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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Tue, 26 May 2020 14:19:18 +0200
Message-ID: <CAK8P3a29BNwvdN1YNzoN966BF4z1QiSxdRXTP+BzhM9H07LoYQ@mail.gmail.com>
Subject: Re: [PATCH -tip v3 09/11] data_race: Avoid nested statement expression
To:     Will Deacon <will@kernel.org>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Marco Elver <elver@google.com>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Dmitry Vyukov <dvyukov@google.com>,
        Alexander Potapenko <glider@google.com>,
        Andrey Konovalov <andreyknvl@google.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Borislav Petkov <bp@alien8.de>
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On Tue, May 26, 2020 at 2:02 PM Will Deacon <will@kernel.org> wrote:
> On Tue, May 26, 2020 at 12:42:16PM +0200, Arnd Bergmann wrote:
> >
> > I find this patch only solves half the problem: it's much faster than
> > without the
> > patch, but still much slower than the current mainline version. As far as I'm
> > concerned, I think the build speed regression compared to mainline is not yet
> > acceptable, and we should try harder.
> >
> > I have not looked too deeply at it yet, but this is what I found from looking
> > at a file in a randconfig build:
> >
> > Configuration: see https://pastebin.com/raw/R9erCwNj
>
> So this .config actually has KCSAN enabled. Do you still see the slowdown
> with that disabled?

Yes, enabling or disabling KCSAN seems to make no difference to
compile speed in this config and source file, I still get the 12 seconds
preprocessing time and 9MB file size with KCSAN disabled, possibly
a few percent smaller/faster. I actually thought that CONFIG_FTRACE
had a bigger impact, but disabling that also just reduces the time
by a few percent rather than getting it down to the expected milliseconds.

> Although not ideal, having a longer compiler time when
> the compiler is being asked to perform instrumentation doesn't seem like a
> show-stopper to me.

I agree in general, but building an allyesconfig kernel is still an important
use case that should not take twice as long after a small kernel change
regardless of whether a new feature is used or not. (I have not actually
compared the overall build speed for allmodconfig, as this takes a really
long time at the moment)

        Arnd