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

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

Hi Leo,

On 22/05/18 10:52, Leo Yan wrote:
> On Tue, May 22, 2018 at 04:39:20PM +0800, Leo Yan wrote:
> 
> [...]
> 
> Rather than the patch I posted in my previous email, I think below new
> patch is more reasonable for me.
> 
> In the below change, 'etmq->prev_packet' is only used to store the
> previous CS_ETM_RANGE packet, we don't need to save CS_ETM_TRACE_ON
> packet into 'etmq->prev_packet'.
> 
> On the other hand, cs_etm__flush() can use 'etmq->period_instructions'
> to indicate if need to generate instruction sample or not.  If it's
> non-zero, then generate instruction sample and
> 'etmq->period_instructions' will be cleared; so next time if there
> have more tracing CS_ETM_TRACE_ON packet, it can skip to generate
> instruction sample due 'etmq->period_instructions' is zero.
> 
> How about you think for this?
> 
> Thanks,
> Leo Yan
> 

I don't think this covers the cases where CS_ETM_TRACE_ON is used to 
indicate a discontinuity in the trace.  For example, there is work in 
progress to configure the ETM so that it only traces a few thousand 
cycles with a gap of many thousands of cycles between each chunk of 
trace - this can be used to sample program execution in the form of 
instruction events with branch stacks for feedback directed optimization 
(AutoFDO).

In this case, the raw trace is something like:

     ...
     I_ADDR_L_64IS0 : Address, Long, 64 bit, IS0.; Addr=0x0000007E7B886908;
     I_ATOM_F3 : Atom format 3.; EEN
     I_ATOM_F1 : Atom format 1.; E
# Trace stops here

# Some time passes, and then trace is turned on again
     I_TRACE_ON : Trace On.
     I_ADDR_CTXT_L_64IS0 : Address & Context, Long, 64 bit, IS0.; 
Addr=0x00000057224322F4; Ctxt: AArch64,EL0, NS;
     I_ATOM_F3 : Atom format 3.; ENN
     I_ATOM_F5 : Atom format 5.; ENENE
     ...

This results in the following packets from the decoder:

CS_ETM_RANGE: [0x7e7b886908-0x7e7b886930] br
CS_ETM_RANGE: [0x7e7b88699c-0x7e7b8869a4] br
CS_ETM_RANGE: [0x7e7b8869d8-0x7e7b8869f0]
CS_ETM_RANGE: [0x7e7b8869f0-0x7e7b8869fc] br
CS_ETM_TRACE_ON
CS_ETM_RANGE: [0x57224322f4-0x5722432304] br
CS_ETM_RANGE: [0x57224320e8-0x57224320ec]
CS_ETM_RANGE: [0x57224320ec-0x57224320f8]
CS_ETM_RANGE: [0x57224320f8-0x572243212c] br
CS_ETM_RANGE: [0x5722439b80-0x5722439bec]
CS_ETM_RANGE: [0x5722439bec-0x5722439c14] br
CS_ETM_RANGE: [0x5722437c30-0x5722437c6c]
CS_ETM_RANGE: [0x5722437c6c-0x5722437c7c] br

Without handling the CS_ETM_TRACE_ON, this would be interpreted as a 
branch from 0x7e7b8869f8 to 0x57224322f4, when there is actually a gap 
of many thousand instructions between these.

I think this patch will break the branch stacks - by removing the 
prev_packet swap from cs_etm__flush(), the next time a CS_ETM_RANGE 
packet is handled, cs_etm__sample() will see prev_packet contains the 
last CS_ETM_RANGE from the previous block of trace, causing an erroneous 
call to cs_etm__update_last_branch_rb().  In the example above, the 
branch stack will contain an erroneous branch from 0x7e7b8869f8 to 
0x57224322f4.

I think what you need to do is add a check for the previous packet being 
a CS_ETM_TRACE_ON when determining the generate_sample value.

Regards

Rob

> 
> diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
> index 822ba91..dd354ad 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;

Also check for etmq->prev_packet->sample_type == CS_ETM_TRACE_ON here 
and set 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) {
> @@ -922,11 +943,12 @@ static int cs_etm__sample(struct cs_etm_queue *etmq)
>   static int cs_etm__flush(struct cs_etm_queue *etmq)
>   {
>   	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) {
> +	    etmq->prev_packet->sample_type == CS_ETM_RANGE &&
> +	    etmq->period_instructions) {
> +

I don't think this is needed.

>   		/*
>   		 * Generate a last branch event for the branches left in the
>   		 * circular buffer at the end of the trace.
> @@ -940,14 +962,6 @@ static int cs_etm__flush(struct cs_etm_queue *etmq)
>   			etmq, addr,
>   			etmq->period_instructions);
>   		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;

This should not be changed as discussed above.

>   	}
>   
>   	return err;
>