Re: [ANNOUNCE] New utility: 'trace'

[Tom Zanussi]

On Wed, 2010-11-17 at 13:36 -0500, Mathieu Desnoyers wrote:
> * Tom Zanussi (tzanussi@xxxxxxxxx) wrote:
> [...]
> > 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.
> 
> Yep, we came up to the same conclusions in UST.
> 
> > 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...
> 
> if we change all assignments to, e.g.:
> 
> _tp_assign(__entry->prev_prio, prev->prio)
> _tp_assign(__entry->prev_state, __trace_sched_switch_state(prev))
> 
> then we can redefine the macros for filtering much more easily than with the
> " = " assignment operator.
> 
> About your comment above, what is the problem with evaluating
> "__trace_sched_switch_state(prev)" twice ? It will typically be cache-hot after
> the first evaluation, so I wonder if, in practice, we really save a significant
> amount of cycles by saving its result between filtering and writing into trace
> buffers. As I pointed out earlier, for my customers, having a very, very fast
> filter "out" case is more important that trying to squeeze a few cycles out of
> the filter passed case.
> 

But the idea is to avoid allocating the trace buffer in the first place,
until we've decided we want the event.  So how do you check the result
of __trace_sched_switch_state(prev) with the filter value if you don't
have it temporarily stored somewhere (not in the trace buffer, which
doesn't exist yet as far as this event is concerned)?

Tom

> Also, how many of these "__trace_sched_switch_state(prev)" are static inlines vs
> actual function calls ? If it's mostly static inlines to dereference a few
> pointers, doing it the second time when the filter passed won't hurt much.
> 
> Thanks,
> 
> Mathieu
> 


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