Re: [PATCH v4 4/6] perf arm-spe: Assign kernel time to synthesized event

[James Clark]

On 15/04/2021 18:23, Leo Yan wrote:
> On Thu, Apr 15, 2021 at 05:46:31PM +0300, James Clark wrote:
>>
>>
>> On 12/04/2021 12:10, Leo Yan wrote:
>>> In current code, it assigns the arch timer counter to the synthesized
>>> samples Arm SPE trace, thus the samples don't contain the kernel time
>>> but only contain the raw counter value.
>>>
>>> To fix the issue, this patch converts the timer counter to kernel time
>>> and assigns it to sample timestamp.
>>>
>>> Signed-off-by: Leo Yan <leo.yan@xxxxxxxxxx>
>>> ---
>>>  tools/perf/util/arm-spe.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/tools/perf/util/arm-spe.c b/tools/perf/util/arm-spe.c
>>> index 23714cf0380e..c13a89f06ab8 100644
>>> --- a/tools/perf/util/arm-spe.c
>>> +++ b/tools/perf/util/arm-spe.c
>>> @@ -234,7 +234,7 @@ static void arm_spe_prep_sample(struct arm_spe *spe,
>>>  	struct arm_spe_record *record = &speq->decoder->record;
>>>  
>>>  	if (!spe->timeless_decoding)
>>> -		sample->time = speq->timestamp;
>>> +		sample->time = tsc_to_perf_time(record->timestamp, &spe->tc);
>>
>>
>> I noticed that in arm_spe_recording_options() the TIME sample bit is set regardless of any options.
>> I don't know of a way to remove this, and if there isn't, does that mean that all the code in this
>> file that looks at spe->timeless_decoding is untested and has never been hit?
>>
>> Unless there is a way to get a perf file with only the AUXTRACE event and no others? I think that one
>> might have no timestamp set. Otherwise other events will always have timestamps so spe->timeless_decoding
>> is always false.
> 
> Good point.  To be honest, I never noticed this issue until you
> mentioned this.
> 
> We should fix for the "timeless" flow; and it's questionable for the
> function arm_spe_recording_options(), except for setting
> PERF_SAMPLE_TIME, it also hard codes for setting
> PERF_SAMPLE_CPU and PERF_SAMPLE_TID.  Might need to carefully go
> through this function.
> 

Yeah, it's not strictly related to your change, which is definitely an improvement.
But maybe we should have a look at the SPE implementation relating to timestamps as a whole.

> Thanks,
> Leo
>