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

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Date: Tue, 7 May 2024 12:05:44 +0100
From: Suzuki K Poulose <suzuki.poulose@....com>
To: James Clark <james.clark@....com>, linux-perf-users@...r.kernel.org,
 gankulkarni@...amperecomputing.com, scclevenger@...amperecomputing.com,
 coresight@...ts.linaro.org, mike.leach@...aro.org
Cc: Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Maxime Coquelin <mcoquelin.stm32@...il.com>,
 Alexandre Torgue <alexandre.torgue@...s.st.com>,
 Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
 Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 Adrian Hunter <adrian.hunter@...el.com>, John Garry
 <john.g.garry@...cle.com>, Will Deacon <will@...nel.org>,
 Leo Yan <leo.yan@...ux.dev>, linux-arm-kernel@...ts.infradead.org,
 linux-kernel@...r.kernel.org, linux-stm32@...md-mailman.stormreply.com
Subject: Re: [PATCH 16/17] coresight: Re-emit trace IDs when the sink changes
 in per-thread mode

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@....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 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