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

[Robert Walker <robert.walker@arm.com>]

Hi Leo,

Following the discussions from your reply to this with a simplified 
patch, this version of the patch works better as you also need to emit a 
branch sample when handling a CS_ETM_TRACE_ON packet to indicate the end 
of a block of trace.

This patch does not break the output from perf inject to generate 
instruction samples for AutoFDO, so I am happy with that.

Regards

Rob

Reviewed-by: Robert Walker <robert.walker@arm.com>


On 22/05/18 09:39, Leo Yan wrote:
> Hi Rob,
>
> On Mon, May 21, 2018 at 12:27:42PM +0100, Robert Walker wrote:
>> 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.
> Thanks for reviewing.  Let's dig more detailed into this issue,
> especially for handling packet CS_ETM_TRACE_ON, I'd like divide into two
> sub cases.
>
> - The first case is for using python script:
>
>    I use python script to analyze packets with below command:
>    ./perf script --itrace=ril128 -s arm-cs-trace-disasm.py -F cpu,event,ip,addr,sym -- -v -d objdump -k ./vmlinux
>
>    What I observe is after we pass python script with parameter '-s
>    arm-cs-trace-disasm.py', then instruction tracing options
>    '--itrace=ril128' isn't really used;  the perf tool creates another
>    new process for launch python script and re-enter cmd_script()
>    function, but at the second time when invoke cmd_script() for python
>    script execution the option '--itrace=ril128' is dropped and all
>    parameters are only valid defined by the python script.
>
>    As result, I can the variable 'etmq->etm->synth_opts.last_branch' is
>    always FALSE for running python script.  So all CS_ETM_TRACE_ON
>    packets will be ignored in the function cs_etm__flush().
>
>    Even the CS_ETM_TRACE_ON packets are missed to handle, the program
>    flow still can work well.  The reason is without the interference by
>    CS_ETM_TRACE_ON, the CS_ETM_RANGE packets can smoothly create
>    instruction range by ignore the middle CS_ETM_TRACE_ON packet.
>
>    Please see below example, in this example there have 3 packets, the
>    first one packet is CS_ETM_RANGE packet which is labelled with
>    'PACKET_1', the first one packet can properly generate branch sample
>    data with previous packet as expected;  the second packet is
>    PACKET_2 which is CS_ETM_TRACE_ON, but
>    'etmq->etm->synth_opts.last_branch' is false so function
>    cs_etm__flush() doesn't handle it and skip the swap operation
>    "etmq->prev_packet = tmp"; the third packet is PACKET_3, which is
>    CS_ETM_RANGE packet and we can see it's smoontly to create
>    continous instruction range between PACKET_1 and PACKET_3.
>
>    cs_etm__sample: prev_packet: sample_type=1 exc=0 exc_ret=0 cpu=1 start_addr=0xffff000008a5f79c end_addr=0xffff000008a5f7bc last_instr_taken_branch=1
>    PACKET_1: cs_etm__sample: packet: sample_type=1 exc=0 exc_ret=0 cpu=1 start_addr=0xffff000008a5f858 end_addr=0xffff000008a5f864 last_instr_taken_branch=1
>    cs_etm__synth_branch_sample: ip=0xffff000008a5f7b8 addr=0xffff000008a5f858 pid=2290 tid=2290 id=1000000021 stream_id=1000000021 period=1 cpu=1 flags=0 cpumode=2
>
>    cs_etm__flush: prev_packet: sample_type=1 exc=0 exc_ret=0 cpu=1 start_addr=0xffff000008a5f858 end_addr=0xffff000008a5f864 last_instr_taken_branch=1
>    PACKET_2: cs_etm__flush: packet: sample_type=2 exc=0 exc_ret=0 cpu=2 start_addr=0xdeadbeefdeadbeef end_addr=0xdeadbeefdeadbeef last_instr_taken_branch=1
>
>    cs_etm__sample: prev_packet: sample_type=1 exc=0 exc_ret=0 cpu=1 start_addr=0xffff000008a5f858 end_addr=0xffff000008a5f864 last_instr_taken_branch=1
>    PACKET_3: cs_etm__sample: packet: sample_type=1 exc=0 exc_ret=0 cpu=2 start_addr=0xffff000008be7528 end_addr=0xffff000008be7538 last_instr_taken_branch=1
>    cs_etm__synth_branch_sample: ip=0xffff000008a5f860 addr=0xffff000008be7528 pid=2290 tid=2290 id=1000000021 stream_id=1000000021 period=1 cpu=2 flags=0 cpumode=2
>
>    So seems to me, the CS_ETM_TRACE_ON packet doesn't introduce trouble
>    for the program flow analysis if we can handle all CS_ETM_RANGE
>    packets and without handling CS_ETM_TRACE_ON packet for branch
>    samples.
>
> - The second case is for --itrace option without python script:
>    ./perf script --itrace=ril -F cpu,event,ip,addr,sym -k ./vmlinux
>
>    In this case, the flag 'etmq->etm->synth_opts.last_branch' is true
>    so CS_ETM_TRACE_ON packet will be handled; but I can observe the
>    CS_ETM_RANGE packet in etmq->prev_packet isn't handled in the
>    function cs_etm__flush() for branch sample, so actually we miss some
>    branch sample for this case.
>
>    So I think we also need handle CS_ETM_RANGE packet in function
>    cs_etm__flush() to generate branch samples.  But this has side
>    effect, we introduce the extra track for CS_ETM_TRACE_ON packet for
>    branch samples, so we will see one branch range like:
>    [ 0xdeadbeefdeadbeef .. 0xdeadbeefdeadbeef ].
>
> Please reivew below change is okay for you?  Thanks a lot for
> suggestions.
>
> diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
> index 822ba91..37d3722 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 +904,28 @@ 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 ||
> +		    etmq->prev_packet->sample_type == CS_ETM_TRACE_ON)
> +			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) {
> @@ -921,12 +943,17 @@ static int cs_etm__sample(struct cs_etm_queue *etmq)
>   
>   static int cs_etm__flush(struct cs_etm_queue *etmq)
>   {
> +	struct cs_etm_auxtrace *etm = etmq->etm;
>   	int err = 0;
>   	struct cs_etm_packet *tmp;
>   
> -	if (etmq->etm->synth_opts.last_branch &&
> -	    etmq->prev_packet &&
> -	    etmq->prev_packet->sample_type == CS_ETM_RANGE) {
> +	if (!etmq->prev_packet)
> +		return 0;
> +
> +	if (etmq->prev_packet->sample_type != CS_ETM_RANGE)
> +		return 0;
> +
> +	if (etmq->etm->synth_opts.last_branch) {
>   		/*
>   		 * Generate a last branch event for the branches left in the
>   		 * circular buffer at the end of the trace.
> @@ -939,18 +966,25 @@ static int cs_etm__flush(struct cs_etm_queue *etmq)
>   		err = cs_etm__synth_instruction_sample(
>   			etmq, addr,
>   			etmq->period_instructions);
> +		if (err)
> +			return err;
>   		etmq->period_instructions = 0;
> +	}
>   
> -		/*
> -		 * Swap PACKET with PREV_PACKET: PACKET becomes PREV_PACKET for
> -		 * the next incoming packet.
> -		 */
> -		tmp = etmq->packet;
> -		etmq->packet = etmq->prev_packet;
> -		etmq->prev_packet = tmp;
> +	if (etm->sample_branches) {
> +		err = cs_etm__synth_branch_sample(etmq);
> +		if (err)
> +			return err;
>   	}
>   
> -	return err;
> +	/*
> +	 * Swap PACKET with PREV_PACKET: PACKET becomes PREV_PACKET for
> +	 * the next incoming packet.
> +	 */
> +	tmp = etmq->packet;
> +	etmq->packet = etmq->prev_packet;
> +	etmq->prev_packet = tmp;
> +	return 0;
>   }
>   
>   static int cs_etm__run_decoder(struct cs_etm_queue *etmq)

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