[tip:perf/core] perf intel-pt: Do not use TSC packets for calculating CPU cycles to TSC

tip-bot for Adrian Hunter
2017-07-01T08:57:12Z

Commit-ID: 38b65b0891dc129dc0a5ce148a21c481e667b395
Gitweb: http://git.kernel.org/tip/38b65b0891dc129dc0a5ce148a21c481e667b395
Author: Adrian Hunter <adrian.hunter@intel.com>
AuthorDate: Fri, 26 May 2017 11:17:37 +0300
Committer: Arnaldo Carvalho de Melo <acme@redhat.com>
CommitDate: Fri, 30 Jun 2017 11:50:55 -0300

perf intel-pt: Do not use TSC packets for calculating CPU cycles to TSC

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>
Cc: Andi Kleen <ak@linux.intel.com>
Link: http://lkml.kernel.org/r/1495786658-18063-37-git-send-email-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.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 5dea062..aa1593c 100644
--- a/tools/perf/util/intel-pt-decoder/intel-pt-decoder.c
+++ b/tools/perf/util/intel-pt-decoder/intel-pt-decoder.c
@@ -711,6 +711,12 @@ 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 &&
@@ -828,6 +834,14 @@ 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);
}