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Re: [Xen-devel] [PATCH] x86/watchdog: Use real timestamps for watchdog timeout

At 10:57 +0100 on 24 May (1369393060), Andrew Cooper wrote:
> On 24/05/13 08:09, Jan Beulich wrote:
> > You can't use NOW() here - while the time updating code is safe
> > against normal interrupts, it's not atomic wrt NMIs.
> But NMIs are latched at the hardware level.  If we get a nested NMI the
> Xen will be toast on the exit path anyway.

The problem is that an NMI can arrive while local_time_calibration() is
writing its results, so calling NOW() in the NMI handler might return

> > Handling this case it nice, but I wonder whether this patch ought to
> > detect and report ludicrous NMI rates rather than silently ignoring
> > them.  I guess that's hard to do in an NMI handler, other than by
> > adjusting the printk when we crash.

Actually on second thoughts it's easier: as well as having this patch
(or near equivalent) to avoid premature watchdog expiry, we cna detect
the NMI rate in, say, the timer softirq and report if it's gone mad.



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