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