Re: [PATCH v2 0/4] riscv: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

Palmer Dabbelt
2023-06-25T20:06:24Z

On Sun, 25 Jun 2023 05:43:13 PDT (-0700), Conor Dooley wrote:
> On Sun, Jun 25, 2023 at 08:24:56PM +0800, Jisheng Zhang wrote:
>> On Fri, Jun 23, 2023 at 10:17:54AM -0700, Nick Desaulniers wrote:
>> > On Thu, Jun 22, 2023 at 11:18:03PM +0000, Nathan Chancellor wrote:
>> > > If you wanted to restrict it to just LD_IS_BFD in arch/riscv/Kconfig,
>> > > that would be fine with me too.
>> > >
>> > > select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if LD_IS_BFD
>> >
>> > Hi Jisheng, would you mind sending a v3 with the attached patch applied
>> > on top / at the end of your series?
>>
>> Hi Nick, Nathan, Palmer,
>>
>> I saw the series has been applied to riscv-next, so I'm not sure which
>> solution would it be, Palmer to apply Nick's patch to riscv-next or
>> I to send out v3, any suggestion is appreciated.
>
> I don't see what you are seeing w/ riscv/for-next. HEAD is currently at
> 4681dacadeef ("riscv: replace deprecated scall with ecall") and there
> are no patches from your series in the branch:
> https://git.kernel.org/pub/scm/linux/kernel/git/riscv/linux.git/log/?h=for-next

It's been in and out of staging a few times as we tracked down the
performance regression, but it shouldn't have ever made it to linux-next
for real.

I'm fine just picking up the patch to disable DCE, I've got a few other
(hopefully small) things to work through first though.

> Cheers,
> Conor.
>
>> > > Nick said he would work on a report for the LLVM side, so as long as
>> > > this issue is handled in some way to avoid regressing LLD builds until
>> > > it is resolved, I don't think there is anything else for the kernel to
>> > > do. We like to have breadcrumbs via issue links, not sure if the report
>> > > will be internal to Google or on LLVM's issue tracker though;
>> > > regardless, we will have to touch this block to add a version check
>> > > later, at which point we can add a link to the fix in LLD.
>> >
>> > https://github.com/ClangBuiltLinux/linux/issues/1881
>>
>> > From 3e5e010958ee41b9fb408cfade8fb017c2fe7169 Mon Sep 17 00:00:00 2001
>> > From: Nick Desaulniers <ndesaulniers@google.com>
>> > Date: Fri, 23 Jun 2023 10:06:17 -0700
>> > Subject: [PATCH] riscv: disable HAVE_LD_DEAD_CODE_DATA_ELIMINATION for LLD
>> >
>> > Linking allyesconfig with ld.lld-17 with CONFIG_DEAD_CODE_ELIMINATION=y
>> > takes hours. Assuming this is a performance regression that can be
>> > fixed, tentatively disable this for now so that allyesconfig builds
>> > don't start timing out. If and when there's a fix to ld.lld, this can
>> > be converted to a version check instead so that users of older but still
>> > supported versions of ld.lld don't hurt themselves by enabling
>> > CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y.
>> >
>> > Link: https://github.com/ClangBuiltLinux/linux/issues/1881
>> > Reported-by: Palmer Dabbelt <palmer@dabbelt.com>
>> > Suggested-by: Nathan Chancellor <nathan@kernel.org>
>> > Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
>> > ---
>> > Hi Jisheng, would you mind sending a v3 with this patch on top/at the
>> > end of your patch series?
>> >
>> > arch/riscv/Kconfig | 3 ++-
>> > 1 file changed, 2 insertions(+), 1 deletion(-)
>> >
>> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
>> > index 8effe5bb7788..0573991e9b78 100644
>> > --- a/arch/riscv/Kconfig
>> > +++ b/arch/riscv/Kconfig
>> > @@ -116,7 +116,8 @@ config RISCV
>> > select HAVE_KPROBES if !XIP_KERNEL
>> > select HAVE_KPROBES_ON_FTRACE if !XIP_KERNEL
>> > select HAVE_KRETPROBES if !XIP_KERNEL
>> > - select HAVE_LD_DEAD_CODE_DATA_ELIMINATION
>> > + # https://github.com/ClangBuiltLinux/linux/issues/1881
>> > + select HAVE_LD_DEAD_CODE_DATA_ELIMINATION if !LD_IS_LLD
>> > select HAVE_MOVE_PMD
>> > select HAVE_MOVE_PUD
>> > select HAVE_PCI
>> > --
>> > 2.41.0.162.gfafddb0af9-goog
>> >
>>