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

[Mathieu Poirier]

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@xxxxxxxxxx>
> Cc: Mike Leach <mike.leach@xxxxxxxxxx>
> Cc: Robert Walker <robert.walker@xxxxxxx>
> Signed-off-by: Leo Yan <leo.yan@xxxxxxxxxx>
> ---
>  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@xxxxxxxxxx>
> -- 
> 2.7.4
>