Am 19.10.2010 15:52, Stefan Hajnoczi wrote: > On Tue, Oct 19, 2010 at 2:36 PM, Daniel P. Berrange <berra...@redhat.com> > wrote: >> 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. > > I think that makes sense for external trace backends. DTrace can > launch a process for you with the probes you want enabled from the > start. The simpletrace backend can't really do this so probes can be > enabled/disabled at compile-time (e.g. early startup tracing).
Once we have "-trace events=...", defining the list of active tracepoints before starting qemu will be trivial (e.g. via a config file). Of course, this requires that all tracepoints are built-in... Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux
