Re: [ANNOUNCE] New utility: 'trace'

[Tom Zanussi <tzanussi@gmail.com>]

On Wed, 2010-11-17 at 15:11 +0100, Peter Zijlstra wrote:
> On Wed, 2010-11-17 at 15:00 +0100, Ingo Molnar wrote:
> > > But yes, it is functional.
> > 
> > I suspect that is what matters mostly - unless you think that it's impossible to 
> > have a sane implementation of it, if the users come. 
> 
> I'm not sure, I raised the point several times, and I think Steve and
> Tom though it was possible to implement sanely, but I'm not sure if it
> was expression invariant (the fix that is).
> 
> It would be very good to have a definite answer on that.
> 
> The idea was to not let the filter engine work on the trace data (once
> its gathered) but on the trace argument right at the beginning of the
> tracepoint callchain, since some of the trace data is an expression of
> the trace argument (say next->prio instead of next), the trace
> expression wouldn't stay invariant, you'd have to write a different
> filter for the same effect.
> 

IIRC, I think the conclusion we came to was that it could be done
mechanically if for example the right-hand-side of an assignment in
TP_fast_assign() only involved a simple variable assignment, but as
Steve pointed out, some assignments are more complicated than that.

For example, in the sched_switch tracepoint assignments:

	__entry->prev_prio      = prev->prio;
        __entry->prev_state     = __trace_sched_switch_state(prev);

so the prev_prio should be able to be tested 'in-line' but the
prev_state would require a temporary buffer to write the value into
before doing the test as mentioned by Steve.  At which point you're no
further ahead (in that case) than the current situation...

Tom

> So I think it would be wise to make this change sooner rather than
> later.