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

[Yang Shi]

On Wed, Mar 29, 2023 at 9:08 AM James Clark <james.clark at arm.com> 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 "--pre-thread" is not passed in, perf record failed with "failed to
mmap with 12 (Cannot allocate memory)". Sorry for not mentioning this
in the first place. I was quite focused on --kcore and didn't realize
they may be related.

>
> 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.