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SubjectRe: [PATCH v4 3/5] perf cs-etm: Correct synthesizing instruction samples
Reviewed by: Mike Leach <mike.leach@linaro.org>

On Thu, 13 Feb 2020 at 09:43, Leo Yan <leo.yan@linaro.org> wrote:
>
> When 'etm->instructions_sample_period' is less than
> 'tidq->period_instructions', the function cs_etm__sample() cannot handle
> this case properly with its logic.
>
> Let's see below flow as an example:
>
> - If we set itrace option '--itrace=i4', then function cs_etm__sample()
> has variables with initialized values:
>
> tidq->period_instructions = 0
> etm->instructions_sample_period = 4
>
> - When the first packet is coming:
>
> packet->instr_count = 10; the number of instructions executed in this
> packet is 10, thus update period_instructions as below:
>
> tidq->period_instructions = 0 + 10 = 10
> instrs_over = 10 - 4 = 6
> offset = 10 - 6 - 1 = 3
> tidq->period_instructions = instrs_over = 6
>
> - When the second packet is coming:
>
> packet->instr_count = 10; in the second pass, assume 10 instructions
> in the trace sample again:
>
> tidq->period_instructions = 6 + 10 = 16
> instrs_over = 16 - 4 = 12
> offset = 10 - 12 - 1 = -3 -> the negative value
> tidq->period_instructions = instrs_over = 12
>
> So after handle these two packets, there have below issues:
>
> The first issue is that cs_etm__instr_addr() returns the address within
> the current trace sample of the instruction related to offset, so the
> offset is supposed to be always unsigned value. But in fact, function
> cs_etm__sample() might calculate a negative offset value (in handling
> the second packet, the offset is -3) and pass to cs_etm__instr_addr()
> with u64 type with a big positive integer.
>
> The second issue is it only synthesizes 2 samples for sample period = 4.
> In theory, every packet has 10 instructions so the two packets have
> total 20 instructions, 20 instructions should generate 5 samples
> (4 x 5 = 20). This is because cs_etm__sample() only calls once
> cs_etm__synth_instruction_sample() to generate instruction sample per
> range packet.
>
> This patch fixes the logic in function cs_etm__sample(); the basic
> idea for handling coming packet is:
>
> - To synthesize the first instruction sample, it combines the left
> instructions from the previous packet and the head of the new
> packet; then generate continuous samples with sample period;
> - At the tail of the new packet, if it has the rest instructions,
> these instructions will be left for the sequential sample.
>
> Suggested-by: Mike Leach <mike.leach@linaro.org>
> Signed-off-by: Leo Yan <leo.yan@linaro.org>
> ---
> tools/perf/util/cs-etm.c | 87 ++++++++++++++++++++++++++++++++--------
> 1 file changed, 70 insertions(+), 17 deletions(-)
>
> diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c
> index b2f31390126a..4b7d6c36ce3c 100644
> --- a/tools/perf/util/cs-etm.c
> +++ b/tools/perf/util/cs-etm.c
> @@ -1356,9 +1356,12 @@ static int cs_etm__sample(struct cs_etm_queue *etmq,
> struct cs_etm_auxtrace *etm = etmq->etm;
> int ret;
> u8 trace_chan_id = tidq->trace_chan_id;
> - u64 instrs_executed = tidq->packet->instr_count;
> + u64 instrs_prev;
>
> - tidq->period_instructions += instrs_executed;
> + /* Get instructions remainder from previous packet */
> + instrs_prev = tidq->period_instructions;
> +
> + tidq->period_instructions += tidq->packet->instr_count;
>
> /*
> * Record a branch when the last instruction in
> @@ -1376,26 +1379,76 @@ static int cs_etm__sample(struct cs_etm_queue *etmq,
> * TODO: allow period to be defined in cycles and clock time
> */
>
> - /* Get number of instructions executed after the sample point */
> - u64 instrs_over = tidq->period_instructions -
> - etm->instructions_sample_period;
> + /*
> + * Below diagram demonstrates the instruction samples
> + * generation flows:
> + *
> + * Instrs Instrs Instrs Instrs
> + * Sample(n) Sample(n+1) Sample(n+2) Sample(n+3)
> + * | | | |
> + * V V V V
> + * --------------------------------------------------
> + * ^ ^
> + * | |
> + * Period Period
> + * instructions(Pi) instructions(Pi')
> + *
> + * | |
> + * \---------------- -----------------/
> + * V
> + * tidq->packet->instr_count
> + *
> + * Instrs Sample(n...) are the synthesised samples occurring
> + * every etm->instructions_sample_period instructions - as
> + * defined on the perf command line. Sample(n) is being the
> + * last sample before the current etm packet, n+1 to n+3
> + * samples are generated from the current etm packet.
> + *
> + * tidq->packet->instr_count represents the number of
> + * instructions in the current etm packet.
> + *
> + * Period instructions (Pi) contains the the number of
> + * instructions executed after the sample point(n) from the
> + * previous etm packet. This will always be less than
> + * etm->instructions_sample_period.
> + *
> + * When generate new samples, it combines with two parts
> + * instructions, one is the tail of the old packet and another
> + * is the head of the new coming packet, to generate
> + * sample(n+1); sample(n+2) and sample(n+3) consume the
> + * instructions with sample period. After sample(n+3), the rest
> + * instructions will be used by later packet and it is assigned
> + * to tidq->period_instructions for next round calculation.
> + */
>
> /*
> - * Calculate the address of the sampled instruction (-1 as
> - * sample is reported as though instruction has just been
> - * executed, but PC has not advanced to next instruction)
> + * Get the initial offset into the current packet instructions;
> + * entry conditions ensure that instrs_prev is less than
> + * etm->instructions_sample_period.
> */
> - u64 offset = (instrs_executed - instrs_over - 1);
> - u64 addr = cs_etm__instr_addr(etmq, trace_chan_id,
> - tidq->packet, offset);
> + u64 offset = etm->instructions_sample_period - instrs_prev;
> + u64 addr;
>
> - ret = cs_etm__synth_instruction_sample(
> - etmq, tidq, addr, etm->instructions_sample_period);
> - if (ret)
> - return ret;
> + while (tidq->period_instructions >=
> + etm->instructions_sample_period) {
> + /*
> + * Calculate the address of the sampled instruction (-1
> + * as sample is reported as though instruction has just
> + * been executed, but PC has not advanced to next
> + * instruction)
> + */
> + addr = cs_etm__instr_addr(etmq, trace_chan_id,
> + tidq->packet, offset - 1);
> + ret = cs_etm__synth_instruction_sample(
> + etmq, tidq, addr,
> + etm->instructions_sample_period);
> + if (ret)
> + return ret;
>
> - /* Carry remaining instructions into next sample period */
> - tidq->period_instructions = instrs_over;
> + offset += etm->instructions_sample_period;
> + tidq->period_instructions -=
> + etm->instructions_sample_period;
> + }
> }
>
> if (etm->sample_branches) {
> --
> 2.17.1
>


--
Mike Leach
Principal Engineer, ARM Ltd.
Manchester Design Centre. UK

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