Re: [PATCH v2 1/4] perf cs-etm: Continuously record last branches

[Robert Walker]

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@xxxxxxxxxx>
> ---
>   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;
>   }
>