Re: [PATCH 16/17] coresight: Re-emit trace IDs when the sink changes in per-thread mode

[Suzuki K Poulose]

On 29/04/2024 16:22, James Clark wrote:
> In per-cpu mode there are multiple aux buffers and each one has a
> fixed sink, so the hw ID mappings which only need to be emitted once
> for each buffer, even with the new per-sink trace ID pools.
>
> But in per-thread mode there is only a single buffer which can be
> written to from any sink with now potentially overlapping trace IDs, so
> hw ID mappings need to be re-emitted every time the sink changes.
>
> This will require a change in Perf to track this so it knows which
> decode tree to use for each segment of the buffer. In theory it's also
> possible to look at the CPU ID on the AUX records, but this is more
> consistent with the existing system, and allows for correct decode using
> either mechanism.
>
> Signed-off-by: James Clark <james.clark@xxxxxxx>
> ---
>   drivers/hwtracing/coresight/coresight-etm-perf.c | 14 ++++++++++++++
>   drivers/hwtracing/coresight/coresight-etm-perf.h |  2 ++
>   2 files changed, 16 insertions(+)
>
> diff --git a/drivers/hwtracing/coresight/coresight-etm-perf.c b/drivers/hwtracing/coresight/coresight-etm-perf.c
> index f07173aa4d66..08f3958f9367 100644
> --- a/drivers/hwtracing/coresight/coresight-etm-perf.c
> +++ b/drivers/hwtracing/coresight/coresight-etm-perf.c
> @@ -499,6 +499,20 @@ static void etm_event_start(struct perf_event *event, int flags)
>   				      &sink->perf_id_map))
>   		goto fail_disable_path;
>   
> +	/*
> +	 * In per-cpu mode there are multiple aux buffers and each one has a
> +	 * fixed sink, so the hw ID mappings which only need to be emitted once
> +	 * for each buffer.
> +	 *
> +	 * But in per-thread mode there is only a single buffer which can be
> +	 * written to from any sink with potentially overlapping trace IDs, so
> +	 * hw ID mappings need to be re-emitted every time the sink changes.
> +	 */
> +	if (event->cpu == -1 && event_data->last_sink_hwid != sink) {
> +		cpumask_clear(&event_data->aux_hwid_done);
> +		event_data->last_sink_hwid = sink;
> +	}
> +

With the traceid caching in the event_data per-cpu , we could avoid this 
step ?

Suzuki