On Wed 28-06-17 13:24:08, Liang, Kan wrote: > > > > From: Kan Liang <Kan.liang@xxxxxxxxx> > > > > > > Some users reported spurious NMI watchdog timeouts. > > > > > > We now have more and more systems where the Turbo range is wide > > enough > > > that the NMI watchdog expires faster than the soft watchdog timer that > > > updates the interrupt tick the NMI watchdog relies on. > > > > AFAIR the watchdog doesn't rely on deferred timers so this would suggest > > that a standard hrtimer can expire much later than programmed, right? > > The softlockup watchdog relies on hrtimers. > The hardlockup watchdog (NMI watchdog) relies on perf subsystem and > using unhalted CPU cycles. > When the softlockup watchdog expires, it updates the hrtimer_interrupts. > When the NMI watchdog expires, it will check the hrtimer_interrupts, and > determine if it's a hardlockup. > The design was to make the softlockup watchdog runs with 2.5 times the > rate of NMI watchdog. So it guarantees that the hrtimer_interrupts is > updated before the NMI watchdog expires. > That works well if Turbo-Mode is disabled. > However, when Turbo-Mode is enabled, unhalted CPU cycles might run > much faster than expected, even faster than softlockup watchdog. > So the softlockup watchdog will not get a chance to update the > hrtimer_interrupts, which will trigger false positives. So it is not the hrtimer which doesn't fire but rather the NMI events fire too quickly, right? -- Michal Hocko SUSE Labs