Messages in this thread |  | | | Date | Wed, 17 Nov 2010 13:36:42 -0500 | | From | Mathieu Desnoyers <> | | Subject | Re: [ANNOUNCE] New utility: 'trace' |
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* Tom Zanussi (tzanussi@gmail.com) 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.
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
-- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com
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