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

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Date:	Fri, 1 Jul 2016 13:27:58 +0900
From:	Namhyung Kim <namhyung@...nel.org>
To:	Steven Rostedt <rostedt@...dmis.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 Wed, Jun 29, 2016 at 09:52:31AM +0900, Namhyung Kim wrote:
> Hi Steve,
> 
> On Tue, Jun 28, 2016 at 09:57:27AM -0400, Steven Rostedt wrote:
> > 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.
> 
> Thanks for sharing.  Yeah, I'd like to know other ways to overcome
> this if possible.  Since I don't have enough knowledge about this
> area, I hope others would have better idea. :)

Now I'm thinking about extending the pstore subsystem.  AFAICS it's
the best fit for my use case.  While it only supports function tracer
with a dedicated ftrace_ops now, it can be used for ftrace dump IMHO.

Does it make sense to add a virtio pstore driver and saves the dump to
files on host?

Thanks,
Namhyung