Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Thu, 1 Dec 2022 13:22:37 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Rik van Riel <riel@surriel.com>
Cc:     Thorsten Leemhuis <regressions@leemhuis.info>,
        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>
Subject: Re: [mm] f35b5d7d67: will-it-scale.per_process_ops -95.5%
 regression
Message-Id: <20221201132237.c55c4bd07ba44463b146882e@linux-foundation.org>
In-Reply-To: <07adee081a70c2b4b44d9bf93a0ad3142e091086.camel@surriel.com>
References: <202210181535.7144dd15-yujie.liu@intel.com>
        <87edv4r2ip.fsf@yhuang6-desk2.ccr.corp.intel.com>
        <Y1DNQaoPWxE+rGce@dev-arch.thelio-3990X>
        <871qr3nkw2.fsf@yhuang6-desk2.ccr.corp.intel.com>
        <366045a27a96e01d0526d63fd78d4f3c5d1f530b.camel@surriel.com>
        <Y1GCYXGtEVZbcv/5@dev-arch.thelio-3990X>
        <Y4RX+JZLddDHrLuQ@dev-arch.thelio-3990X>
        <e919e5a3-1118-043e-8bbd-5ee35f2ab411@leemhuis.info>
        <07adee081a70c2b4b44d9bf93a0ad3142e091086.camel@surriel.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Precedence: bulk

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:
> > Hi, this is your Linux kernel regression tracker.
> > 
> > On 28.11.22 07:40, Nathan Chancellor wrote:
> > > Hi Rik,
> > 
> > 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.