linux-kernel - Re: [BUG] perf: No samples found when using kcore + coresight

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Date:   Wed, 29 Mar 2023 16:25:24 -0700
From:   Yang Shi <shy828301@...il.com>
To:     James Clark <james.clark@....com>
Cc:     Leo Yan <leo.yan@...aro.org>, linux-perf-users@...r.kernel.org,
        LAK <linux-arm-kernel@...ts.infradead.org>,
        coresight@...ts.linaro.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        mathieu.poirier@...aro.org, adrian.hunter@...el.com,
        Jiri Olsa <jolsa@...nel.org>, acme@...hat.com,
        mike.leach@...aro.org, Will Deacon <will@...nel.org>,
        suzuki.poulose@....com, yang@...amperecomputing.com
Subject: Re: [BUG] perf: No samples found when using kcore + coresight

On Wed, Mar 29, 2023 at 9:08 AM James Clark <james.clark@....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/@..._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/@..._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.