perf intel-pt: Fix premature IPC

BugLink: https://bugs.launchpad.net/bugs/1918974

[ Upstream commit 20aa397 ]

The code assumed a change in cycle count means accurate IPC. That is not
correct, for example when sampling both branches and instructions, or at
a FUP packet (which is not CYC-eligible) address. Fix by using an explicit
flag to indicate when IPC can be sampled.

Fixes: 5b1dc0f ("perf intel-pt: Add support for samples to contain IPC ratio")
Signed-off-by: Adrian Hunter <[email protected]>
Reviewed-by: Andi Kleen <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>
Signed-off-by: Kamal Mostafa <[email protected]>
Signed-off-by: Kelsey Skunberg <[email protected]>

Author: ahunter6

tools/perf/util/intel-pt-decoder/intel-pt-decoder.c

@@ -2637,9 +2637,18 @@ const struct intel_pt_state *intel_pt_decode(struct intel_pt_decoder *decoder)
 		}
 		if (intel_pt_sample_time(decoder->pkt_state)) {
 			intel_pt_update_sample_time(decoder);
-			if (decoder->sample_cyc)
+			if (decoder->sample_cyc) {
 				decoder->sample_tot_cyc_cnt = decoder->tot_cyc_cnt;
+				decoder->state.flags |= INTEL_PT_SAMPLE_IPC;
+				decoder->sample_cyc = false;
+			}
 		}
+		/*
+		 * When using only TSC/MTC to compute cycles, IPC can be
+		 * sampled as soon as the cycle count changes.
+		 */
+		if (!decoder->have_cyc)
+			decoder->state.flags |= INTEL_PT_SAMPLE_IPC;
 	}
 
 	decoder->state.timestamp = decoder->sample_timestamp;

tools/perf/util/intel-pt-decoder/intel-pt-decoder.h

@@ -17,6 +17,7 @@
 #define INTEL_PT_ABORT_TX	(1 << 1)
 #define INTEL_PT_ASYNC		(1 << 2)
 #define INTEL_PT_FUP_IP		(1 << 3)
+#define INTEL_PT_SAMPLE_IPC	(1 << 4)
 
 enum intel_pt_sample_type {
 	INTEL_PT_BRANCH		= 1 << 0,

tools/perf/util/intel-pt.c

@@ -1304,7 +1304,8 @@ static int intel_pt_synth_branch_sample(struct intel_pt_queue *ptq)
 		sample.branch_stack = (struct branch_stack *)&dummy_bs;
 	}
 
-	sample.cyc_cnt = ptq->ipc_cyc_cnt - ptq->last_br_cyc_cnt;
+	if (ptq->state->flags & INTEL_PT_SAMPLE_IPC)
+		sample.cyc_cnt = ptq->ipc_cyc_cnt - ptq->last_br_cyc_cnt;
 	if (sample.cyc_cnt) {
 		sample.insn_cnt = ptq->ipc_insn_cnt - ptq->last_br_insn_cnt;
 		ptq->last_br_insn_cnt = ptq->ipc_insn_cnt;
@@ -1366,7 +1367,8 @@ static int intel_pt_synth_instruction_sample(struct intel_pt_queue *ptq)
 	sample.stream_id = ptq->pt->instructions_id;
 	sample.period = ptq->state->tot_insn_cnt - ptq->last_insn_cnt;
 
-	sample.cyc_cnt = ptq->ipc_cyc_cnt - ptq->last_in_cyc_cnt;
+	if (ptq->state->flags & INTEL_PT_SAMPLE_IPC)
+		sample.cyc_cnt = ptq->ipc_cyc_cnt - ptq->last_in_cyc_cnt;
 	if (sample.cyc_cnt) {
 		sample.insn_cnt = ptq->ipc_insn_cnt - ptq->last_in_insn_cnt;
 		ptq->last_in_insn_cnt = ptq->ipc_insn_cnt;
@@ -1901,14 +1903,8 @@ static int intel_pt_sample(struct intel_pt_queue *ptq)
 
 	ptq->have_sample = false;
 
-	if (ptq->state->tot_cyc_cnt > ptq->ipc_cyc_cnt) {
-		/*
-		 * Cycle count and instruction count only go together to create
-		 * a valid IPC ratio when the cycle count changes.
-		 */
-		ptq->ipc_insn_cnt = ptq->state->tot_insn_cnt;
-		ptq->ipc_cyc_cnt = ptq->state->tot_cyc_cnt;
-	}
+	ptq->ipc_insn_cnt = ptq->state->tot_insn_cnt;
+	ptq->ipc_cyc_cnt = ptq->state->tot_cyc_cnt;
 
 	/*
 	 * Do PEBS first to allow for the possibility that the PEBS timestamp