped to
> ping the watchdog timer itself, then a kernel coredump will not be collected
> before the watchdog fires. This change registers a panic notifier and
> callback so the watchdog timeout can be extended if a kernel panic occurs.
> This timeout extension would give the crash kernel enough time to collect
> a coredump before the CPU resets. The watchdog timeout is extended if and only
> if a crash kernel image is loaded in memory, the watchdog is active at the
> time of the panic, and the kconfig setting is set.
> 
> A Kconfig option has been added to configure the timeout duration at
> compile-time. Default is zero seconds.
> 
> Signed-off-by: JP Ertola <jp.ertola@hpe.com>
> ---
> v4: Remove optional callback mechanism alltogether. I agree with Guenter, 
> not widely used.
> v3: Fix logic so timeout extension is not longer than wdd->max_timeout
> v2: Remove dead code and comments.
>  drivers/watchdog/Kconfig        | 13 ++++++
>  drivers/watchdog/watchdog_dev.c | 80 ++++++++++++++++++++++++++++++++-
>  include/linux/watchdog.h        |  1 +
>  3 files changed, 93 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/watchdog/Kconfig b/drivers/watchdog/Kconfig
> index fd7968635e6d..f1055985e100 100644
> --- a/drivers/watchdog/Kconfig
> +++ b/drivers/watchdog/Kconfig
> @@ -141,6 +141,19 @@ comment "Watchdog Device Drivers"
>  
>  # Architecture Independent
>  
> +config DEFAULT_WATCHDOG_CRASH_KERNEL_TIMEOUT
> +	int "Default timeout for watchdog timer before crash kernel starts (seconds)"
> +	default 0
> +	help
> +	  This option allows an extended timeout to be used for the watchdog when
> +	  the kernel panics and a crash kernel is about to start. This is helpful
> +	  when the existing WDT timeout value is less than the time required for
> +	  crash kernel to run and the crash kernel is unable to handle the
> +	  the watchdog itself. The timeout extension happens last in chain of
> +	  kernel panic handler callbacks just in case another panic handler
> +	  hangs unexpectedly. When this value is set to 0, the watchdog timeout
> +	  will not be changed.
> +
>  config SOFT_WATCHDOG
>  	tristate "Software watchdog"
>  	select WATCHDOG_CORE
> diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
> index 2946f3a63110..cf91c08b0606 100644
> --- a/drivers/watchdog/watchdog_dev.c
> +++ b/drivers/watchdog/watchdog_dev.c
> @@ -34,6 +34,7 @@
>  #include <linux/init.h>		/* For __init/__exit/... */
>  #include <linux/hrtimer.h>	/* For hrtimers */
>  #include <linux/kernel.h>	/* For printk/panic/... */
> +#include <linux/kexec.h>	/* For checking if crash kernel is loaded */
>  #include <linux/kthread.h>	/* For kthread_work */
>  #include <linux/miscdevice.h>	/* For handling misc devices */
>  #include <linux/module.h>	/* For module stuff/... */
> @@ -82,6 +83,8 @@ static bool handle_boot_enabled =
>  
>  static unsigned open_timeout = CONFIG_WATCHDOG_OPEN_TIMEOUT;
>  
> +static unsigned int wdt_panic_timeout = CONFIG_DEFAULT_WATCHDOG_CRASH_KERNEL_TIMEOUT;
> +
>  static bool watchdog_past_open_deadline(struct watchdog_core_data *data)
>  {
>  	return ktime_after(ktime_get(), data->open_deadline);
> @@ -658,6 +661,59 @@ static int watchdog_ioctl_op(struct watchdog_device *wdd, unsigned int cmd,
>   *	off the watchdog (if 'nowayout' is not set).
>   */
>  
> +static int watchdog_panic_notifier(struct notifier_block *nb,
> +	unsigned long code, void *data)
> +{
> +	struct watchdog_device *wdd;
> +	int ret = 0;
> +	unsigned int time_out = wdt_panic_timeout;

Please use timeout as variable name.

> +
> +	if (wdt_panic_timeout == 0)
> +		return NOTIFY_DONE;
> +
> +	wdd = container_of(nb, struct watchdog_device, panic_nb);
> +
> +	if (watchdog_timeout_invalid(wdd, wdt_panic_timeout)) {
> +		time_out = min(wdt_panic_timeout, wdd->max_timeout);
> +		pr_err("watchdog%d: timeout extension value "
> +			" invalid. Falling back to %d seconds\n", wdd->id,

String in single line, please.

> +			time_out);
> +	}
> +
> +	if (kexec_crash_image && watchdog_active(wdd)) {
> +		int ping_ret = 0;

Unnecessary assignment.

> +
> +		pr_info("watchdog%d: Extending watchdog timeout to "
> +			"%d seconds\n", wdd->id, time_out);
> +
> +		ret = watchdog_set_timeout(wdd, time_out);
> +
Unnecessary empty line

> +		if (ret) {
> +			pr_err("watchdog%d: Unable to extend watchdog timeout "
> +				"before starting crash kernel (%d)",
> +				wdd->id, ret);
> +		}
> +
> +		/* Many watchdog implementations will reset the timer
> +		* when the timeout is changed, but ping again to be
> +		* safe.
> +		*/

This needs to call __watchdog_ping(). Some watchdog ping functions have
a minimum time between calls of the ping function, and others may not
have a ping function but use the start function instead.

> +		if (wdd->ops->ping) {
> +			ping_ret = wdd->ops->ping(wdd);
> +			if (ping_ret) {
> +				pr_warn("watchdog%d: Unable to ping "
> +					"watchdog before starting "
> +					"crash kernel (%d)\n",
> +					wdd->id, ping_ret);
> +			}
> +		}
> +
> +
Please drop those unnecessary empty lines.

> +	}
> +
> +	return notifier_from_errno(ret);

Are you sure about that ? I don't know the implication when NOTIFY_BAD is returned.

> +}
> +
>  static ssize_t watchdog_write(struct file *file, const char __user *data,
>  						size_t len, loff_t *ppos)
>  {
> @@ -1118,8 +1174,25 @@ int watchdog_dev_register(struct watchdog_device *wdd)
>  		return ret;
>  
>  	ret = watchdog_register_pretimeout(wdd);
> -	if (ret)
> +	if (ret) {
>  		watchdog_cdev_unregister(wdd);
> +		return ret;
> +	}
> +
> +	/*
> +	 * Setting panic_nb priority to minimum ensures the watchdog device
> +	 * panic callback runs last in the chain of kernel panic callbacks.
> +	 * This way, the watchdog device will still fire in the event another
> +	 * panic callback hangs.
> +	 */
> +	wdd->panic_nb.priority = INT_MIN;
> +	wdd->panic_nb.notifier_call = watchdog_panic_notifier;
> +
> +	ret = atomic_notifier_chain_register(&panic_notifier_list,
> +					&wdd->panic_nb);
> +	if (ret)
> +		pr_err("watchdog%d: Cannot register panic notifier (%d)\n",
> +			wdd->id, ret);

Either make this a warning and ignore the error (return 0),
or this needs a proper cleanup.

>  
>  	return ret;>  }
> @@ -1228,3 +1301,8 @@ module_param(open_timeout, uint, 0644);
>  MODULE_PARM_DESC(open_timeout,
>  	"Maximum time (in seconds, 0 means infinity) for userspace to take over a running watchdog (default="
>  	__MODULE_STRING(CONFIG_WATCHDOG_OPEN_TIMEOUT) ")");
> +
> +module_param(wdt_panic_timeout, uint, 0444);
> +MODULE_PARM_DESC(wdt_panic_timeout,
> +	"Watchdog timeout extension duration upon kernel panic. (default="
> +	__MODULE_STRING(CONFIG_DEFAULT_WATCHDOG_CRASH_KERNEL_TIMEOUT) " seconds)");
> diff --git a/include/linux/watchdog.h b/include/linux/watchdog.h
> index 9b19e6bb68b5..bc7e6e8aa7ab 100644
> --- a/include/linux/watchdog.h
> +++ b/include/linux/watchdog.h
> @@ -107,6 +107,7 @@ struct watchdog_device {
>  	unsigned int max_hw_heartbeat_ms;
>  	struct notifier_block reboot_nb;
>  	struct notifier_block restart_nb;
> +	struct notifier_block panic_nb;
>  	void *driver_data;
>  	struct watchdog_core_data *wd_data;
>  	unsigned long status;
>