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Re: Help with decoding a NMI Watchdog interrupt on an Octeon

To: "Kevin D. Kissell" <kevink@paralogos.com>
Subject: Re: Help with decoding a NMI Watchdog interrupt on an Octeon
From: David Daney <ddaney@caviumnetworks.com>
Date: Thu, 17 Jun 2010 14:51:40 -0700
Cc: Jan Rovins <janr@adax.com>, linux-mips@linux-mips.org
In-reply-to: <4C1A9319.1020202@paralogos.com>
References: <4C1A8D86.60005@adax.com> <4C1A9319.1020202@paralogos.com>
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On 06/17/2010 02:26 PM, Kevin D. Kissell wrote:
NMI is just an input pin, so you'd really need to know what it's
connected to in the system you're working on.

In this case, the NMI is likely being asserted by the watchdog. So if you are stuck in a loop with interrupts disabled, the register dump might help you figure out where things are stuck. But as you say below, knowing the value of the ErrorEPC register is critical.

David Daney

Typically, it's tied to
some kind of memory bus time-out, but it could be other things.
Depending on what it's hooked up to, knowing what code was executing
when it came in may be completely useless. *If* it's hooked up to a bus
time-out, *and* the instruction that caused the time-out was a load,
*and* the time-out and NMI occurred *after* the processor got to the
instruction that consumes the load value (pretty likely if the first two
conditions are met), *then* looking at disassembled kernel code
(mips-linux-objdump --disassemble vmlinux) at the ErrorEPC address,
*not* the address in EPC, which will have latched the address of the
last recoverable exception (which NMI is not, strictly speaking). That
instruction should be the consumer of the bad load, so one of its input
registers should be the target of that load. If it's a two-input
instruction, e.g. add r1,r2,r3, then it could be either r2 or r3, and
you have to work your way backwords up the code flow to find out where
the r2 and r3 values came from, respectively. *Usually* it's possible to
identify the load, thus the register used as a base address, and see
that the base address register was trashed, at which point you can start
forming hypotheses as to how that could have happened.

Of course, in the dump below, we don't see ErrorEPC. I've never been
able to figure out why so many kernel register dumps skip that register,
especially for NMI reporting. But unless you're able to reproduce this
with a kernel that you build yourself, so that you can fix the
instrumentation, it's going to be tough. So "Plan B" would be to make
sure that any removable memory DIMMs have been properly seated, and
double-check that the actual memory capacity corresponds to whatever
boot parameters are being passed to the kernel. In otherwords, if you
can't debug the kernel, pray that it's a hardware or operator error. ;o)

Regards,

Kevin K.

Jan Rovins wrote:
Hi, I need some tips on how to go about deciphering the following NMI
dump.

This is from a 2.6.21.7 kernel that came with the Cavium Networks
1.8.1 toolchain.
Is there any way to get some kind of back trace from this, or just
find out which function it was in?

I have been playing around with objdump -x vmlinux but I cant zero in
on anything this way.

Thanks in advance,

Jan
*** NMI Watchdog interrupt on Core 0x6 ***
$0 0x0000000000000000 at 0x000000001010cce0
v0 0x000000000000003d v1 0x000000000000024a
a0 0xffffffff807d7b70 a1 0x0000000000000000
a2 0x000000000000024a a3 0x0000000000000000
a4 0xffffffff807d7b60 a5 0x0000000000000080
a6 0x0000000000000001 a7 0xa800000411c62578
t0 0x0000000000000001 t1 0xa80000048ef3e880
t2 0xffffffff82d40000 t3 0xa80000041f48c000
s0 0xc0000000000d9640 s1 0xc000000000088028
s2 0x0000000000000000 s3 0x0000000000000180
s4 0x0000000000000000 s5 0x0000000000000000
s6 0xb7a89c196f513832 s7 0x0000000000000000
t8 0xffffffff807d0000 t9 0xffffffff807d0000
k0 0x0000000000000000 k1 0x00000000104dbcbf
gp 0xa80000041f48c000 sp 0xa80000041f48fcf0
s8 0x0000000000000000 ra 0xc0000000023c5004
epc 0xffffffff802b10b8
status 0x000000001058cce4 cause 0x0000000040008c08
sum0 0x0000002100000000 en0 0x0000009300008000
Code around epc
0xffffffff802b10a8 000000002406ffff
0xffffffff802b10ac 0000000064a5ffff
0xffffffff802b10b0 0000000010a60005
0xffffffff802b10b4 0000000000000000
0xffffffff802b10b8 0000000080620000
0xffffffff802b10bc 000000001440fffb
0xffffffff802b10c0 0000000064630001
0xffffffff802b10c4 000000006463ffff
0xffffffff802b10c8 0000000003e00008







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