On 01/11/2019 02:07, Leo Yan wrote:
Every time synthesize instruction sample, the last branches recording will be reset. This would be fine if the instruction period is big enough, for example if we use the option '--itrace=i100000', the last branch array is reset for every instruction sample (10000 instructions per period); before generate the next instruction sample, there has the enough packets coming to fill 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 if the last branch array is reset for the previous instruction sample, it's almost empty for the next instruction sample.
To allow the last branches to work for any instruction periods, this patch avoids to reset the last branches 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; thus it can continuously record last branches.
Is this the right thing to do? This would cause profiling tools to count the same branch several times if it appears in multiple instruction samples, which could result in a biased profile.
The current implementation matches the behavior of intel_pt where the branch buffer is reset after each sample, so the instruction sample only includes branches since the previous sample.
However x86 lbr (perf record -b) does appear to repeat the same full branch stack on several samples until a new stack is captured.
I'm not sure what the right or wrong answer is here. For AutoFDO, we're likely to use a much bigger period (>10000 instructions) so won't be affected, but other tools might be.
Regards
Rob
Signed-off-by: Leo Yan leo.yan@linaro.org
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 f5f855fff412..8be6d010ae84 100644 --- a/tools/perf/util/cs-etm.c +++ b/tools/perf/util/cs-etm.c @@ -1153,9 +1153,6 @@ static int cs_etm__synth_instruction_sample(struct cs_etm_queue *etmq, "CS ETM Trace: failed to deliver instruction event, error %d\n", ret);
- if (etm->synth_opts.last_branch)
cs_etm__reset_last_branch_rb(tidq);- return ret; }
@@ -1486,6 +1483,10 @@ static int cs_etm__flush(struct cs_etm_queue *etmq, tidq->prev_packet = tmp; }
- /* Reset last branches after flush the trace */
- if (etm->synth_opts.last_branch)
cs_etm__reset_last_branch_rb(tidq);- return err; }