Re: [PATCH v2 09/13] perf: cs-etm: Update record event to use new Trace ID protocol

[James Clark <james.clark@arm.com>]

On 04/07/2022 09:11, Mike Leach wrote:
> Trace IDs are now dynamically allocated.
> 
> Previously used the static association algorithm that is no longer
> used. The 'cpu * 2 + seed' was outdated and broken for systems with high
> core counts (>46). as it did not scale and was broken for larger
> core counts.
> 
> Trace ID is  as unknown in AUXINFO record, and the ID / CPU association
> will now be sent in PERF_RECORD_AUX_OUTPUT_HW_ID record.
> 
> Remove legacy Trace ID allocation algorithm.
> 
> Signed-off-by: Mike Leach <mike.leach@linaro.org>
> ---
>  include/linux/coresight-pmu.h       | 19 +++++++------------
>  tools/include/linux/coresight-pmu.h | 19 +++++++------------

I usually see mentions that these header updates need to be separate commits
because they are merged through different trees.

>  tools/perf/arch/arm/util/cs-etm.c   | 21 ++++++++++++---------
>  3 files changed, 26 insertions(+), 33 deletions(-)
> 
> diff --git a/include/linux/coresight-pmu.h b/include/linux/coresight-pmu.h
> index 4ac5c081af93..9f7ee380266b 100644
> --- a/include/linux/coresight-pmu.h
> +++ b/include/linux/coresight-pmu.h
> @@ -8,7 +8,13 @@
>  #define _LINUX_CORESIGHT_PMU_H
>  
>  #define CORESIGHT_ETM_PMU_NAME "cs_etm"
> -#define CORESIGHT_ETM_PMU_SEED  0x10
> +
> +/*
> + * Metadata now contains an unused trace ID - IDs are transmitted using a
> + * PERF_RECORD_AUX_OUTPUT_HW_ID record.
> + * Value architecturally defined as reserved in CoreSight.
> + */
> +#define CS_UNUSED_TRACE_ID 0x7F
>  
>  /*
>   * Below are the definition of bit offsets for perf option, and works as
> @@ -32,15 +38,4 @@
>  #define ETM4_CFG_BIT_RETSTK	12
>  #define ETM4_CFG_BIT_VMID_OPT	15
>  
> -static inline int coresight_get_trace_id(int cpu)
> -{
> -	/*
> -	 * A trace ID of value 0 is invalid, so let's start at some
> -	 * random value that fits in 7 bits and go from there.  Since
> -	 * the common convention is to have data trace IDs be I(N) + 1,
> -	 * set instruction trace IDs as a function of the CPU number.
> -	 */
> -	return (CORESIGHT_ETM_PMU_SEED + (cpu * 2));
> -}
> -
>  #endif
> diff --git a/tools/include/linux/coresight-pmu.h b/tools/include/linux/coresight-pmu.h
> index 6c2fd6cc5a98..31d007fab3a6 100644
> --- a/tools/include/linux/coresight-pmu.h
> +++ b/tools/include/linux/coresight-pmu.h
> @@ -8,7 +8,13 @@
>  #define _LINUX_CORESIGHT_PMU_H
>  
>  #define CORESIGHT_ETM_PMU_NAME "cs_etm"
> -#define CORESIGHT_ETM_PMU_SEED  0x10
> +
> +/*
> + * Metadata now contains an unused trace ID - IDs are transmitted using a
> + * PERF_RECORD_AUX_OUTPUT_HW_ID record.
> + * Value architecturally defined as reserved in CoreSight.
> + */
> +#define CS_UNUSED_TRACE_ID 0x7F
>  

minor nit: this isn't used in the kernel so only needs to be defined on the
tools side.

>  /*
>   * Below are the definition of bit offsets for perf option, and works as
> @@ -34,15 +40,4 @@
>  #define ETM4_CFG_BIT_RETSTK	12
>  #define ETM4_CFG_BIT_VMID_OPT	15
>  
> -static inline int coresight_get_trace_id(int cpu)
> -{
> -	/*
> -	 * A trace ID of value 0 is invalid, so let's start at some
> -	 * random value that fits in 7 bits and go from there.  Since
> -	 * the common convention is to have data trace IDs be I(N) + 1,
> -	 * set instruction trace IDs as a function of the CPU number.
> -	 */
> -	return (CORESIGHT_ETM_PMU_SEED + (cpu * 2));
> -}
> -
>  #endif
> diff --git a/tools/perf/arch/arm/util/cs-etm.c b/tools/perf/arch/arm/util/cs-etm.c
> index 1b54638d53b0..2d68e6a722ed 100644
> --- a/tools/perf/arch/arm/util/cs-etm.c
> +++ b/tools/perf/arch/arm/util/cs-etm.c
> @@ -421,13 +421,16 @@ static int cs_etm_recording_options(struct auxtrace_record *itr,
>  	evlist__to_front(evlist, cs_etm_evsel);
>  
>  	/*
> -	 * In the case of per-cpu mmaps, we need the CPU on the
> -	 * AUX event.  We also need the contextID in order to be notified
> +	 * get the CPU on the sample - need it to associate trace ID in the
> +	 * AUX_OUTPUT_HW_ID event, and the AUX event for per-cpu mmaps.
> +	 */
> +	evsel__set_sample_bit(cs_etm_evsel, CPU);
> +
> +	/*
> +	 * Also the case of per-cpu mmaps, need the contextID in order to be notified
>  	 * when a context switch happened.
>  	 */
>  	if (!perf_cpu_map__empty(cpus)) {
> -		evsel__set_sample_bit(cs_etm_evsel, CPU);
> -
>  		err = cs_etm_set_option(itr, cs_etm_evsel,
>  					BIT(ETM_OPT_CTXTID) | BIT(ETM_OPT_TS));
>  		if (err)
> @@ -633,8 +636,9 @@ static void cs_etm_save_etmv4_header(__u64 data[], struct auxtrace_record *itr,
>  
>  	/* Get trace configuration register */
>  	data[CS_ETMV4_TRCCONFIGR] = cs_etmv4_get_config(itr);
> -	/* Get traceID from the framework */
> -	data[CS_ETMV4_TRCTRACEIDR] = coresight_get_trace_id(cpu);
> +	/* traceID set to unused */
> +	data[CS_ETMV4_TRCTRACEIDR] = CS_UNUSED_TRACE_ID;
> +
>  	/* Get read-only information from sysFS */
>  	data[CS_ETMV4_TRCIDR0] = cs_etm_get_ro(cs_etm_pmu, cpu,
>  					       metadata_etmv4_ro[CS_ETMV4_TRCIDR0]);
> @@ -681,9 +685,8 @@ static void cs_etm_get_metadata(int cpu, u32 *offset,
>  		magic = __perf_cs_etmv3_magic;
>  		/* Get configuration register */
>  		info->priv[*offset + CS_ETM_ETMCR] = cs_etm_get_config(itr);
> -		/* Get traceID from the framework */
> -		info->priv[*offset + CS_ETM_ETMTRACEIDR] =
> -						coresight_get_trace_id(cpu);
> +		/* traceID set to unused */
> +		info->priv[*offset + CS_ETM_ETMTRACEIDR] = CS_UNUSED_TRACE_ID;
>  		/* Get read-only information from sysFS */
>  		info->priv[*offset + CS_ETM_ETMCCER] =
>  			cs_etm_get_ro(cs_etm_pmu, cpu,