Hi Leo,
There are a couple of typos in the comments below, but I also believe that the sample loop could be considerably simplified
On Mon, 3 Feb 2020 at 01:52, 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 is to divide into three parts for handling coming packet:
- The first part is for synthesizing the first instruction sample, it combines the instructions from the tail of previous packet and the instructions from the head of the new packet;
- The second part is to simply generate samples with sample period aligned;
- The third part is the tail of new packet, the rest instructions will be left for the sequential sample handling.
Suggested-by: Mike Leach mike.leach@linaro.org Signed-off-by: Leo Yan leo.yan@linaro.org
tools/perf/util/cs-etm.c | 105 ++++++++++++++++++++++++++++++++++----- 1 file changed, 92 insertions(+), 13 deletions(-)
diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c index 3e28462609e7..c5a05f728eac 100644 --- a/tools/perf/util/cs-etm.c +++ b/tools/perf/util/cs-etm.c @@ -1360,23 +1360,102 @@ 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* instrs_executed** Period instructions (Pi) contains the the number of* instructions executed after the sample point(n). When a new* instruction packet is coming and generate for the next sample* (n+1), it combines with two parts instructions, one is the* tail of the old packet and another is the head of the new* coming packet. So 'head' variable is used to cauclate the
typo : s/cauclate/calculate
* instruction numbers in the new packet for sample(n+1).** Sample(n+2) and sample(n+3) consume the instructions with* sample period, so directly generate samples based on the* sampe period.*
typo: s/sampe/sample
* After sample(n+3), the rest instructions will be used by* later packet; so use 'instrs_over' to track the rest* instruction number and it is assigned to* 'tidq->period_instructions' for next round calculation.*/u64 head, offset = 0;u64 addr; /*
* 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)
* 'instrs_over' is the number of instructions executed after* sample points, initialise it to 'instrs_executed' and will* decrease it for consumed instructions in every synthesized* instruction sample. */
u64 offset = (instrs_executed - instrs_over - 1);u64 addr = cs_etm__instr_addr(etmq, trace_chan_id,tidq->packet, offset);
u64 instrs_over = instrs_executed;
ret = cs_etm__synth_instruction_sample(etmq, tidq, addr, etm->instructions_sample_period);if (ret)return ret;
/** 'head' is the instructions number of the head in the new* packet, it combines with the tail of previous packet to* generate a sample. So 'head' uses the sample period to* decrease the instruction number introduced by the previous* packet.*/head = etm->instructions_sample_period -(tidq->period_instructions - instrs_executed);if (head) {offset = head;/** 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;instrs_over -= head;}while (instrs_over >= etm->instructions_sample_period) {offset += 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;instrs_over -= etm->instructions_sample_period;} /* Carry remaining instructions into next sample period */ tidq->period_instructions = instrs_over;-- 2.17.1
I believe the following change would work and make for easier reading...
.... at the start of the function remove instrs_executed and replace .... /* get instructions remainder from previous packet */ u64 instrs_prev = tidq->period_instructions;
/* set available instructions to previous packet remainder + the current packet count */ tidq->period_instructions += tidq->packet->instr_count;
.... within the if(etm->sample_instructions && ...) statement I would be more explicit what the elements of the diagram are ....
/* * 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 occuring every etm->instructions_sample_period * instructions - as defined on the perf command line. Sample(n) being the last sample before the * current etm packet, n+1 to n+3 samples 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. *
.... continue with explanation here ....
.... then we can simplify the loop code removing some of the temporary variables ....
/* get the initial offset into the current packet instructions (entry conditions ensure that instrs_prev < etm->instructions_sample_period) */ u64 offset = etm->instructions_sample_period - instrs_prev; u64 addr;
/* Prepare last branches for instruction sample */ if (etm->synth_opts.last_branch) cs_etm__copy_last_branch_rb(etmq, tidq);
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;
offset += etm->instructions_sample_period; tidq->period_instructions -= etm->instructions_sample_period; }
..... I believe the above should work, but cannot claim to have tried it out. What do you think?
Regards
Mike