Re: [Qemu-devel] [PATCH] kvm: add flightrecorder script

[Stefan Hajnoczi]

On Mon, Mar 12, 2012 at 02:08:10PM +0200, Avi Kivity wrote:
> On 03/09/2012 04:13 PM, Stefan Hajnoczi wrote:
> > The kvm kernel module includes a number of trace events which can be
> > useful when debugging system behavior.  Even on production systems these
> > trace events can be used to observe guest behavior and identify the
> > source of problems.
> >
> > The kvm_flightrecorder script is a command-line wrapper for the
> > /sys/kernel/debug/tracing interface.  Kernel symbols do not need to be
> > installed.
> >
> > This script captures a fixed-size buffer of KVM trace events.  Recent
> > events overwrite the oldest events when the buffer size is exceeded and
> > it is possible to leave KVM tracing enabled for any period of time with
> > just a fixed-size buffer.  If the buffer is large enough this script is
> > a useful tool for collecting detailed information after an issue occurs
> > with a guest.  Hence the name "flight recorder".
> >
> > The script can also be used in 'tail' mode to simply view KVM trace
> > events as they occur.  This is handy for development and to ensure that
> > the guest is indeed running.
> >
> >
> 
> Have you considered updating trace-cmd or perf-something instead?  While
> each of these tools are useful we miss out on combining capabilities. 
> For example you could 'perf probe' a dynamic tracepoint and flightrecord
> it together with kvm tracepoints.

This script is ideal in situations where you want to ask someone who is
not a developer to collect information.  They sometimes don't want to
install software but running a small Python script is okay.  So that's
the background of this tool.

That said, I agree that integrating flightrecorder mode into perf would
be very nice.  I'm not sure I'll be able to come up with patches soon
but if someone is interested in doing this that would be fantastic.

Stefan