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

From: Jisheng Zhang <jszhang@kernel.org>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>,
	bjorn@kernel.org, Conor Dooley <conor@kernel.org>,
	llvm@lists.linux.dev, Paul Walmsley <paul.walmsley@sifive.com>,
	aou@eecs.berkeley.edu, Arnd Bergmann <arnd@arndb.de>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	linux-arch@vger.kernel.org
Subject: Re: [PATCH v2 0/4] riscv: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
Date: Sun, 25 Jun 2023 20:24:56 +0800	[thread overview]
Message-ID: <ZJgyGD8ZSjVmiprB@xhacker> (raw)
In-Reply-To: <ZJXTwqZIkXLxXaSi@google.com>

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.

Thanks
> 
> > 
> > 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
> 

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