linux-kernel - Re: [QUESTION] Is there a better way to get ftrace dump on guest?

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Message-ID: <20160629014903.GB1628@sejong>
Date:	Wed, 29 Jun 2016 10:49:03 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Rabin Vincent <rabin@....in>
CC:	<kvm@...r.kernel.org>, Steven Rostedt <rostedt@...dmis.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Minchan Kim <minchan@...nel.org>
Subject: Re: [QUESTION] Is there a better way to get ftrace dump on guest?

Hello,

On Tue, Jun 28, 2016 at 06:46:34PM +0200, Rabin Vincent wrote:
> On Tue, Jun 28, 2016 at 03:33:18PM +0900, Namhyung Kim wrote:
> > On Tue, Jun 28, 2016 at 3:25 PM, Namhyung Kim <namhyung@...nel.org> wrote:
> > > I'm running some guest machines for kernel development.  For debugging
> > > purpose, I use lots of trace_printk() since it's faster than normal
> > > printk().  When kernel crash happens the trace buffer is printed on
> > > console (I set ftrace_dump_on_oops) but it takes too much time.  I
> > > don't want to reduce the size of ring buffer as I want to collect the
> > > debug info as much as possible.  And I also want to see trace from all
> > > cpu so 'ftrace_dump_on_oop = 2' is not an option.
> > >
> > > I know the kexec/kdump (and the crash tool) can dump and analyze the
> > > trace buffer later.  But it's cumbersome to do it everytime and more
> > > importantly, I don't want to spend the memory for the crashkernel.
> 
> Assuming you're using QEMU:
> 
> QEMU has a dump-guest-memory command which can be used to dump the
> guest's entire memory to an ELF which can be loaded by the crash utility
> to extract the trace buffer.  This doesn't require kexec/kdump or any
> other support from the guest kernel.

Thanks for the info.  Not requiring kexec/kdump step is a big win for
me.  Although I mostly use kvmtool (lkvm), I'll give it a try.


> 
> It's apparently even possible to run QEMU with the guest memory in a
> file and load that to crash directly, although this is not something
> I've had a chance to try out myself:
> 
> https://github.com/crash-utility/crash/commit/89ed9d0a7f7da4578294a492c1ad857244ce7352

Interesting, I'll take a look but wouldn't it impact the performance?

And even if the crash tool is good, it'd be great if I can work
without it (if possible).


Thanks,
Namhyung