X-Git-Url: http://git.onelab.eu/?a=blobdiff_plain;f=Documentation%2Fnmi_watchdog.txt;h=c025a4561c103381cb14f77880e3d95037061e5c;hb=6a77f38946aaee1cd85eeec6cf4229b204c15071;hp=6cad46e8ac2d4006d66811c8b987599ee23516b0;hpb=87fc8d1bb10cd459024a742c6a10961fefcef18f;p=linux-2.6.git

diff --git a/Documentation/nmi_watchdog.txt b/Documentation/nmi_watchdog.txt
index 6cad46e8a..c025a4561 100644
--- a/Documentation/nmi_watchdog.txt
+++ b/Documentation/nmi_watchdog.txt
@@ -54,6 +54,20 @@ then the system has crashed so hard (eg. hardware-wise) that either it
 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.