Re: [Qemu-devel] Static tracepoint control via trace-event

["Daniel P. Berrange" <berrange@redhat.com>]

On Tue, Oct 19, 2010 at 03:08:08PM +0200, Jan Kiszka wrote:
> Hi Stefan,
> 
> just had a closer look at qemu's new tracing framework. Looks cool,
> though it leaves a bit room for improvements. ;)
> 
> One quirk I stumbled over quickly was the "disable" tag in trace-events.
> It confused me first as qemu starts without any tracepoint enabled by
> default and I thought I had to hack the file. Then I read the doc and
> wondered which exiting or future backend would come without sufficiently
> fast dynamic tracepoint control. Do you have any in mind?
> 
> Instead of making it a compile-time switch (except for simpletrace), I
> would vote for declaring the simpletrace usage as the only one: disable
> sets the default state of the dynamic tracepoint. That way we could use
> trace-events to define a useful set of standard, moderate-impact
> tracepoints that shall be on. Others will still be available once a
> backend is configured, but remain off until enabled during runtime.
> Anything else looks like overkill to me.

FYI with the DTrace/SystemTAP backend I posted yesterday, the 'disable'
keyword is effectively completely ignored. All tracepoints are disabled
when QEMU is running normally. Only when a end user runs a dtrace script
that references a QEMU tracepoint, is that specific tracepoint enabled.

Regards,
Daniel
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