Re: [PATCH] NMI watchdog: when booting with reset_devices, clear the performance counters

[Aristeu Rozanski <aris@redhat.com>]

> > P4s have a quirk that makes necessary to clear P4_CCCR_OVF bit on the CCCR
> > everytime the PMI is triggered. When booting the kernel with reset_devices
> > (more specific kdump case), the counters reach zero and the PMI will be
> > generated. This is not a problem on other processors but on P4s, it'll
> > continue to generate NMIs until that bit is cleared. Since there may be
> > other users of the performance counters, clear and disable all of them
> > when booting with reset_devices option.
> > 
> > We have a P4 box here that crashes because of this problem. Since the kdump
> > kernel usually boots with only one processor active, the second logical
> > unit won't be set up, therefore, MSR_P4_IQ_CCCR1 (and other performance
> > counter registers) won't be cleared and P4_CCCR_OVF may be still set because
> > the previous kernel was using this register. An NMI is triggered because of
> > the MSR_P4_IQ_CCCR1 right after the NMI delivery is enabled, triggering the
> > race fixed on my previous email.
> > 
> > Signed-off-by: Aristeu Rozanski <aris@redhat.com>
> > Acked-by: Don Zickus <dzickus@redhat.com>
> > Acked-by: Prarit Bhargava <prarit@redhat.com>
> > Acked-by: Vivek Goyal <vgoyal@redhat.com>
> 
> applied to tip/x86/nmi-watchdog, thanks.
> 
> i'm wondering, is this fix a v2.6.27 candidate?
I believe so. Even being a small affected group (P4 based boxes using NMI
watchdog and kdump), it's fully reproducible.

-- 
Aristeu