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Date:   Thu, 24 Mar 2022 17:09:54 +0000
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From:   John Garry <john.garry@huawei.com>
Subject: Re: perf version issues
To:     Arnaldo Carvalho de Melo <acme@kernel.org>,
        Ingo Molnar <mingo@redhat.com>
CC:     Ian Rogers <irogers@google.com>, Jiri Olsa <jolsa@kernel.org>,
        "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
        <linux-perf-users@vger.kernel.org>
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+ Ingo

On 23/03/2022 12:44, John Garry wrote:
>> $ git cherry-pick cb66befccba18fac
>>    [perf/core 8ff6a6c06a90a362] perf tools: Fix version kernel tag
>>     Author: John Garry<john.garry@huawei.com>
>>     Date: Mon Feb 21 21:16:49 2022 +0800
>>     1 file changed, 4 insertions(+), 9 deletions(-)
>>    $ make -C tools/perf O=/tmp/build/perf install-bin
>>    $ perf -v
>>    perf version 5.17.rc8.g4e666cdb06ee
>>
>> Now it doesn´t notice it and there is no automatic rebuild triggered, we
>> stay with the cset from before the cherry-pick:-\
>>
>> $ git log --oneline -2
>> 8ff6a6c06a90a362 (HEAD -> perf/core) perf tools: Fix version kernel tag
>> 4e666cdb06eede20 perf tools: Fix dependency for version file creation
>> $
>>
>> Anyway, your patch works for some cases, so its an improvement and I'll
>> apply it, we can continue from there.
> 
> After some experimentation, I find that only .git/HEAD changes for a 
> cherry-pick depending on whether we're on a branch or not. As such, as 
> you have seen, we may not rebuild after a cherry-pick, which is no good.
> 
> As far as I can see, only .git/index changes in all circumstances, but 
> we don't have a dependency on that.
> 
> TBH, using .git files as Makefile dependencies doesn't seem to work well.
> 
> Maybe we should output "git log" and check versus what we have in 
> PERF-VERSION-FILE and re-build depending on that.
> 
> Let me check this further now.

Just an update on this...

I did some research and found that Ingo introduced the change to make 
.git/HEAD a dependency as a build optimisation. See commit commit 
c72e3f04b45fb2e50cdd81a50c3778c6a57251d8 ("tools/perf/build: Speed up 
git-version test on re-make")

According to the commit log it's quicker to trigger a rebuild of the 
version string depending on a change in a git internal file rather than 
run "git describe" to see any change in tag/commit every times.

However, I asked the git community about this approach, and their 
general idea is that a dependency on .git/HEAD or other git internal 
file is just not a good idea.

See 
https://lore.kernel.org/git/87wngkpddp.fsf@igel.home/T/#m4a4dd6de52fdbe21179306cd57b3761eb07f45f8

So to get this working robustly we may need to roll back that named 
commit in part.

Thanks,
John