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Date:   Fri, 2 Dec 2022 09:46:48 +0100
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Subject: Re: [mm] f35b5d7d67: will-it-scale.per_process_ops -95.5% regression
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To:     Andrew Morton <akpm@linux-foundation.org>,
        Rik van Riel <riel@surriel.com>
Cc:     Yang Shi <shy828301@gmail.com>,
        "Huang, Ying" <ying.huang@intel.com>,
        kernel test robot <yujie.liu@intel.com>, lkp@lists.01.org,
        lkp@intel.com, Matthew Wilcox <willy@infradead.org>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        feng.tang@intel.com, zhengjun.xing@linux.intel.com,
        fengwei.yin@intel.com, Nathan Chancellor <nathan@kernel.org>
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From:   Thorsten Leemhuis <regressions@leemhuis.info>
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On 01.12.22 22:22, Andrew Morton wrote:
> On Thu, 01 Dec 2022 15:29:41 -0500 Rik van Riel <riel@surriel.com> wrote:
>> On Thu, 2022-12-01 at 19:33 +0100, Thorsten Leemhuis wrote:
>>> On 28.11.22 07:40, Nathan Chancellor wrote:
>>> I wonder what we should do about below performance regression. Is
>>> reverting the culprit now and reapplying it later together with a fix
>>> a
>>> viable option? Or was anything done/is anybody doing something
>>> already
>>> to address the problem and I just missed it?
>>
>> The changeset in question speeds up kernel compiles with
>> GCC, as well as the runtime speed of other programs, due
>> to being able to use THPs more. However, it slows down kernel
>> compiles with clang, due to ... something clang does.
>>
>> I have not figured out what that something is yet.
>>
>> I don't know if I have the wrong version of clang here,
>> but I have not seen any smoking gun at all when tracing
>> clang system calls. I see predominantly small mmap and
>> unmap calls, and nothing that even triggers 2MB alignment.
> 
> 2.8% speedup for gcc is nice.  Massive slowdown in the malloc banchmark
> and in LLVM/clang is very bad - we don't know what other userspace will
> be so affected.
> 
> So I think we revert until this is fully understood.

Andrew, many thx for taking care of this. While at it let me please get
a small process issue of my chest:

What beverage of choice do I have to offer you to make you in the future
include 'Link:' tags linking to the report(s) when you add reverts like
the one for this issue?

My regression tracking bot heavily relies on them, that's why I care.
And our documentation (see [1]) also says that they should be used. But
the main reason why I ask in this particular case is different:

They in cases like afaics are especially helpful, as they make life a
whole lot easier for future code archeologists. And I guess that's not
something theoretical in this case, as I assume the patch that triggered
the issue will come back sooner or later -- and then those links will
help a lot to find this thread. Which is also why Linus really wants to
see them:
https://lore.kernel.org/all/CAHk-=wjMmSZzMJ3Xnskdg4+GGz=5p5p+GSYyFBTh0f-DgvdBWg@mail.gmail.com/
https://lore.kernel.org/all/CAHk-=wgs38ZrfPvy=nOwVkVzjpM3VFU1zobP37Fwd_h9iAD5JQ@mail.gmail.com/
https://lore.kernel.org/all/CAHk-=wjxzafG-=J8oT30s7upn4RhBs6TX-uVFZ5rME+L5_DoJA@mail.gmail.com/

Ciao, Thorsten

[1] see Documentation/process/submitting-patches.rst
(http://docs.kernel.org/process/submitting-patches.html) and
Documentation/process/5.Posting.rst
(https://docs.kernel.org/process/5.Posting.html)