[PATCH 2/4] perf cs-etm: Use previous thread for branch sample source IP

[Leo Yan]

On Wed, May 24, 2023 at 02:19:56PM +0100, James Clark wrote:
> Branch samples currently use the IP of the previous packet as the from
> IP, and the IP of the current packet as the to IP. But it incorrectly
> uses the current thread. In some cases like a jump into a different
> exception level this will attribute to the incorrect process.

It's about the timing that branch has taken or not taken :)

If we think the branch sample as 'branch has taken', then current code
is doning right thing, otherwise, we need this fix.

> Fix it by tracking the previous thread in the same way the previous
> packet is tracked.
> 
> Signed-off-by: James Clark <james.clark at arm.com>
> ---
>  tools/perf/util/cs-etm.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
> index ebffc9052561..a997fe79d458 100644
> --- a/tools/perf/util/cs-etm.c
> +++ b/tools/perf/util/cs-etm.c
> @@ -86,6 +86,7 @@ struct cs_etm_traceid_queue {
>  	size_t last_branch_pos;
>  	union perf_event *event_buf;
>  	struct thread *thread;
> +	struct thread *prev_thread;
>  	struct branch_stack *last_branch;
>  	struct branch_stack *last_branch_rb;
>  	struct cs_etm_packet *prev_packet;
> @@ -480,6 +481,7 @@ static int cs_etm__init_traceid_queue(struct cs_etm_queue *etmq,
>  	tidq->trace_chan_id = trace_chan_id;
>  	tidq->thread = machine__findnew_thread(&etm->session->machines.host, -1,
>  					       queue->tid);
> +	tidq->prev_thread = machine__idle_thread(&etm->session->machines.host);
>  
>  	tidq->packet = zalloc(sizeof(struct cs_etm_packet));
>  	if (!tidq->packet)
> @@ -616,6 +618,8 @@ static void cs_etm__packet_swap(struct cs_etm_auxtrace *etm,
>  		tmp = tidq->packet;
>  		tidq->packet = tidq->prev_packet;
>  		tidq->prev_packet = tmp;
> +		thread__put(tidq->prev_thread);
> +		tidq->prev_thread = thread__get(tidq->thread);

Maybe cs_etm__packet_swap() is not the best place to update
"tidq->prev_thread", since swapping packet doesn't mean it's necessarily
thread switching; can we move this change into the cs_etm__set_thread()?

Thanks,
Leo

>  	}
>  }
>  
> @@ -791,6 +795,7 @@ static void cs_etm__free_traceid_queues(struct cs_etm_queue *etmq)
>  		/* Free this traceid_queue from the array */
>  		tidq = etmq->traceid_queues[idx];
>  		thread__zput(tidq->thread);
> +		thread__zput(tidq->prev_thread);
>  		zfree(&tidq->event_buf);
>  		zfree(&tidq->last_branch);
>  		zfree(&tidq->last_branch_rb);
> @@ -1450,8 +1455,8 @@ static int cs_etm__synth_branch_sample(struct cs_etm_queue *etmq,
>  	sample.time = cs_etm__resolve_sample_time(etmq, tidq);
>  
>  	sample.ip = ip;
> -	sample.pid = tidq->thread->pid_;
> -	sample.tid = tidq->thread->tid;
> +	sample.pid = tidq->prev_thread->pid_;
> +	sample.tid = tidq->prev_thread->tid;
>  	sample.addr = cs_etm__first_executed_instr(tidq->packet);
>  	sample.id = etmq->etm->branches_id;
>  	sample.stream_id = etmq->etm->branches_id;
> -- 
> 2.34.1
>