[BUG] perf: No samples found when using kcore + coresight

[James Clark]

On 29/03/2023 17:11, James Clark wrote:
> 
> 
> On 14/03/2023 00:36, Leo Yan wrote:
>> On Mon, Mar 13, 2023 at 11:15:44AM -0700, Yang Shi wrote:
>>
>> [...]
>>
>>>> Just a quick summary, here we have two issues:
>>>>
>>>> - With command:
>>>>   perf record -e cs_etm/@tmc_etf63/k --kcore --per-thread \
>>>>   -- taskset --cpu-list 1 uname",
>>>>
>>>>   perf doesn't enable "text poke" attribution.
>>>
>>> No, it enables "text poke" and perf fails to decode coresight trace
>>> data too. It doesn't matter whether "--kcore" is after or before "-e
>>> cs/etm/@tmc_etf63/k".
>>
>> Understand now.  Thanks for correction, if so we can ignore this one.
>>
>> Leo
> 
> To me it looks like it's only --per-thread and --kcore together that
> cause the issue. I can't see if that was mentioned previously in this
> thread.
> 
> If it is --per-thread that's causing the issue then I think I have an
> idea why it might be. There are some assumptions and different paths
> taken in decoding in that mode that aren't correct. It causes some other
> issues to do with ordering and timestamps as well and I wanted to fix it
> previously. I wouldn't say that the text-poke change has caused a
> regression, as decoding in this mode was always a bit buggy.
> 
> Maybe this is another reason to fix it properly.

Hi Yang,

I found a temporary workaround to the issue. Whenever you use
--per-thread mode, force timeless mode when decoding with --itrace=Z and
it works the same as it did before. For example:

  perf report --itrace=Zi1000i

The reason is that the new text poke event was added with timestamps
enabled. The Coresight decoder slightly incorrectly assumed that no
timestamps == per-thread mode so when it sees the new event it goes into
the wrong decode mode.

The assumption was never really correct so I will make a proper fix. But
in the mean time the workaround should be fine.