Re: [PATCH] check nmi watchdog is broken

Re: [PATCH] check nmi watchdog is broken

[Dave Jones]

On Sun, May 01, 2005 at 10:04:28AM -0700, Linux Kernel wrote:
 > tree 6adb8d33585f8eee20794827c79e40991aeeaee5
 > parent fd51f666fa591294bd7462447512666e61c56ea0
 > author Jack F Vogel <jfv@bluesong.net> Sun, 01 May 2005 22:58:48 -0700
 > committer Linus Torvalds <torvalds@ppc970.osdl.org> Sun, 01 May 2005 22:58:48 -0700
 > 
 > [PATCH] check nmi watchdog is broken
 > 
 > A bug against an xSeries system showed up recently noting that the
 > check_nmi_watchdog() test was failing.
 > 
 > I have been investigating it and discovered in both i386 and x86_64 the
 > recent change to the routine to use the cpu_callin_map has uncovered a
 > problem.  Prior to that change, on an SMP box, the test was trivally
 > passing because all cpu's were found to not yet be online, but now with the
 > callin_map they are discovered, it goes on to test the counter and they
 > have not yet begun to increment, so it announces a CPU is stuck and bails
 > out.
 > 
 > On all the systems I have access to test, the announcement of failure is
 > also bougs...  by the time you can login and check /proc/interrupts, the
 > NMI count is happily incrementing on all CPUs.  Its just that the test is
 > being done too early.
 > 
 > I have tried moving the call to the test around a bit, and it was always
 > too early.  I finally hit on this proposed solution, it delays the routine
 > via a late_initcall(), seems like the right solution to me.

My EM64T Dell Precision 470 seems to have problems both before
and after this change, though the behaviour changes.

With -rc3..
testing NMI watchdog ... CPU#1: NMI appears to be stuck (0)!

With -rc4...
Testing NMI watchdog ... CPU#4: NMI appears to be stuck (0)!

The CPU #'s are consistent across reboots (Ie, they're always the same).
Though the rc4 behaviour seems odd, as my CPUs are numbered 0-3.

Bill (Cc'd) also reports that with -rc4, he sees around
a 10 second delay when it does that 'Testing NMI watchdog'
message.

		Dave

Re: [PATCH] check nmi watchdog is broken

[Mikael Pettersson]

Dave Jones writes:
 > My EM64T Dell Precision 470 seems to have problems both before
 > and after this change, though the behaviour changes.
 > 
 > With -rc3..
 > testing NMI watchdog ... CPU#1: NMI appears to be stuck (0)!
 > 
 > With -rc4...
 > Testing NMI watchdog ... CPU#4: NMI appears to be stuck (0)!
 > 
 > The CPU #'s are consistent across reboots (Ie, they're always the same).
 > Though the rc4 behaviour seems odd, as my CPUs are numbered 0-3.

There is known breakage in x86-64's nmi watchdog recently.
One of the problems is that the checking code inspects
all possible CPU numbers [0..NR_CPUS[ and not just the
ones actually present, which leads to false failure reports.

 > Bill (Cc'd) also reports that with -rc4, he sees around
 > a 10 second delay when it does that 'Testing NMI watchdog'
 > message.

And that's another of the known x86-64 breakages.

/Mikael