Date: Mon, 23 Nov 2015 19:26:08 +0100
From: Uwe =?iso-8859-1?Q?Kleine-K=F6nig?= 
	<u.kleine-koenig@pengutronix.de>
To: Guenter Roeck <linux@roeck-us.net>
Cc: linux-watchdog@vger.kernel.org, Wim Van Sebroeck <wim@iguana.be>,
        linux-kernel@vger.kernel.org, Timo Kokkonen <timo.kokkonen@offcode.fi>,
        linux-doc@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>
Subject: Re: [PATCH v5 1/8] watchdog: Introduce hardware maximum timeout in
 watchdog core
Message-ID: <20151123182608.GA19888@pengutronix.de>
References: <1448248865-21684-1-git-send-email-linux@roeck-us.net>
 <1448248865-21684-2-git-send-email-linux@roeck-us.net>
 <20151123075304.GA5369@pengutronix.de>
 <56533B80.4030400@roeck-us.net>
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Hello Guenter,

On Mon, Nov 23, 2015 at 08:14:56AM -0800, Guenter Roeck wrote:
> On 11/22/2015 11:53 PM, Uwe Kleine-K?nig wrote:
> >On Sun, Nov 22, 2015 at 07:20:58PM -0800, Guenter Roeck wrote:
> >>@@ -160,7 +176,11 @@ they are supported. These optional routines/operations are:
> >>    and -EIO for "could not write value to the watchdog". On success this
> >>    routine should set the timeout value of the watchdog_device to the
> >>    achieved timeout value (which may be different from the requested one
> >>-  because the watchdog does not necessarily has a 1 second resolution).
> >>+  because the watchdog does not necessarily have a 1 second resolution).
> >>+  Drivers implementing hw_max_timeout_ms set the hardware watchdog timeout
> >>+  to the minimum of timeout and hw_max_timeout_ms. Those drivers set the
> >
> >Actually this is something that the wdg core could abstract for drivers.
> >Oh well, apart from hw_max_timeout_ms having ms accuracy.
> >
> Not that sure. about the abstraction. The actual timeout to set depends on
> the hardware, and may have an unknown granularity. The watchdog core can

What I wanted to say is that the driver core can do (ignoring scaling
due to different units):

	if (timeout > wdd->hw_max_timeout_ms)
		wdd->set_timeout(wdd->hw_max_timeout_ms);
	else
		wdd->set_timeout(timeout)

So that the device specific driver can assume that timeout passed to its
set_timeout callback is always less than or equal to hw_max_timeout_ms.
And so it doesn't need to compare against hw_max_timeout_ms again.

> not predict what that granularity would be. We can play with it, and try
> if it can be done, but I really would like this to be a separate patch.
> 
> hw_max_timeout is in ms because some watchdogs have a very low maximum
> HW timeout.
> 
> >>+  timeout value of the watchdog_device either to the requested timeout value
> >>+  (if it is larger than hw_max_timeout_ms), or to the achieved timeout value.
> >>    (Note: the WDIOF_SETTIMEOUT needs to be set in the options field of the
> >>    watchdog's info structure).
> >>  * get_timeleft: this routines returns the time that's left before a reset.
> >>diff --git a/drivers/watchdog/watchdog_dev.c b/drivers/watchdog/watchdog_dev.c
> >>index 56a649e66eb2..1dba3f57dba3 100644
> >>--- a/drivers/watchdog/watchdog_dev.c
> >>+++ b/drivers/watchdog/watchdog_dev.c
> >>[...]
> >>+static long watchdog_next_keepalive(struct watchdog_device *wdd)
> >>+{
> >>+	unsigned int timeout_ms = wdd->timeout * 1000;
> >>+	unsigned long keepalive_interval;
> >>+	unsigned long last_heartbeat;
> >>+	unsigned long virt_timeout;
> >>+	unsigned int hw_timeout_ms;
> >>+
> >>+	virt_timeout = wdd->last_keepalive + msecs_to_jiffies(timeout_ms);
> >
> >I think it's sensible to store
> >
> >	last_keepalive + timeout
> >
> >(i.e. the expected expiration time) in struct watchdog_device instead of
> >last_keepalive. This moves the (maybe expensive) calculation to a
> >context that has userspace interaction anyhow. On the other hand this
> >complicates the set_timeout call. Hmm.
> >
> 
> I would rather keep the code simple. It is not as if this is called
> all the time. Also, I need last_keepalive later in the series
> to determine if the minimum timeout between hardware pings has elapsed,
> so we would need both.

I'm not sure my suggestion improves the situation, but I will keep my
idea in mind and check once the dust settled.

> >>+	hw_timeout_ms = min(timeout_ms, wdd->max_hw_timeout_ms);
> >>+	keepalive_interval = msecs_to_jiffies(hw_timeout_ms / 2);
> >>+
> >>+	/*
> >>+	 * To ensure that the watchdog times out wdd->timeout seconds
> >>+	 * after the most recent ping from userspace, the last
> >>+	 * worker ping has to come in hw_timeout_ms before this timeout.
> >>+	 */
> >>+	last_heartbeat = virt_timeout - msecs_to_jiffies(hw_timeout_ms);
> >>+	return min_t(long, last_heartbeat - jiffies, keepalive_interval);
> >>+}
> >>[...]
> >>@@ -61,26 +137,25 @@ static struct watchdog_device *old_wdd;
> >>
> >>  static int watchdog_ping(struct watchdog_device *wdd)
> >>  {
> >>-	int err = 0;
> >>+	int err;
> >>
> >>  	mutex_lock(&wdd->lock);
> >>+	wdd->last_keepalive = jiffies;
> >>+	err = _watchdog_ping(wdd);
> >>+	mutex_unlock(&wdd->lock);
> >>
> >>-	if (test_bit(WDOG_UNREGISTERED, &wdd->status)) {
> >>-		err = -ENODEV;
> >>-		goto out_ping;
> >>-	}
> >>+	return err;
> >>+}
> >>
> >>-	if (!watchdog_active(wdd))
> >>-		goto out_ping;
> >>+static void watchdog_ping_work(struct work_struct *work)
> >>+{
> >>+	struct watchdog_device *wdd;
> >>
> >>-	if (wdd->ops->ping)
> >>-		err = wdd->ops->ping(wdd);	/* ping the watchdog */
> >>-	else
> >>-		err = wdd->ops->start(wdd);	/* restart watchdog */
> >>+	wdd = container_of(to_delayed_work(work), struct watchdog_device, work);
> >>
> >>-out_ping:
> >>+	mutex_lock(&wdd->lock);
> >>+	_watchdog_ping(wdd);
> >>  	mutex_unlock(&wdd->lock);
> >>-	return err;
> >
> >Calling this function might come after last_keepalive + timeout in which
> >case the watchdog shouldn't be pinged.
> >
> Unless the code is wrong, the last time this function is called should be
> at (timeout - max_hw_timeout), ie well before the timeout elapsed.
> Given that, I don't think this is something to be concerned about.

Depends on max_hw_timeout and the load of the machine if that can
happen. And thinking again, if the ping didn't come in until
last_keepalive + timeout, the machine is reset anyhow ...

> >>  }
> >>
> >>  /*
> >>@@ -107,8 +182,11 @@ static int watchdog_start(struct watchdog_device *wdd)
> >>  		goto out_start;
> >>
> >>  	err = wdd->ops->start(wdd);
> >>-	if (err == 0)
> >>+	if (err == 0) {
> >>  		set_bit(WDOG_ACTIVE, &wdd->status);
> >>+		wdd->last_keepalive = jiffies;
> >>+		watchdog_update_worker(wdd, true);
> >>+	}
> >
> >I think it's more correct to sample jiffies before calling .start.
> >Something like:
> >
> >	unsigned long started_at = jiffies;
> >
> >	err = wdd->ops->start(wdd);
> >	if (err == 0)
> >		wdd->last_keepalive = jiffies;
> >
> I assume you mean
> 		wdd->last_keepalive = started_at;

right.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-K?nig            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |
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