On Thu, May 5, 2011 at 1:08 PM, David Christensen <davi...@broadcom.com> wrote:
> I was looking at using dtrace to help characterize performance
> for the new bxe(4) driver but I'm having problems with the very
> simple task of capturing time spent in a function. The D script
> I'm using looks like the following:
>
> #pragma D option quiet
>
> fbt:if_bxe::entry
> {
> self->in = timestamp;
> }
>
> fbt:if_bxe::return
> {
>
> @callouts[((struct callout *)arg0)->c_func] = sum(timestamp -
> self->in);
> }
>
> tick-10sec
> {
> printa("%40a %10@d\n", @callouts);
> clear(@callouts);
> printf("\n");
> }
>
> BEGIN
> {
> printf("%40s | %s\n", "function", "nanoseconds per second");
> }
>
> After building dtrace into the kernel and loading the dtraceall
> kernel module, when I load my bxe kernel module and run "dtrace -l"
> to list all supported probes I notice that many functions have an
> entry probe but no exit probe. This effectively prevents me from
> calculating timestamps on "fbt:if_bxe::return" probes. Why am I
> seeing this behavior?
Tail call optimization could do that to you:
http://en.wikipedia.org/wiki/Tail_call
--Artem
>
> Dave
>
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