linux-kernel - Re: [mm] f35b5d7d67: will-it-scale.per_process

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Date:   Thu, 1 Dec 2022 13:44:57 -0800
From:   Yang Shi <shy828301@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Rik van Riel <riel@...riel.com>,
        Thorsten Leemhuis <regressions@...mhuis.info>,
        "Huang, Ying" <ying.huang@...el.com>,
        kernel test robot <yujie.liu@...el.com>, lkp@...ts.01.org,
        lkp@...el.com, Matthew Wilcox <willy@...radead.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        feng.tang@...el.com, zhengjun.xing@...ux.intel.com,
        fengwei.yin@...el.com, Nathan Chancellor <nathan@...nel.org>
Subject: Re: [mm] f35b5d7d67: will-it-scale.per_process_ops -95.5% regression

On Thu, Dec 1, 2022 at 1:22 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Thu, 01 Dec 2022 15:29:41 -0500 Rik van Riel <riel@...riel.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.

+1

>
>