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

[Yang Shi]

On Thu, Mar 30, 2023 at 3:36 AM James Clark <james.clark at arm.com> wrote:
>
>
>
> 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.

Hi James,

Thanks for figuring out the workaround. I tested both (using "-C"
instead of "--per-thread" for record or using "--itrace=Z" for report)
works. I think the workaround is fine before we have a proper fix.

>