Hi Mike,
On Wed, Oct 23, 2019 at 12:36:39AM +0100, Mike Leach wrote:
Hi Leo,
Two points here - both related.
On Tue, 22 Oct 2019 at 06:10, Leo Yan leo.yan@linaro.org wrote:
Hi Mathieu,
On Fri, Oct 11, 2019 at 02:16:06PM -0600, Mathieu Poirier wrote:
On Sat, Oct 05, 2019 at 05:16:09PM +0800, Leo Yan wrote:
If the u64 variable 'offset' is a negative integer, comparison it with bigger than zero is always going to be true because it is unsigned. Fix this by using s64 type for variable 'offset'.
Signed-off-by: Leo Yan leo.yan@linaro.org
tools/perf/util/cs-etm.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/tools/perf/util/cs-etm.c b/tools/perf/util/cs-etm.c index 4ba0f871f086..4bc2d9709d4f 100644 --- a/tools/perf/util/cs-etm.c +++ b/tools/perf/util/cs-etm.c @@ -940,7 +940,7 @@ u64 cs_etm__last_executed_instr(const struct cs_etm_packet *packet) static inline u64 cs_etm__instr_addr(struct cs_etm_queue *etmq, u64 trace_chan_id, const struct cs_etm_packet *packet,
u64 offset)
s64 offset)Issue 1:
OK - it appears that cs_etm__instr_addr() is supposed to be returning the address within the current trace sample of the instruction related to offset. For T32 - then if offset < 0, packet->start_addr is returned - not good but at least within the current trace range For A32/A64 - if offset < 0 then an address _before_ packet->start_addr is returned - clearly wrong and possibly a completely invalid address that was never actually traced.
Exactly, if offset < 0 it might output the incorrect trace.
In Suzuki's reply there was two choices, 1) move the while(offset > 0) to while (offset) or change the type of @offset to an s64. Here we know offset can't be negative because of the tidq->period_instructions >= etm->instructions_sample_period
in function cs_etm__sample(). As such I think option #1 is the right way to deal with this rather than changing the type of the variable.
I took sometime to use formulas to prove that 'offset' is possible to be a negative value :)
Just paste the updated commit log at here for review:
Pi: period_instructions Ie: instrs_executed Io: instrs_over Ip: instructions_sample_period
Pi' = Pi + Ie -> New period_instructions equals to the old period_instructions + instrs_executed Io = Pi' - Ip -> period_instructions - instructions_sample_period
offset = Ie - Io - 1 = Ie - (Pi' - Ip) -1 = Ie - (Pi + Ie - Ip) -1 = Ip - Pi - 1
In theory, if Ip (instructions_sample_period) is small enough and Pi (period_instructions) is bigger than Ip, then it will lead to the negative value for 'offset'.
So let's see below command:
perf inject --itrace=i1il128 -i perf.data -o perf.data.new
With this command, 'offset' is very easily to be a negative value when handling packets; this is because if use the inject option 'i1', then instructions_sample_period equals to 1; so:
offset = 1 - Pi - 1 = -Pi
Any Pi bigger than zero leads 'offset' to a negative value.
Thanks, Leo Yan
Issue 2:
Assuming I have understood the logic of this code correctly - there is an issue were sample_period < period_instructions as you say - but I believe the problem is in the logic of the sampling function itself.
Suppose we have a sample_period of 4.
Now on an initial pass through the function, period_instructions must be 0. (i.e. none left over from the previous pass.) Suppose also that the number of instructions executed in this sample is 10 - thus updating period_instructions. Therefore: instr_over = 10 - 4 -> 6 offset = 10 - 6 - 1 -> 3. We therefore call cs_etm_instr_addr to find the address an offset of 3 instructions from the start of the trace sample and synthesize the sample. After this we set period_instructions to the instr_over value of 6.
Next pass, assume 10 instructions in the trace sample again. period_instructions = 6 + 10 -> 16 instr_over = 16 - 4 -> 12 offset = 10 - 12 - 1 -> -3 - the negative value your formulae predict.
This implies that the sample we want is actually in the previous trace packet - which I believe is in fact the case - as explained below.
My reading of the code is that cs_etm__sample() is called once per trace range packet extracted from the decoder - and a trace range packet represents N instructions_executed. Further I am assuming that instructions_sample_period represents the desired periodicity of the instruction samples - i.e. 1 sample every instructions_sample_period number of instructions.
Good point. Yeah, this is the root cause.
Thus my conclusion here is that where M = instructions_executed + period_instructions, the function should generate quotient ( M / instructions_sample_period ) samples and set period_instructions to M mod instructions_sample_period on exit, ensuring period_instructions is never larger than the sample_period.
Totally agree with this; we should generate synthetic samples without dropping trace data.
i.e. loop to generate multiple samples until instr_over and therefore the output value of period_instructions is less than the value of instructions_sample_period - for the example above, with 10 instructions and a periodicity of 4, we generate 2 samples with a remainder of 2 instructions carried forwards.
In short leave offset as unsigned and fix the logic of the cs_etm__sample() function.
Will follow up this suggestion.
Very appreciate your time to review and gave out much reasonable solution!
Thanks, Leo Yan
{ if (packet->isa == CS_ETM_ISA_T32) { u64 addr = packet->start_addr; @@ -1372,7 +1372,7 @@ static int cs_etm__sample(struct cs_etm_queue *etmq, * sample is reported as though instruction has just been * executed, but PC has not advanced to next instruction) */
u64 offset = (instrs_executed - instrs_over - 1);
s64 offset = (instrs_executed - instrs_over - 1); u64 addr = cs_etm__instr_addr(etmq, trace_chan_id, tidq->packet, offset);-- 2.17.1
-- Mike Leach Principal Engineer, ARM Ltd. Manchester Design Centre. UK