Re: [PATCH 2/2] perf cs-etm: Set time on synthesised samples to preserve ordering

[Leo Yan]

Hi James,

On Wed, Apr 14, 2021 at 05:39:19PM +0300, James Clark wrote:
> The following attribute is set when synthesising samples in
> timed decoding mode:
> 
>     attr.sample_type |= PERF_SAMPLE_TIME;
> 
> This results in new samples that appear to have timestamps but
> because we don't assign any timestamps to the samples, when the
> resulting inject file is opened again, the synthesised samples
> will be on the wrong side of the MMAP or COMM events.
> 
> For example this results in the samples being associated with
> the perf binary, rather than the target of the record:
> 
>     perf record -e cs_etm/@tmc_etr0/u top
>     perf inject -i perf.data -o perf.inject --itrace=i100il
>     perf report -i perf.inject
> 
> Where 'Command' == perf should show as 'top':
> 
>     # Overhead  Command  Source Shared Object  Source Symbol           Target Symbol           Basic Block Cycles
>     # ........  .......  ....................  ......................  ......................  ..................
>     #
>         31.08%  perf     [unknown]             [.] 0x000000000040c3f8  [.] 0x000000000040c3e8  -
> 
> If the perf.data file is opened directly with perf, without the
> inject step, then this already works correctly because the
> events are synthesised after the COMM and MMAP events and
> no second sorting happens. Re-sorting only happens when opening
> the perf.inject file for the second time so timestamps are
> needed.
> 
> Using the timestamp from the AUX record mirrors the current
> behaviour when opening directly with perf, because the events
> are generated on the call to cs_etm__process_queues().
> 
> Signed-off-by: James Clark <james.clark@xxxxxxx>
> Co-developed-by: Al Grant <al.grant@xxxxxxx>
> Signed-off-by: Al Grant <al.grant@xxxxxxx>
> ---
>  tools/perf/util/cs-etm.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
> index c25da2ffa8f3..d0fa9dce47f1 100644
> --- a/tools/perf/util/cs-etm.c
> +++ b/tools/perf/util/cs-etm.c
> @@ -54,6 +54,7 @@ struct cs_etm_auxtrace {
>  	u8 sample_instructions;
>  
>  	int num_cpu;
> +	u64 latest_kernel_timestamp;
>  	u32 auxtrace_type;
>  	u64 branches_sample_type;
>  	u64 branches_id;
> @@ -1192,6 +1193,8 @@ static int cs_etm__synth_instruction_sample(struct cs_etm_queue *etmq,
>  	event->sample.header.misc = cs_etm__cpu_mode(etmq, addr);
>  	event->sample.header.size = sizeof(struct perf_event_header);
>  
> +	if (!etm->timeless_decoding)
> +		sample.time = etm->latest_kernel_timestamp;
>  	sample.ip = addr;
>  	sample.pid = tidq->pid;
>  	sample.tid = tidq->tid;
> @@ -1248,6 +1251,8 @@ static int cs_etm__synth_branch_sample(struct cs_etm_queue *etmq,
>  	event->sample.header.misc = cs_etm__cpu_mode(etmq, ip);
>  	event->sample.header.size = sizeof(struct perf_event_header);
>  
> +	if (!etm->timeless_decoding)
> +		sample.time = etm->latest_kernel_timestamp;
>  	sample.ip = ip;
>  	sample.pid = tidq->pid;
>  	sample.tid = tidq->tid;
> @@ -2412,9 +2417,10 @@ static int cs_etm__process_event(struct perf_session *session,
>  	else if (event->header.type == PERF_RECORD_SWITCH_CPU_WIDE)
>  		return cs_etm__process_switch_cpu_wide(etm, event);
>  
> -	if (!etm->timeless_decoding &&
> -	    event->header.type == PERF_RECORD_AUX)
> +	if (!etm->timeless_decoding && event->header.type == PERF_RECORD_AUX) {
> +		etm->latest_kernel_timestamp = sample_kernel_timestamp;
>  		return cs_etm__process_queues(etm);
> +	}

The change looks good to me, I went through these two patches for at
least twice, and didn't find issue.

And given the trace data might be overflow and overwritten, it's
reasonable for me to use the PERF_RECORD_AUX timestamp from the tail of
trace data.

Reviewed-by: Leo Yan <leo.yan@xxxxxxxxxx>

>  	return 0;
>  }
> -- 
> 2.28.0
>