On Wed, Mar 16, 2016 at 06:27:48PM +0000, Dr. David Alan Gilbert wrote: > * Stefan Hajnoczi (stefa...@redhat.com) wrote: > > On Tue, Mar 15, 2016 at 01:56:47PM +0000, Dr. David Alan Gilbert wrote: > > > > This would put trace_foo() in generated-tracers-virtio-blk.h and > > > > trace_bar() in generated-tracers-memory.h. Source files using tracing > > > > would need to include headers for relevant sections. > > > > > > > > This way we can narrow the scope of tracing headers and prevent global > > > > recompilation. > > > > > > > > The fly in the ointment is that trace/control.h defines enum > > > > TraceEventID, a global numbering of all trace events. The enum is used > > > > in trace/contro.h APIs and also in the simpletrace file format. > > > > > > > > If ./trace-event is modified the numbering of trace events could change. > > > > This would require global recompilation :(. > > > > > > > > So in order to avoid global recompilation we need to eliminate enum > > > > TraceEventID. Perhaps it's possible to use TraceEvent* instead of > > > > TraceEventID. For the simpletrace backend we would continue to use a > > > > global ordering but that only affects generated-tracers.c and not header > > > > files (thankfully!). > > > > > > I think the hardest part is going to be understanding all of the different > > > tracers and the things they need to know; like that thing about > > > simpletrace > > > needing the single ordering; I don't know what the other backends need > > > but I bet they've each got their own surprise. > > > > > > (And yes, I'd love to split them up - I tend to use traceing quite a bit > > > in migration now and trying to do a bisect over a big migration patch > > > series takes ages mostly because of the trace.h changes) > > > > Yes, this is the hard part. The simpletrace.py pretty-printing script > > takes a trace-events file as input. I suppose it could alphabetically > > sort the trace-events filenames from the command-line to produce a > > global ordering of trace event IDs. Of course that doesn't work > > properly if the script is invoked with relative paths from a > > sub-directory... > > > > Perhaps we need a new script during the QEMU build process that produces > > a trace-event-ids.csv file. Then simpletrace.py could take that instead > > of processing ./trace-events. > > Well that sounds easy - but is it easy to avoid using those fixed IDs > in any of the trace.h functions that are included everywhere (without > making them more expensive).
How about this: Instead of using TraceEventID in tracing APIs, pass the TraceEvent pointer directly. The tracetool.py code generator should produce "extern TraceEvent trace_event_foo;" definitions in headers so that the users have direct access to the TraceEvents. Stefan
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