[PATCH v2 2/4] perf cs-etm: Correct synthesizing instruction samples

[Leo Yan]

When 'etm->instructions_sample_period' is less than
'tidq->period_instructions', the function cs_etm__sample() cannot handle
this case properly with its logic.

Let's see below flow as an example:

- If we set itrace option '--itrace=i4', then function cs_etm__sample()
  has variables with initialized values:

  tidq->period_instructions = 0
  etm->instructions_sample_period = 4

- When the first packet is coming:

  packet->instr_count = 10; the number of instructions executed in this
  packet is 10, thus update period_instructions as below:

  tidq->period_instructions = 0 + 10 = 10
  instrs_over = 10 - 4 = 6
  offset = 10 - 6 - 1 = 3
  tidq->period_instructions = instrs_over = 6

- When the second packet is coming:

  packet->instr_count = 10; in the second pass, assume 10 instructions
  in the trace sample again:

  tidq->period_instructions = 6 + 10 = 16
  instrs_over = 16 - 4 = 12
  offset = 10 - 12 - 1 = -3  -> the negative value
  tidq->period_instructions = instrs_over = 12

So after handle these two packets, there have below issues:

The first issue is that cs_etm__instr_addr() returns the address within
the current trace sample of the instruction related to offset, so the
offset is supposed to be always unsigned value.  But in fact, function
cs_etm__sample() might calculate a negative offset value (in handling
the second packet, the offset is -3) and pass to cs_etm__instr_addr()
with u64 type with a big positive integer.

The second issue is it only synthesizes 2 samples for sample period = 4.
In theory, every packet has 10 instructions so the two packets have
total 20 instructions, 20 instructions should generate 5 samples
(4 x 5 = 20).  This is because cs_etm__sample() only calls once
cs_etm__synth_instruction_sample() to generate instruction sample per
range packet.

This patch fixes the logic in function cs_etm__sample(); the basic
idea is to divide into three parts for handling coming packet:

- The first part is for synthesizing the first instruction sample, it
  combines the instructions from the tail of previous packet and the
  instructions from the head of the new packet;
- The second part is to simply generate samples with sample period
  aligned;
- The third part is the tail of new packet, the rest instructions will
  be left to next time handling with sequential packet.

Suggested-by: Mike Leach <mike.leach@xxxxxxxxxx>
Signed-off-by: Leo Yan <leo.yan@xxxxxxxxxx>
---
 tools/perf/util/cs-etm.c | 106 ++++++++++++++++++++++++++++++++++-----
 1 file changed, 93 insertions(+), 13 deletions(-)

diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
index 8be6d010ae84..8e9eb7583bcd 100644
--- a/tools/perf/util/cs-etm.c
+++ b/tools/perf/util/cs-etm.c
@@ -1360,23 +1360,103 @@ static int cs_etm__sample(struct cs_etm_queue *etmq,
 		 * TODO: allow period to be defined in cycles and clock time
 		 */
 
-		/* Get number of instructions executed after the sample point */
-		u64 instrs_over = tidq->period_instructions -
-			etm->instructions_sample_period;
+		/*
+		 * Below diagram is used to demonstrate the instruction samples
+		 * generation flows:
+		 *
+		 *    Instrs     Instrs       Instrs       Instrs
+		 *   Sample(n)  Sample(n+1)  Sample(n+2)  Sample(n+3)
+		 *    |            |            |            |
+		 *    V            V            V            V
+		 *   --------------------------------------------------
+		 *            ^                                  ^
+		 *            |                                  |
+		 *         Period                             Period
+		 *    instructions(Pi)                   instructions(Pi')
+		 *
+		 *            |                                  |
+		 *            \---------------- -----------------/
+		 *                             V
+		 *                      instrs_executed
+		 *
+		 * When the new instruction packet is coming, period
+		 * instructions (Pi) contains the the number of instructions
+		 * executed after the sample point(n).  So for the next sample
+		 * point(n+1), it is combined the two parts instructions, one
+		 * is the tail of the old packet and another is the head of
+		 * the new coming packet.  So we use 'head' variable to cauclate
+		 * the instruction numbers in the new packet for sample(n+1).
+		 *
+		 * For sample(n+2) and sample(n+3), they consume the instruction
+		 * for sample period, so we directly generate samples based on
+		 * the sampe period.
+		 *
+		 * After sample(n+3), there still leave some instructions which
+		 * will be used by later packet; so we use 'instrs_over' to
+		 * track the rest instruction number and its final value
+		 * presents the tail of the packet, it will be assigned to
+		 * 'tidq->period_instructions' for next round calculation.
+		 */
+		u64 head, offset = 0;
+		u64 addr;
 
 		/*
-		 * Calculate the address of the sampled instruction (-1 as
-		 * sample is reported as though instruction has just been
-		 * executed, but PC has not advanced to next instruction)
+		 * 'instrs_over' is the number of instructions executed after
+		 * sample points, initialise it to 'instrs_executed' and will
+		 * decrease it for consumed instructions in every synthesized
+		 * instruction sample.
 		 */
-		u64 offset = (instrs_executed - instrs_over - 1);
-		u64 addr = cs_etm__instr_addr(etmq, trace_chan_id,
-					      tidq->packet, offset);
+		u64 instrs_over = instrs_executed;
 
-		ret = cs_etm__synth_instruction_sample(
-			etmq, tidq, addr, etm->instructions_sample_period);
-		if (ret)
-			return ret;
+		/*
+		 * 'head' is the instructions number of the head in the new
+		 * packet, it combines with the tail of previous packet to
+		 * generate a sample.  So 'head' uses the sample period to
+		 * decrease the instruction number introduced by the previous
+		 * packet.
+		 */
+		head = etm->instructions_sample_period -
+				  (tidq->period_instructions - instrs_executed);
+
+		if (head) {
+			offset = head;
+
+			/*
+			 * Calculate the address of the sampled instruction (-1
+			 * as sample is reported as though instruction has just
+			 * been executed, but PC has not advanced to next
+			 * instruction)
+			 */
+			addr = cs_etm__instr_addr(etmq, trace_chan_id,
+						  tidq->packet, offset - 1);
+			ret = cs_etm__synth_instruction_sample(
+				etmq, tidq, addr,
+				etm->instructions_sample_period);
+			if (ret)
+				return ret;
+
+			instrs_over -= head;
+		}
+
+		while (instrs_over >= etm->instructions_sample_period) {
+			offset += etm->instructions_sample_period;
+
+			/*
+			 * Calculate the address of the sampled instruction (-1
+			 * as sample is reported as though instruction has just
+			 * been executed, but PC has not advanced to next
+			 * instruction)
+			 */
+			addr = cs_etm__instr_addr(etmq, trace_chan_id,
+						  tidq->packet, offset - 1);
+			ret = cs_etm__synth_instruction_sample(
+				etmq, tidq, addr,
+				etm->instructions_sample_period);
+			if (ret)
+				return ret;
+
+			instrs_over -= etm->instructions_sample_period;
+		}
 
 		/* Carry remaining instructions into next sample period */
 		tidq->period_instructions = instrs_over;
-- 
2.17.1