On Thu, Apr 18, 2013 at 02:04:00PM +0200, Stephane Eranian wrote: > On Thu, Apr 18, 2013 at 1:42 PM, Peter Zijlstra <a.p.zijls...@chello.nl> > wrote: > > On Tue, 2013-04-16 at 06:57 +0000, Pan, Zhenjie wrote: > >> Watchdog use performance monitor of cpu clock cycle to generate NMI to > >> detect hard lockup. > >> But when cpu's frequency changes, the event period will also change. > >> It's not as expected as the configration. > >> For example, set the NMI event handler period is 10 seconds when the cpu > >> is 2.0GHz. > >> If the cpu changes to 800MHz, the period will be 10*(2000/800)=25 seconds. > >> So it may make hard lockup detect not work if the watchdog timeout is not > >> long enough. > >> Now, set a notifier to listen to the cpu frequency change. > >> And dynamic re-config the NMI event to make the event period correct. > >> > > > > > > Urgh,. does this really matter.. all we really want is for that NMI to > > hit eventually in the not too distant future. Does the frequency really > > matter _that_ much? > > > I agree, it does not really matter. Set the watchdog to a couple of minutes > and it should be fine, shouldn't it?
I believe it mattered to the Chrome folks. They want the watchdog to be as tight as possible so the user experience isn't a hang but a quick reboot instead. They like setting the watchdog to something like 2 seconds. There was a patch a few months ago that tried to hack around this issue and I suggested this approach as a better solution. I forgot what the original problem was. Perhaps someone can jump in and explain the problem being solved (other than the watchdog isn't always 10 seconds)? Cheers, Don > > > Also, can't we simply pick an event that's invariant to the cpufreq > > nonsense? Something like CPU_CLK_UNHALTED.REF -- or better the > > fixed_ctr2 which nobody ever uses anyway. > > > You don't want to use fixed counter 2 for NMI watchdog because it's pinned. > No other counter can count this event. And it is very useful. I use it often. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
