Re: Unified tracing buffer

[Jason Baron <jbaron@redhat.com>]

On Fri, Oct 03, 2008 at 12:11:54PM -0400, Mathieu Desnoyers wrote:
> > > > > How about :
> > > > > 
> > > > >   trace_mark(ftrace_evname, "size %lu binary %pW",
> > > > >     sizeof(mystruct), mystruct);
> > > > >   or
> > > > >   trace_mark(sched_wakeup, "target_pid %ld", task->pid);
> > > > > 
> > > > > Note the namespacing with buffers being "ftrace" and "sched" here.
> > > > > 
> > > > > That would encapsulate the whole
> > > > >   - Event ID registration
> > > > >   - Event type registration
> > > > >   - Sending data out
> > > > >   - Enabling the event source directly at the source
> > > > > 
> > > > > We can then export the markers through a debugfs file and let userland
> > > > > enable them one by one and possibly connect systemtap filters on them
> > > > > (one table of registered filters, one table for the markers, a command
> > > > > file to connect/disconnect filters to/from markers).
> > > > 
> > > > I would like to ask for the following from the start: have a field for
> > > > a longer description of the marker that describes it's usage and
> > > > context. Getting this there from the start is critical, because only
> > > > when adding the marker point do people still really remember why/what
> > > > (and having to type a good description also helps them to realize if
> > > > this is the right point or not). This can then be exposed to the user
> > > > so he has a standing chance of knowing what the marker is about.
> > > > 
> > > > It also has a standing chance of being updated when the code changes
> > > > this way
> > > > 
> > > 
> > > I agree, and I think it might be required in both markers and
> > > tracepoints.
> > > 
> > > Given that tracepoints are declared in a global header
> > > (DECLARE_TRACE()), I would add this kind of description here. Tracepoint
> > > uses within the kernel code (statements like :
> > >   trace_sched_switch(prev, next);
> > > added to the scheduler) would therefore be tied to the description
> > > without having to contain it in the core kernel code.
> > > 
> > > Markers, on the other hand, could become the "event description"
> > > interface which is exported to userspace. Considering that, I guess it's
> > > as important to let a precise description follow the markers.
> > > 
> > > Mathieu
> > > 
> > >
> > 
> > hi,
> > 
> > Tracepoints and markers seem to both have their place, with tracepoints
> > being integral to kernel users, and markers being important for
> > userspace. However, it seems to me like there is overlap in the
> > code and an extra level of indirection when markers are layered on
> > tracespoints. could they be merged a bit more?
> > 
> > What if we extended DEFINE_TRACE() to also create a
> > 'set_marker(marker_cb)' function where 'marker_cb' has the function signature:
> > 
> > marker_cb(<tracepoint prototype>, *marker_probe_func);
> > 
> > We then also create 'register_marker_##name' function in DEFINE_TRACE(),
> > which allows one to regiser marker callbacks in the usual way.
> > 
> > Then 'marker_cb' function is then called in '__DO_TRACE' if anybody has
> > registered a marker (which can set the tracepoint.state appropriately).
> > 
> > The 'marker_cb' function then marshalls its arguemnts and passes them
> > through to the marker functions that were registered.
> > 
> > I think in this way we can simplify the tracepoints and markers by
> > combining them to a large extent.
> > 
> > thanks,
> > 
> > -Jason
> > 
> 
> I think what you propose here is already in y LTTng tree in a different
> form. It's a patch to markers to allow declaring a marker which enables
> an associated tracepoint when enabled. This way, we can have a marker
> (exposed to userspace) connecting itself automatically to a tracepoint
> when enabled.
> 
> It's here :
> http://git.kernel.org/?p=linux/kernel/git/compudj/linux-2.6-lttng.git;a=commitdiff;h=d52ea7c48f47a1179aee01636d515cfea4ff6ede;hp=0a7b5c02209f3582ed1369ec818a1b389bd45a09
> 
> Note that locking depends on the psrwlock patch so we can have nested
> module list readers. Otherwise locking becomes _really_ messy. :-(
> 
> Mathieu
> 

That patch simplifies using markers with tracepoints and couples
markers and tracepoints much more closely. But I was proposing to make
the coupling tighter...

Couldn't 'marker_probe_register()' register the marker directly with
the tracepoint callsite? Have DEFINE_TRACE() take an additional argument
which references a marker callback funtion. That function would look 
like (very loose C code):

marker_blah_callback(TPPROTO(arg1, arg2), marker_probe_func *probe,
private_data)
{
	probe(private_data, "%arg1 %arg2", arg1->a, arg2->b);
}

The 'marker_blah_callback()' would be invoked from within DO_TRACE() for
each marker that has been registered with the associated tracepoint, in
a similar way to how we iterate over the tracepoint callbacks, we can
iterate over the registered markers and pass them to the
'marker_blah_callback()' function. 

By associating the marker_blah_callback() in DEFINE_TRACE(), we only
need to look in one file to understand what is associated with a
particular tracepoint. I think marker.c and tracepoint.c could also be
consolidated at that point.

thanks,

-Jason