[V3,2/2] perf: cs-etm: Store previous timestamp in packet queue

Commit Message

Tanmay Jagdale March 27, 2025, 11:11 a.m. UTC

Since logic in cs_etm__sample is changed to synthesizing the previous
packet (tidq->prev_packet) instead of current packet (tidq->packet),
the first time this function is called, tidq->prev_packet is NULL
and we return without processing anything.

This is as expected but, in the first call, we would have a valid
timestamp (stored in tidq->packet_queue.cs_timestamp) which belongs
to tidq->packet. This would be lost due to no processing.

Losing this timestamp results in all the synthesized packets being
associated with the next timestamp and not their corresponding one.

To fix this, introduce a new variable (prev_cs_timestamp) to store the
queue's timestamp in cs_etm__sample(). When we start synthesizing the
prev_packet, use this cached value instead of the current timestamp
(cs_timestamp).

Signed-off-by: Tanmay Jagdale <[e-mail]>
Reviewed-by: James Clark <[e-mail]>
---
 tools/perf/util/cs-etm.c | 17 +++++++++++++----
 tools/perf/util/cs-etm.h |  1 +
 2 files changed, 14 insertions(+), 4 deletions(-)

Patch

diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
index ebed5b98860e..b52b58328ca0 100644
--- a/tools/perf/util/cs-etm.c
+++ b/tools/perf/util/cs-etm.c
@@ -1542,13 +1542,15 @@  u64 cs_etm__convert_sample_time(struct cs_etm_queue *etmq, u64 cs_timestamp)
 }
 
 static inline u64 cs_etm__resolve_sample_time(struct cs_etm_queue *etmq,
-					       struct cs_etm_traceid_queue *tidq)
+					       struct cs_etm_traceid_queue *tidq,
+					       bool instruction_sample)
 {
 	struct cs_etm_auxtrace *etm = etmq->etm;
 	struct cs_etm_packet_queue *packet_queue = &tidq->packet_queue;
 
 	if (!etm->timeless_decoding && etm->has_virtual_ts)
-		return packet_queue->cs_timestamp;
+		return instruction_sample ? packet_queue->prev_cs_timestamp :
+					    packet_queue->cs_timestamp;
 	else
 		return etm->latest_kernel_timestamp;
 }
@@ -1567,7 +1569,7 @@  static int cs_etm__synth_instruction_sample(struct cs_etm_queue *etmq,
 	event->sample.header.size = sizeof(struct perf_event_header);
 
 	/* Set time field based on etm auxtrace config. */
-	sample.time = cs_etm__resolve_sample_time(etmq, tidq);
+	sample.time = cs_etm__resolve_sample_time(etmq, tidq, true);
 
 	sample.ip = addr;
 	sample.pid = thread__pid(tidq->thread);
@@ -1643,7 +1645,7 @@  static int cs_etm__synth_branch_sample(struct cs_etm_queue *etmq,
 	event->sample.header.size = sizeof(struct perf_event_header);
 
 	/* Set time field based on etm auxtrace config. */
-	sample.time = cs_etm__resolve_sample_time(etmq, tidq);
+	sample.time = cs_etm__resolve_sample_time(etmq, tidq, false);
 
 	sample.ip = ip;
 	sample.pid = thread__pid(tidq->prev_packet_thread);
@@ -1903,6 +1905,13 @@  static int cs_etm__sample(struct cs_etm_queue *etmq,
 		}
 	}
 
+	/*
+	 * Since we synthesize the prev_packet, store the current timestamp
+	 * here in prev_cs_timestamp so that when we *actually* synthesize
+	 * the prev_packet, we use this timestamp and not the future one.
+	 */
+	tidq->packet_queue.prev_cs_timestamp = tidq->packet_queue.cs_timestamp;
+
 	cs_etm__packet_swap(etm, tidq);
 
 	return 0;
diff --git a/tools/perf/util/cs-etm.h b/tools/perf/util/cs-etm.h
index a8caeea720aa..359d6233dd9b 100644
--- a/tools/perf/util/cs-etm.h
+++ b/tools/perf/util/cs-etm.h
@@ -205,6 +205,7 @@  struct cs_etm_packet_queue {
 	u32 instr_count;
 	u64 cs_timestamp; /* Timestamp from trace data, converted to ns if possible */
 	u64 next_cs_timestamp;
+	u64 prev_cs_timestamp;
 	struct cs_etm_packet packet_buffer[CS_ETM_PACKET_MAX_BUFFER];
 };