Re: Unified tracing buffer

[Jason Baron <jbaron@redhat.com>]

On Mon, Sep 22, 2008 at 02:52:09PM -0400, Mathieu Desnoyers wrote:
> > On Sat, 20 Sep 2008 09:55:48 -0400
> > Mathieu Desnoyers <compudj@krystal.dyndns.org> 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