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

[Palmer Dabbelt]

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@xxxxxxxxxx>
> > > 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@xxxxxxxxxxx>
> > > Suggested-by: Nathan Chancellor <nathan@xxxxxxxxxx>
> > > Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
> > > ---
> > > 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
> > >