Re: [PATCH v2 15/16] coresight: Re-emit trace IDs when the sink changes in per-thread mode

[Suzuki K Poulose <suzuki.poulose@arm.com>]

On 04/06/2024 15:30, 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@arm.com>
> ---
>   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 17cafa1a7f18..b6f505b50e67 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_sink_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;
> +	}

I am wondering if we really need this patch, as we have the sinkid in 
the HWID already ? We would emit the packet for each CPU only once and
that wouldn't change the HWID ?

Suzuki


> +
>   	/*
>   	 * output cpu / trace ID in perf record, once for the lifetime
>   	 * of the event.
> diff --git a/drivers/hwtracing/coresight/coresight-etm-perf.h b/drivers/hwtracing/coresight/coresight-etm-perf.h
> index 744531158d6b..bd4553b2a1ec 100644
> --- a/drivers/hwtracing/coresight/coresight-etm-perf.h
> +++ b/drivers/hwtracing/coresight/coresight-etm-perf.h
> @@ -52,6 +52,7 @@ struct etm_filters {
>    * @snk_config:		The sink configuration.
>    * @cfg_hash:		The hash id of any coresight config selected.
>    * @path:		An array of path, each slot for one CPU.
> + * @last_sink_hwid:	Last sink that a hwid was emitted for.
>    */
>   struct etm_event_data {
>   	struct work_struct work;
> @@ -60,6 +61,7 @@ struct etm_event_data {
>   	void *snk_config;
>   	u32 cfg_hash;
>   	struct list_head * __percpu *path;
> +	struct coresight_device *last_sink_hwid;
>   };
>   
>   int etm_perf_symlink(struct coresight_device *csdev, bool link);