linux-kernel - Re: [PATCH -tip] compiler_types.h: Optimize __unqual_scalar

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Message-ID: <CA+icZUWzPMOj+qsDz-5Z3tD-hX5gcowjBkwYyiy8SL36Jg+2Nw@mail.gmail.com>
Date:   Wed, 27 May 2020 15:37:19 +0200
From:   Sedat Dilek <sedat.dilek@...il.com>
To:     Marco Elver <elver@...gle.com>
Cc:     Arnd Bergmann <arnd@...db.de>, Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH -tip] compiler_types.h: Optimize __unqual_scalar_typeof
 compilation time

On Wed, May 27, 2020 at 3:30 PM Marco Elver <elver@...gle.com> wrote:
>
> On Wed, 27 May 2020 at 15:11, Sedat Dilek <sedat.dilek@...il.com> wrote:
> >
> > On Wed, May 27, 2020 at 2:50 PM Arnd Bergmann <arnd@...db.de> wrote:
> > >
> > > On Wed, May 27, 2020 at 2:35 PM Sedat Dilek <sedat.dilek@...il.com> wrote:
> > > > On Wed, May 27, 2020 at 2:31 PM Arnd Bergmann <arnd@...db.de> wrote:
> > > > > On Wed, May 27, 2020 at 1:36 PM Sedat Dilek <sedat.dilek@...il.com> wrote:
> > > > > > On Wed, May 27, 2020 at 1:27 PM Arnd Bergmann <arnd@...db.de> wrote:
> > > > > > > On Wed, May 27, 2020 at 12:33 PM Marco Elver <elver@...gle.com> wrote:
> > > > > > >
> > > > > > > This gives us back 80% of the performance drop on clang, and 50%
> > > > > > > of the drop I saw with gcc, compared to current mainline.
> > > > > > >
> > > > > > > Tested-by: Arnd Bergmann <arnd@...db.de>
> > > > > > >
> > > > > >
> > > > > > Hi Arnd,
> > > > > >
> > > > > > with "mainline" you mean Linux-next aka Linux v5.8 - not v5.7?
> > > > >
> > > > > I meant v5.7.
> > > > >
> > > > > > I have not seen __unqual_scalar_typeof(x) in compiler_types.h in Linux v5.7.
> > > > > >
> > > > > > Is there a speedup benefit also for Linux v5.7?
> > > > > > Which patches do I need?
> > > > >
> > > > > v5.7-rc is the baseline and is the fastest I currently see. On certain files,
> > > > > I saw an intermittent 10x slowdown that was already fixed earlier, now
> > > > > linux-next
> > > > > is more like 2x slowdown for me and 1.2x with this patch on top, so we're
> > > > > almost back to the speed of linux-5.7.
> > > > >
> > > >
> > > > Which clang version did you use - and have you set KCSAN kconfigs -
> > > > AFAICS this needs clang-11?
> > >
> > > I'm currently using clang-11, but I see the same problem with older
> > > versions, and both with and without KCSAN enabled. I think the issue
> > > is mostly the deep nesting of macros that leads to code bloat.
> > >
> >
> > Thanks.
> >
> > With clang-10:
> >
> > $ scripts/diffconfig /boot/config-5.7.0-rc7-2-amd64-clang .config
> >  BUILD_SALT "5.7.0-rc7-2-amd64-clang" -> "5.7.0-rc7-3-amd64-clang"
> > +HAVE_ARCH_KCSAN y
>
> Clang 10 doesn't support KCSAN (HAVE_KCSAN_COMPILER unset).
>
> > With clang-11:
> >
> > $ scripts/diffconfig /boot/config-5.7.0-rc7-2-amd64-clang .config
> >  BUILD_SALT "5.7.0-rc7-2-amd64-clang" -> "5.7.0-rc7-3-amd64-clang"
> >  CLANG_VERSION 100001 -> 110000
> > +CC_HAS_ASM_INLINE y
> > +HAVE_ARCH_KCSAN y
> > +HAVE_KCSAN_COMPILER y
> > +KCSAN n
> >
> > Which KCSAN kconfigs did you enable?
>
> To clarify: as said in [1], KCSAN (or any other instrumentation) is no
> longer relevant to the issue here, and the compile-time regression is
> observable with most configs. The problem is due to pre-processing and
> parsing, which came about due to new READ_ONCE() and the
> __unqual_scalar_typeof() macro (which this patch optimizes).
>
> KCSAN and new ONCEs got tangled up because we first attempted to
> annotate {READ,WRITE}_ONCE() with data_race(), but that turned out to
> have all kinds of other issues (explanation in [2]). So we decided to
> drop all the KCSAN-specific bits from ONCE, and require KCSAN to be
> Clang 11. Those fixes were applied to the first version of new
> {READ,WRITE}_ONCE() in -tip, which actually restored the new ONCEs to
> the pre-KCSAN version (now that KCSAN can deal with them without
> annotations).
>
> Hope this makes more sense now.
>
> [1] https://lore.kernel.org/lkml/CANpmjNOUdr2UG3F45=JaDa0zLwJ5ukPc1MMKujQtmYSmQnjcXg@mail.gmail.com/
> [2] https://lore.kernel.org/lkml/20200521142047.169334-1-elver@google.com/
>

Thanks, Marco.

I pulled tip.git#locking/kcsan on top of Linux v5.7-rc7 and applied this patch.
Just wanted to try KCSAN for the first time and it will also be my
first building with clang-11.
That's why I asked.

- Sedat -