[Bug 206669] Little-endian kernel crashing on POWER8 on heavy big-endian PowerKVM load

[Nicholas Piggin]

bugzilla-daemon at bugzilla.kernel.org's on February 26, 2020 8:28 pm:
> https://bugzilla.kernel.org/show_bug.cgi?id=206669
> 
> --- Comment #4 from John Paul Adrian Glaubitz (glaubitz at physik.fu-berlin.de) ---
> (In reply to npiggin from comment #3)
>> Do you have tracing / ftrace enabled in the host kernel for any
>> reason? Turning that off might let the oops message get printed.
> 
> Seems that this is the case in the Debian kernel, yes:
> 
> root at watson:~# grep -i ftrace /boot/config-5.4.0-0.bpo.3-powerpc64le 
> CONFIG_KPROBES_ON_FTRACE=y
> CONFIG_HAVE_KPROBES_ON_FTRACE=y
> CONFIG_HAVE_DYNAMIC_FTRACE=y
> CONFIG_HAVE_DYNAMIC_FTRACE_WITH_REGS=y
> CONFIG_HAVE_FTRACE_MCOUNT_RECORD=y
> CONFIG_FTRACE=y
> CONFIG_FTRACE_SYSCALLS=y
> CONFIG_DYNAMIC_FTRACE=y
> CONFIG_DYNAMIC_FTRACE_WITH_REGS=y
> CONFIG_FTRACE_MCOUNT_RECORD=y
> # CONFIG_FTRACE_STARTUP_TEST is not set
> root at watson:~#
> 
> Do you have the kernel command option at hand which disables ftrace on the
> command line? Is it just ftrace=off?

Hmm, not sure, Documentation/admin-guide/kernel-parameters.txt seems
to say that wouldn't work.

I thought it might only be going down that path if you have already done
some tracing. Perhaps ensure /sys/kernel/debug/tracing/tracing_on is set
to 0, and then `echo 1 > /sys/kernel/debug/tracing/free_buffer` before
you start the test.

>> > FWIW, the kernel image comes from this Debian package:
>> > 
>> >>
>> >>
>> >>
>> http://snapshot.debian.org/archive/debian/20200211T210433Z/pool/main/l/linux/linux-image-5.4.0-0.bpo.3-powerpc64le_5.4.13-1%7Ebpo10%2B1_ppc64el.deb
>> 
>> Okay. Any chance you could test an upstream kernel? 
> 
> Sure, absolutely. Any preference on the version number?

Current head if you're feeling lucky, but v5.5 if not. But you can
give the ftrace test a try with the debian kernel first if you've got
it ready to go.

>> Don't bother testing that after the above -- panic_on_oops happens
>> after oops_begin(), so it won't help unfortunately.
> 
> Okay.
> 
>> Attmepting to get into xmon might though, if you boot with xmon=on.
>> Try that if tracing wasn't enabled, or disabling it doesn't help.
> 
> Okay. I will try to disable ftrace first, then retrigger the crash.

Cool

Thanks,
Nick