Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20201201213707.541432-1-samitolvanen@google.com>
 <CAK8P3a1WEAo2SEgKUEs3SB7n7QeeHa0=cx_nO==rDK0jjDArow@mail.gmail.com>
 <CABCJKueCHo2RYfx_A21m+=d1gQLR9QsOOxCsHFeicCqyHkb-Kg@mail.gmail.com>
 <CAK8P3a1Xfpt7QLkvxjtXKcgzcWkS8g9bmxD687+rqjTafTzKrg@mail.gmail.com> <CAKwvOd=hL=Vt1ATYqky9jmv+tM5hpTnLRuZudG-7ki0EYoFGJQ@mail.gmail.com>
In-Reply-To: <CAKwvOd=hL=Vt1ATYqky9jmv+tM5hpTnLRuZudG-7ki0EYoFGJQ@mail.gmail.com>
From:   Arnd Bergmann <arnd@kernel.org>
Date:   Tue, 8 Dec 2020 23:20:32 +0100
Message-ID: <CAK8P3a1k_cq3NOUeuQC4-uKDBaGq49GSjAMSiS_M9AVMBxv51g@mail.gmail.com>
Subject: Re: [PATCH v8 00/16] Add support for Clang LTO
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     Sami Tolvanen <samitolvanen@google.com>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Will Deacon <will@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        linux-pci <linux-pci@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Tue, Dec 8, 2020 at 10:10 PM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@googlegroups.com> wrote:
>
> On Tue, Dec 8, 2020 at 1:00 PM Arnd Bergmann <arnd@kernel.org> wrote:
> >
> > On Tue, Dec 8, 2020 at 5:43 PM 'Sami Tolvanen' via Clang Built Linux
> > <clang-built-linux@googlegroups.com> wrote:
> > >
> > > On Tue, Dec 8, 2020 at 4:15 AM Arnd Bergmann <arnd@kernel.org> wrote:
> > > >
> > > > - one build seems to take even longer to link. It's currently at 35GB RAM
> > > >   usage and 40 minutes into the final link, but I'm worried it might
> > > > not complete
> > > >   before it runs out of memory.  I only have 128GB installed, and google-chrome
> > > >   uses another 30GB of that, and I'm also doing some other builds in parallel.
> > > >   Is there a minimum recommended amount of memory for doing LTO builds?
> > >
> > > When building arm64 defconfig, the maximum memory usage I measured
> > > with ThinLTO was 3.5 GB, and with full LTO 20.3 GB. I haven't measured
> > > larger configurations, but I believe LLD can easily consume 3-4x that
> > > much with full LTO allyesconfig.
> >
> > Ok, that's not too bad then. Is there actually a reason to still
> > support full-lto
> > in your series? As I understand it, full LTO was the initial approach and
> > used to work better, but thin LTO is actually what we want to use in the
> > long run. Perhaps dropping the full LTO option from your series now
> > that thin LTO works well enough and uses less resources would help
> > avoid some of the problems.
>
> While all developers agree that ThinLTO is a much more palatable
> experience than full LTO; our product teams prefer the excessive build
> time and memory high water mark (at build time) costs in exchange for
> slightly better performance than ThinLTO in <benchmarks that I've been
> told are important>.  Keeping support for full LTO in tree would help
> our product teams reduce the amount of out of tree code they have.  As
> long as <benchmarks that I've been told are important> help
> sell/differentiate phones, I suspect our product teams will continue
> to ship full LTO in production.

Ok, fair enough. How about marking FULL_LTO as 'depends on
!COMPILE_TEST' then? I'll do that locally for my randconfig tests,
but it would help the other build bots that also force-enable
COMPILE_TEST.

       Arnd