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Subject[PATCH v4 3/5] perf cs-etm: Correct synthesizing instruction samples
Date
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 for handling coming packet is:

- To synthesize the first instruction sample, it combines the left
instructions from the previous packet and the head of the new
packet; then generate continuous samples with sample period;
- At the tail of the new packet, if it has the rest instructions,
these instructions will be left for the sequential sample.

Suggested-by: Mike Leach <mike.leach@linaro.org>
Signed-off-by: Leo Yan <leo.yan@linaro.org>
---
tools/perf/util/cs-etm.c | 87 ++++++++++++++++++++++++++++++++--------
1 file changed, 70 insertions(+), 17 deletions(-)

diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
index b2f31390126a..4b7d6c36ce3c 100644
--- a/tools/perf/util/cs-etm.c
+++ b/tools/perf/util/cs-etm.c
@@ -1356,9 +1356,12 @@ static int cs_etm__sample(struct cs_etm_queue *etmq,
struct cs_etm_auxtrace *etm = etmq->etm;
int ret;
u8 trace_chan_id = tidq->trace_chan_id;
- u64 instrs_executed = tidq->packet->instr_count;
+ u64 instrs_prev;

- tidq->period_instructions += instrs_executed;
+ /* Get instructions remainder from previous packet */
+ instrs_prev = tidq->period_instructions;
+
+ tidq->period_instructions += tidq->packet->instr_count;

/*
* Record a branch when the last instruction in
@@ -1376,26 +1379,76 @@ 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 demonstrates 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
+ * tidq->packet->instr_count
+ *
+ * Instrs Sample(n...) are the synthesised samples occurring
+ * every etm->instructions_sample_period instructions - as
+ * defined on the perf command line. Sample(n) is being the
+ * last sample before the current etm packet, n+1 to n+3
+ * samples are generated from the current etm packet.
+ *
+ * tidq->packet->instr_count represents the number of
+ * instructions in the current etm packet.
+ *
+ * Period instructions (Pi) contains the the number of
+ * instructions executed after the sample point(n) from the
+ * previous etm packet. This will always be less than
+ * etm->instructions_sample_period.
+ *
+ * When generate new samples, it combines with two parts
+ * instructions, one is the tail of the old packet and another
+ * is the head of the new coming packet, to generate
+ * sample(n+1); sample(n+2) and sample(n+3) consume the
+ * instructions with sample period. After sample(n+3), the rest
+ * instructions will be used by later packet and it is assigned
+ * to tidq->period_instructions for next round calculation.
+ */

/*
- * 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)
+ * Get the initial offset into the current packet instructions;
+ * entry conditions ensure that instrs_prev is less than
+ * etm->instructions_sample_period.
*/
- u64 offset = (instrs_executed - instrs_over - 1);
- u64 addr = cs_etm__instr_addr(etmq, trace_chan_id,
- tidq->packet, offset);
+ u64 offset = etm->instructions_sample_period - instrs_prev;
+ u64 addr;

- ret = cs_etm__synth_instruction_sample(
- etmq, tidq, addr, etm->instructions_sample_period);
- if (ret)
- return ret;
+ while (tidq->period_instructions >=
+ 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;

- /* Carry remaining instructions into next sample period */
- tidq->period_instructions = instrs_over;
+ offset += etm->instructions_sample_period;
+ tidq->period_instructions -=
+ etm->instructions_sample_period;
+ }
}

if (etm->sample_branches) {
--
2.17.1
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