Messages in this thread | | | Subject | Re: [ANNOUNCE] New utility: 'trace' | From | Tom Zanussi <> | Date | Wed, 17 Nov 2010 13:25:29 -0600 |
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On Wed, 2010-11-17 at 14:02 -0500, Mathieu Desnoyers wrote: > * Tom Zanussi (tzanussi@gmail.com) wrote: > > On Wed, 2010-11-17 at 13:36 -0500, Mathieu Desnoyers wrote: > > > * 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. > > > > > > > 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)? > > It seems I might be missing something important, but what's wrong with using > registers or the stack to hold the value for comparison ? In this case, it's a
Nothing, it shouldn't matter where the temporary storage is, as long as it's not in the trace buffer.
Tom
> "long", so a register seems perfectly reasonable. But again, I feel I'm missing > a key point -- what is it ? > > Thanks, > > Mathieu > > > > > 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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