Re: [PATCH v4 3/5] perf cs-etm: Correct synthesizing instruction samples

[Mike Leach]

Reviewed by: Mike Leach <mike.leach@xxxxxxxxxx>

On Thu, 13 Feb 2020 at 09:43, Leo Yan <leo.yan@xxxxxxxxxx> 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@xxxxxxxxxx>
> Signed-off-by: Leo Yan <leo.yan@xxxxxxxxxx>
> ---
>  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