Re: [RFC 2/2] watchdog: update watchdog_tresh properly

[Michal Hocko <mhocko@suse.cz>]

On Fri 19-07-13 12:08:52, Don Zickus wrote:
> On Fri, Jul 19, 2013 at 11:04:59AM +0200, Michal Hocko wrote:
> > watchdog_tresh controls how often nmi perf event counter checks per-cpu
> > hrtimer_interrupts counter and blows up if the counter hasn't changed
> > since the last check. The counter is updated by per-cpu watchdog_hrtimer
> > hrtimer which is scheduled with 2/5 watchdog_thresh period which
> > guarantees that hrtimer is scheduled 2 times per the main period. Both
> > hrtimer and perf event are started together when the watchdog is
> > enabled.
> > 
> > So far so good. But...
> > 
> > But what happens when watchdog_thresh is updated from sysctl handler?
> > 
> > proc_dowatchdog will set a new sampling period and hrtimer callback
> > (watchdog_timer_fn) will use the new value in the next round.
> > The problem, however, is that nobody tells the perf event that the
> > sampling period has changed so it is ticking with the period configured
> > when it has been set up.
> > 
> > This might result in an ear riping dissonance between perf and hrtimer
> > parts if the watchdog_thresh is increased. And even worse it might lead
> > to KABOOM if the watchdog is configured to panic on such a spurious
> > lockup.
> 
> Heh.  Good point.
> 
> What if we keep it simpler.
> 
> if (old_thresh != watchdog_thresh)
>   watchdog_disable_all_cpus()
>   wathcdog_enable_all_cpus()

Those would do nothing because of watchdog_disabled checks. It is also
much more heavy than necessary. I hope we can get a perf API which just
update the perf event period directly and we do not have to call even
watchdog_nmi_{en,dis}able

-- 
Michal Hocko
SUSE Labs