Re: [PATCH] watchdog: don't run proc_watchdog_update if new value is same as old

[Josh Hunt]

On 03/14/2016 09:34 AM, Don Zickus wrote:
> On Sat, Mar 12, 2016 at 06:50:26PM -0500, Joshua Hunt wrote:
> > While working on a script to restore all sysctl params before a series of
> > tests I found that writing any value into the
> > /proc/sys/kernel/{nmi_watchdog,soft_watchdog,watchdog,watchdog_thresh}
> > causes them to call proc_watchdog_update(). Not only that, but when I
> > wrote to these proc files in a loop I could easily trigger a soft lockup.
> >
> > [  955.756196] NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
> > [  955.765994] NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
> > [  955.774619] NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
> > [  955.783182] NMI watchdog: enabled on all CPUs, permanently consumes one hw-PMU counter.
> > [  959.788319] NMI watchdog: BUG: soft lockup - CPU#4 stuck for 30s! [swapper/4:0]
> > [  959.788325] NMI watchdog: BUG: soft lockup - CPU#5 stuck for 30s! [swapper/5:0]
> >
> > There doesn't appear to be a reason for doing this work other every time a
> > write occurs, so only do the work when the values change.
>
> Hi Josh,
>
> Thanks for the patch.  I have no objections to it, but Uli and myself were
> interested in the reason for the softlockups.  Uli is going to provide a
> test patch to see if his theory is correct.  That way we fix the underlying
> issue and then apply your patch on top. Make sense?

Yep. Sounds good. I meant to mention I didn't diagnose the soft-lockup. 
If you provide a patch I'm happy to test. I can also attempt to debug 
that part more if needed.

Josh

> Cheers,
> Don
>
> > Signed-off-by: Josh Hunt <johunt@xxxxxxxxxx>
> > ---
> >   kernel/watchdog.c |    9 ++++++++-
> >   1 file changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/watchdog.c b/kernel/watchdog.c
> > index b3ace6e..9acb29f 100644
> > --- a/kernel/watchdog.c
> > +++ b/kernel/watchdog.c
> > @@ -923,6 +923,9 @@ static int proc_watchdog_common(int which, struct ctl_table *table, int write,
> >   		 * both lockup detectors are disabled if proc_watchdog_update()
> >   		 * returns an error.
> >   		 */
> > +		if (old == new)
> > +			goto out;
> > +
> >   		err = proc_watchdog_update();
> >   	}
> >   out:
> > @@ -967,7 +970,7 @@ int proc_soft_watchdog(struct ctl_table *table, int write,
> >   int proc_watchdog_thresh(struct ctl_table *table, int write,
> >   			 void __user *buffer, size_t *lenp, loff_t *ppos)
> >   {
> > -	int err, old;
> > +	int err, old, new;
> >
> >   	get_online_cpus();
> >   	mutex_lock(&watchdog_proc_mutex);
> > @@ -987,6 +990,10 @@ int proc_watchdog_thresh(struct ctl_table *table, int write,
> >   	/*
> >   	 * Update the sample period. Restore on failure.
> >   	 */
> > +	new = ACCESS_ONCE(watchdog_thresh);
> > +	if (old == new)
> > +		goto out;
> > +
> >   	set_sample_period();
> >   	err = proc_watchdog_update();
> >   	if (err) {
> > --
> > 1.7.9.5