The hwpmc support Brendan's using here is in stock FreeBSD, but the default callchain capture depth is only 8 frames in FreeBSD 10.0, so you'd want to set in loader.conf:
kern.hwpmc.callchaindepth=32
The default is 32 in FreeBSD-Current and the upcoming 10.1 release.
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[ 2.5 ms ] story [ 16.2 ms ] threadkern.hwpmc.callchaindepth=32
The default is 32 in FreeBSD-Current and the upcoming 10.1 release.