[PATCH 35/37] perf intel-pt: Do not use TSC packets for calculating CPU cycles to TSC

Adrian Hunter
2017-05-19T09:02:04Z

CBR (core-to-bus ratio) packets provide an indication of CPU frequency. A
more accurate measure can be made by counting the cycles (given by CYC
packets) in between other timing packets (either MTC or TSC). Using TSC
packets has at least 2 issues: 1) timing might have stopped (e.g. mwait) or
2) TSC packets within PSB+ might slip past CYC packets. For now, simply do
not use TSC packets for calculating CPU cycles to TSC. That leaves the case
where 2 MTC packets are used, otherwise falling back to the CBR value.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
---
tools/perf/util/intel-pt-decoder/intel-pt-decoder.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)

diff --git a/tools/perf/util/intel-pt-decoder/intel-pt-decoder.c b/tools/perf/util/intel-pt-decoder/intel-pt-decoder.c
index 9bb694290a28..a6f41a0e1ae8 100644
--- a/tools/perf/util/intel-pt-decoder/intel-pt-decoder.c
+++ b/tools/perf/util/intel-pt-decoder/intel-pt-decoder.c
@@ -690,6 +690,12 @@ static int intel_pt_calc_cyc_cb(struct intel_pt_pkt_info *pkt_info)
break;

case INTEL_PT_TSC:
+ /*
+ * For now, do not support using TSC packets - refer
+ * intel_pt_calc_cyc_to_tsc().
+ */
+ if (data->from_mtc)
+ return 1;
timestamp = pkt_info->packet.payload |
(data->timestamp & (0xffULL << 56));
if (data->from_mtc && timestamp < data->timestamp &&
@@ -807,6 +813,14 @@ static void intel_pt_calc_cyc_to_tsc(struct intel_pt_decoder *decoder,
.cbr_cyc_to_tsc = 0,
};

+ /*
+ * For now, do not support using TSC packets for at least the reasons:
+ * 1) timing might have stopped
+ * 2) TSC packets within PSB+ can slip against CYC packets
+ */
+ if (!from_mtc)
+ return;
+
intel_pt_pkt_lookahead(decoder, intel_pt_calc_cyc_cb, &data);
}

--
1.9.1