Re: [PATCH v2 2/6] perf cs-etm: Avoid stale branch samples when flush packet

Mathieu Poirier
2018-12-10T22:48:21Z

On Mon, Dec 10, 2018 at 04:52:57PM +0800, Leo Yan wrote:
> At the end of trace buffer handling, function cs_etm__flush() is invoked
> to flush any remaining branch stack entries. As a side effect, it also
> generates branch sample, because the 'etmq->packet' doesn't contains any
> new coming packet but point to one stale packet after packets swapping,
> so it wrongly makes synthesize branch samples with stale packet info.
>
> We could review below detailed flow which causes issue:
>
> Packet1: start_addr=0xffff000008b1fbf0 end_addr=0xffff000008b1fbfc
> Packet2: start_addr=0xffff000008b1fb5c end_addr=0xffff000008b1fb6c
>
> step 1: cs_etm__sample():
> sample: ip=(0xffff000008b1fbfc-4) addr=0xffff000008b1fb5c
>
> step 2: flush packet in cs_etm__run_decoder():
> cs_etm__run_decoder()
> `-> err = cs_etm__flush(etmq, false);
> sample: ip=(0xffff000008b1fb6c-4) addr=0xffff000008b1fbf0
>
> Packet1 and packet2 are two continuous packets, when packet2 is the new
> coming packet, cs_etm__sample() generates branch sample for these two
> packets and use [packet1::end_addr - 4 => packet2::start_addr] as branch
> jump flow, thus we can see the first generated branch sample in step 1.
> At the end of cs_etm__sample() it swaps packets so 'etm->prev_packet'=
> packet2 and 'etm->packet'=packet1, so far it's okay for branch sample.
>
> If packet2 is the last one packet in trace buffer, even there have no
> any new coming packet, cs_etm__run_decoder() invokes cs_etm__flush() to
> flush branch stack entries as expected, but it also generates branch
> samples by taking 'etm->packet' as a new coming packet, thus the branch
> jump flow is as [packet2::end_addr - 4 => packet1::start_addr]; this
> is the second sample which is generated in step 2. So actually the
> second sample is a stale sample and we should not generate it.
>
> This patch introduces a new function cs_etm__end_block(), at the end of
> trace block this function is invoked to only flush branch stack entries
> and thus can avoid to generate branch sample for stale packet.
>
> Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
> Cc: Mike Leach <mike.leach@linaro.org>
> Cc: Robert Walker <robert.walker@arm.com>
> Signed-off-by: Leo Yan <leo.yan@linaro.org>
> ---
> tools/perf/util/cs-etm.c | 35 ++++++++++++++++++++++++++++++++++-
> 1 file changed, 34 insertions(+), 1 deletion(-)
>
> diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
> index 789707b..ffc4fe5 100644
> --- a/tools/perf/util/cs-etm.c
> +++ b/tools/perf/util/cs-etm.c
> @@ -1055,6 +1055,39 @@ static int cs_etm__flush(struct cs_etm_queue *etmq)
> return err;
> }
>
> +static int cs_etm__end_block(struct cs_etm_queue *etmq)
> +{
> + int err;
> +
> + /*
> + * It has no new packet coming and 'etmq->packet' contains the stale
> + * packet which was set at the previous time with packets swapping;
> + * so skip to generate branch sample to avoid stale packet.
> + *
> + * For this case only flush branch stack and generate a last branch
> + * event for the branches left in the circular buffer at the end of
> + * the trace.
> + */
> + if (etmq->etm->synth_opts.last_branch &&
> + etmq->prev_packet->sample_type == CS_ETM_RANGE) {
> + /*
> + * Use the address of the end of the last reported execution
> + * range.
> + */
> + u64 addr = cs_etm__last_executed_instr(etmq->prev_packet);
> +
> + err = cs_etm__synth_instruction_sample(
> + etmq, addr,
> + etmq->period_instructions);
> + if (err)
> + return err;
> +
> + etmq->period_instructions = 0;
> + }
> +
> + return 0;
> +}
> +
> static int cs_etm__run_decoder(struct cs_etm_queue *etmq)
> {
> struct cs_etm_auxtrace *etm = etmq->etm;
> @@ -1137,7 +1170,7 @@ static int cs_etm__run_decoder(struct cs_etm_queue *etmq)
>
> if (err == 0)
> /* Flush any remaining branch stack entries */
> - err = cs_etm__flush(etmq);
> + err = cs_etm__end_block(etmq);
> }
>
> return err;

Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
> --
> 2.7.4
>