linux-kernel - Re: [PATCH v2 0/4] riscv: enable HAVE_LD_DEAD_CODE_DATA

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Date:   Thu, 22 Jun 2023 14:40:59 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Palmer Dabbelt <palmer@...belt.com>
Cc:     nathan@...nel.org, bjorn@...nel.org,
        Conor Dooley <conor@...nel.org>, jszhang@...nel.org,
        llvm@...ts.linux.dev, Paul Walmsley <paul.walmsley@...ive.com>,
        aou@...s.berkeley.edu, Arnd Bergmann <arnd@...db.de>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arch@...r.kernel.org
Subject: Re: [PATCH v2 0/4] riscv: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

On Wed, Jun 21, 2023 at 12:46 PM Palmer Dabbelt <palmer@...belt.com> wrote:
>
> On Wed, 21 Jun 2023 11:19:31 PDT (-0700), Palmer Dabbelt wrote:
> > On Wed, 21 Jun 2023 10:51:15 PDT (-0700), bjorn@...nel.org wrote:
> >> Conor Dooley <conor@...nel.org> writes:
> >>
> >> [...]
> >>
> >>>> So I'm no longer actually sure there's a hang, just something slow.
> >>>> That's even more of a grey area, but I think it's sane to call a 1-hour
> >>>> link time a regression -- unless it's expected that this is just very
> >>>> slow to link?
> >>>
> >>> I dunno, if it was only a thing for allyesconfig, then whatever - but
> >>> it's gonna significantly increase build times for any large kernels if LLD
> >>> is this much slower than LD. Regression in my book.
> >>>
> >>> I'm gonna go and experiment with mixed toolchain builds, I'll report
> >>> back..
> >>
> >> I took palmer/for-next (1bd2963b2175 ("Merge patch series "riscv: enable
> >> HAVE_LD_DEAD_CODE_DATA_ELIMINATION"")) for a tuxmake build with llvm-16:
> >>
> >>   | ~/src/tuxmake/run -v --wrapper ccache --target-arch riscv \
> >>   |     --toolchain=llvm-16 --runtime docker --directory . -k \
> >>   |     allyesconfig
> >>
> >> Took forever, but passed after 2.5h.
> >
> > Thanks.  I just re-ran mine 17/trunk LLD under time (rather that just
> > checking top sometimes), it's at 1.5h but even that seems quite long.
> >
> > I guess this is sort of up to the LLVM folks: if it's expected that DCE
> > takes a very long time to link then I'm not opposed to allowing it, but
> > if this is probably a bug in LLD then it seems best to turn it off until
> > we sort things out over there.
> >
> > I think maybe Nick or Nathan is the best bet to know?
>
> Looks like it's about 2h for me.  I'm going to drop these from my
> staging tree in the interest of making progress on other stuff, but if
> this is just expected behavior them I'm OK taking them (though that's
> too much compute for me to test regularly):
>
> $ time ../../../../llvm/install/bin/ld.lld -melf64lriscv -z noexecstack -r -o vmlinux.o --whole-archive vmlinux.a --no-whole-archive --start-group ./drivers/firmware/efi/libstub/lib.a --end-group
>
> real    111m50.678s
> user    111m18.739s
> sys     1m13.147s

Ah, I think you meant s/allmodconfig/allyesconfig/ in your initial
report.  That makes more sense, and I can reproduce.  Let me work on a
report.

>
> >> CONFIG_CC_VERSION_TEXT="Debian clang version 16.0.6 (++20230610113307+7cbf1a259152-1~exp1~20230610233402.106)"
> >>
> >>
> >> Björn



--
Thanks,
~Nick Desaulniers