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

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Date:   Wed, 29 Mar 2023 17:11:24 +0100
From:   James Clark <james.clark@....com>
To:     Leo Yan <leo.yan@...aro.org>, Yang Shi <shy828301@...il.com>
Cc:     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
Subject: Re: [BUG] perf: No samples found when using kcore + coresight

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 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.