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

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Message-ID: <20160628095727.436925cb@gandalf.local.home>
Date:	Tue, 28 Jun 2016 09:57:27 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Namhyung Kim <namhyung@...nel.org>
Cc:	kvm@...r.kernel.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?

On Tue, 28 Jun 2016 15:33:18 +0900
Namhyung Kim <namhyung@...nel.org> wrote:

> Send again to correct addresses, sorry!
> 
> On Tue, Jun 28, 2016 at 3:25 PM, Namhyung Kim <namhyung@...nel.org> wrote:
> > Hello,
> >
> > 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.
> >
> > So what is the best way to handle this?  I'd like to know how others
> > setup the debugging environment..  

Heh, I'd say something helpful but you basically already shot down all
of my advice, because what I do is...

1) Reduce the size of the ring buffer

2) Dump out just one CPU

3) use kexec/kdump and make a crash kernel to extract trace.dat from


That's my debugging environment, but it looks like you want something
else.

-- Steve