linux-kernel - [PATCH V2 36/37] perf intel-pt: Do not use TSC packets for calculating CPU cycles to TSC

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Message-Id: <1495786658-18063-37-git-send-email-adrian.hunter@intel.com>
Date:   Fri, 26 May 2017 11:17:37 +0300
From:   Adrian Hunter <adrian.hunter@...el.com>
To:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH V2 36/37] 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@...el.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 5dea06289db5..aa1593ce551d 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);
 }
 
-- 
1.9.1