linux-kernel - Re: [RFT v2 1/4] perf cs-etm: Generate sample for missed packets

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Message-ID: <bc320d6d-a21d-b5a4-e30d-511bb69aba32@arm.com>
Date:   Mon, 21 May 2018 12:27:42 +0100
From:   Robert Walker <robert.walker@....com>
To:     Leo Yan <leo.yan@...aro.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Jonathan Corbet <corbet@....net>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, Tor Jeremiassen <tor@...com>,
        mike.leach@...aro.org, kim.phillips@....com,
        coresight@...ts.linaro.org
Cc:     Mike Leach <mike.leach@....com>
Subject: Re: [RFT v2 1/4] perf cs-etm: Generate sample for missed packets

Hi Leo,

On 21/05/18 09:52, Leo Yan wrote:
> Commit e573e978fb12 ("perf cs-etm: Inject capabilitity for CoreSight
> traces") reworks the samples generation flow from CoreSight trace to
> match the correct format so Perf report tool can display the samples
> properly.  But the change has side effect for packet handling, it only
> generate samples when 'prev_packet->last_instr_taken_branch' is true,
> this results in the start tracing packet and exception packets are
> dropped.
> 
> This patch checks extra two conditions for complete samples:
> 
> - If 'prev_packet->sample_type' is zero we can use this condition to
>    get to know this is the start tracing packet; for this case, the start
>    packet's end_addr is zero as well so we need to handle it in the
>    function cs_etm__last_executed_instr();
> 

I think you also need to add something in to handle discontinuities in
trace - for example it is possible to configure the ETM to only trace
execution in specific code regions or to trace a few cycles every so
often. In these cases, prev_packet->sample_type will not be zero, but 
whatever the previous packet was.  You will get a CS_ETM_TRACE_ON packet 
in such cases, generated by an I_TRACE_ON element in the trace stream.
You also get this on exception return.

However, you should also keep the test for prev_packet->sample_type == 0
as you may not see a CS_ETM_TRACE_ON when decoding a buffer that has
wrapped.

Regards

Rob

> - If 'prev_packet->exc' is true, we can know the previous packet is
>    exception handling packet so need to generate sample for exception
>    flow.
> 
> Fixes: e573e978fb12 ("perf cs-etm: Inject capabilitity for CoreSight traces")
> Cc: Mike Leach <mike.leach@....com>
> Cc: Robert Walker <robert.walker@....com>
> Cc: Mathieu Poirier <mathieu.poirier@...aro.org>
> Signed-off-by: Leo Yan <leo.yan@...aro.org>
> ---
>   tools/perf/util/cs-etm.c | 35 ++++++++++++++++++++++++++++-------
>   1 file changed, 28 insertions(+), 7 deletions(-)
> 
> diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
> index 822ba91..378953b 100644
> --- a/tools/perf/util/cs-etm.c
> +++ b/tools/perf/util/cs-etm.c
> @@ -495,6 +495,13 @@ static inline void cs_etm__reset_last_branch_rb(struct cs_etm_queue *etmq)
>   static inline u64 cs_etm__last_executed_instr(struct cs_etm_packet *packet)
>   {
>   	/*
> +	 * The packet is the start tracing packet if the end_addr is zero,
> +	 * returns 0 for this case.
> +	 */
> +	if (!packet->end_addr)
> +		return 0;
> +
> +	/*
>   	 * The packet records the execution range with an exclusive end address
>   	 *
>   	 * A64 instructions are constant size, so the last executed
> @@ -897,13 +904,27 @@ static int cs_etm__sample(struct cs_etm_queue *etmq)
>   		etmq->period_instructions = instrs_over;
>   	}
>   
> -	if (etm->sample_branches &&
> -	    etmq->prev_packet &&
> -	    etmq->prev_packet->sample_type == CS_ETM_RANGE &&
> -	    etmq->prev_packet->last_instr_taken_branch) {
> -		ret = cs_etm__synth_branch_sample(etmq);
> -		if (ret)
> -			return ret;
> +	if (etm->sample_branches && etmq->prev_packet) {
> +		bool generate_sample = false;
> +
> +		/* Generate sample for start tracing packet */
> +		if (etmq->prev_packet->sample_type == 0)
> +			generate_sample = true;
> +
> +		/* Generate sample for exception packet */
> +		if (etmq->prev_packet->exc == true)
> +			generate_sample = true;
> +
> +		/* Generate sample for normal branch packet */
> +		if (etmq->prev_packet->sample_type == CS_ETM_RANGE &&
> +		    etmq->prev_packet->last_instr_taken_branch)
> +			generate_sample = true;
> +
> +		if (generate_sample) {
> +			ret = cs_etm__synth_branch_sample(etmq);
> +			if (ret)
> +				return ret;
> +		}
>   	}
>   
>   	if (etm->sample_branches || etm->synth_opts.last_branch) {
>