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Subject[PATCH v4 2/5] perf cs-etm: Continuously record last branch
Every time synthesize instruction sample, the last branch recording
will be reset. This is fine if the instruction period is big enough,
for example if use the option '--itrace=i100000', the last branch
array is reset for every sample with 100000 instructions per period;
before generate the next instruction sample, there has the sufficient
packets coming to fill the last branch array.

On the other hand, if set a very small period, the packets will be
significantly reduced between two continuous instruction samples, thus
the last branch array is almost empty for new instruction sample by
frequently resetting.

To allow the last branches to work properly for any instruction periods,
this patch avoids to reset the last branch for every instruction sample
and only reset it when flush the trace data. The last branches will
be reset only for two cases, one is for trace starting, another case
is for discontinuous trace; other cases can keep recording last branches
for continuous instruction samples.

Signed-off-by: Leo Yan <>
Reviewed-by: Mike Leach <>
tools/perf/util/cs-etm.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
index 84f30c2de185..b2f31390126a 100644
--- a/tools/perf/util/cs-etm.c
+++ b/tools/perf/util/cs-etm.c
@@ -1170,9 +1170,6 @@ static int cs_etm__synth_instruction_sample(struct cs_etm_queue *etmq,
"CS ETM Trace: failed to deliver instruction event, error %d\n",

- if (etm->synth_opts.last_branch)
- cs_etm__reset_last_branch_rb(tidq);
return ret;

@@ -1485,6 +1482,10 @@ static int cs_etm__flush(struct cs_etm_queue *etmq,
cs_etm__packet_swap(etm, tidq);

+ /* Reset last branches after flush the trace */
+ if (etm->synth_opts.last_branch)
+ cs_etm__reset_last_branch_rb(tidq);
return err;

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