[PATCH v3 6/6] perf cs-etm: Synchronize instruction sample with the thread stack

[Leo Yan]

The synthesized flow use 'tidq->packet' for instruction samples; on the
other hand, 'tidp->prev_packet' is used to generate the thread stack and
the branch samples, this results in the instruction samples using one
packet ahead than thread stack and branch samples ('tidp->prev_packet'
vs 'tidq->packet').

This leads to an instruction's callchain error as shows in below
example:

  main  1579        100      instructions:
  	ffff000010214854 perf_event_update_userpage+0x4c ([kernel.kallsyms])
  	ffff000010214850 perf_event_update_userpage+0x48 ([kernel.kallsyms])
  	ffff000010219360 perf_swevent_add+0x88 ([kernel.kallsyms])
  	ffff0000102135f4 event_sched_in.isra.57+0xbc ([kernel.kallsyms])
  	ffff0000102137a0 group_sched_in+0x60 ([kernel.kallsyms])
  	ffff000010213b84 flexible_sched_in+0xfc ([kernel.kallsyms])
  	ffff00001020c0b4 visit_groups_merge+0x12c ([kernel.kallsyms])

In the callchain log, for the two continuous lines the up line contains
one child function info and the followed line contains the caller
function info, and so forth.  So the first two lines are:

  perf_event_update_userpage+0x4c  => the sampled instruction
  perf_event_update_userpage+0x48  => the parent function's calling

The child function and parent function both are the same function
perf_event_update_userpage(), but this isn't a recursive function, thus
the sequence for perf_event_update_userpage() calling itself shouldn't
never happen.  This callchain error is caused by the instruction sample
using an ahead packet than the thread stack, the thread stack is deferred
to process the new packet and misses to pop stack if it is just a return
packet.

To fix this issue, we can simply change to use 'tidq->prev_packet' to
generate the instruction samples, this allows the thread stack to push
and pop synchronously with instruction sample.  Finally, the callchain
can be displayed correctly as below:

  main  1579        100      instructions:
	ffff000010214854 perf_event_update_userpage+0x4c ([kernel.kallsyms])
	ffff000010219360 perf_swevent_add+0x88 ([kernel.kallsyms])
	ffff0000102135f4 event_sched_in.isra.57+0xbc ([kernel.kallsyms])
	ffff0000102137a0 group_sched_in+0x60 ([kernel.kallsyms])
	ffff000010213b84 flexible_sched_in+0xfc ([kernel.kallsyms])
	ffff00001020c0b4 visit_groups_merge+0x12c ([kernel.kallsyms])

Signed-off-by: Leo Yan <leo.yan@xxxxxxxxxx>
---
 tools/perf/util/cs-etm.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
index 56e501cd2f5f..fa969dcb45d2 100644
--- a/tools/perf/util/cs-etm.c
+++ b/tools/perf/util/cs-etm.c
@@ -1419,7 +1419,7 @@ static int cs_etm__sample(struct cs_etm_queue *etmq,
 	struct cs_etm_packet *tmp;
 	int ret;
 	u8 trace_chan_id = tidq->trace_chan_id;
-	u64 instrs_executed = tidq->packet->instr_count;
+	u64 instrs_executed = tidq->prev_packet->instr_count;
 
 	tidq->period_instructions += instrs_executed;
 
@@ -1450,7 +1450,7 @@ static int cs_etm__sample(struct cs_etm_queue *etmq,
 		 */
 		s64 offset = (instrs_executed - instrs_over - 1);
 		u64 addr = cs_etm__instr_addr(etmq, trace_chan_id,
-					      tidq->packet, offset);
+					      tidq->prev_packet, offset);
 
 		ret = cs_etm__synth_instruction_sample(
 			etmq, tidq, addr, etm->instructions_sample_period);
-- 
2.17.1