Re: 2.6.10-rc1-mm5

[Maciej W. Rozycki]

On Thu, 18 Nov 2004, Stas Sergeev wrote:

> Ah, thanks for explanations!
> Indeed it works as you say.
> Very usefull info, perhaps worth
> being added to documentation?
> So if you ACK the attached patch,
> I ask Andrew to please apply it.

 Sure, no problem with that.

> Andrew, the attached patch adds
> the information about the NMI
> watchdog frequency caveats to the
> Documentation/nmi_watchdog.txt
> Unless there are any objections,
> please apply.

Signed-off-by: Maciej W. Rozycki <macro@xxxxxxxxxxxxxx>

--- linux/Documentation/nmi_watchdog.txt.old	2004-10-21 21:21:56.000000000 +0400
+++ linux/Documentation/nmi_watchdog.txt	2004-11-18 20:16:37.972370552 +0300
@@ -54,6 +54,20 @@
 cannot even accept NMI interrupts, or the crash has made the kernel
 unable to print messages.
 
+Be aware that when using local APIC, the frequency of NMI interrupts
+it generates, depends on the system load. The local APIC NMI watchdog,
+lacking a better source, uses the "cycles unhalted" event. As you may
+guess it doesn't tick when the CPU is in the halted state (which happens
+when the system is idle), but if your system locks up on anything but the
+"hlt" processor instruction, the watchdog will trigger very soon as the
+"cycles unhalted" event will happen every clock tick. If it locks up on
+"hlt", then you are out of luck -- the event will not happen at all and the
+watchdog won't trigger. This is a shortcoming of the local APIC watchdog
+-- unfortunately there is no "clock ticks" event that would work all the
+time. The I/O APIC watchdog is driven externally and has no such shortcoming.  
+But its NMI frequency is much higher, resulting in a more significant hit
+to the overall system performance.
+
 NOTE: starting with 2.4.2-ac18 the NMI-oopser is disabled by default,
 you have to enable it with a boot time parameter.  Prior to 2.4.2-ac18
 the NMI-oopser is enabled unconditionally on x86 SMP boxes.
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