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

Nick Desaulniers
2023-06-20T20:06:16Z

On Mon, Jun 19, 2023 at 6:06 PM Palmer Dabbelt <palmer@dabbelt.com> wrote:
>
> On Thu, 15 Jun 2023 06:54:33 PDT (-0700), Palmer Dabbelt wrote:
> > On Wed, 14 Jun 2023 09:25:49 PDT (-0700), jszhang@kernel.org wrote:
> >>
> >> On Wed, Jun 14, 2023 at 07:49:17AM -0700, Palmer Dabbelt wrote:
> >>> On Tue, 23 May 2023 09:54:58 PDT (-0700), jszhang@kernel.org wrote:
> >>> > When trying to run linux with various opensource riscv core on
> >>> > resource limited FPGA platforms, for example, those FPGAs with less
> >>> > than 16MB SDRAM, I want to save mem as much as possible. One of the
> >>> > major technologies is kernel size optimizations, I found that riscv
> >>> > does not currently support HAVE_LD_DEAD_CODE_DATA_ELIMINATION, which
> >>> > passes -fdata-sections, -ffunction-sections to CFLAGS and passes the
> >>> > --gc-sections flag to the linker.
> >>> >
> >>> > This not only benefits my case on FPGA but also benefits defconfigs.
> >>> > Here are some notable improvements from enabling this with defconfigs:
> >>> >
> >>> > nommu_k210_defconfig:
> >>> > text data bss dec hex
> >>> > 1112009 410288 59837 1582134 182436 before
> >>> > 962838 376656 51285 1390779 1538bb after
> >>> >
> >>> > rv32_defconfig:
> >>> > text data bss dec hex
> >>> > 8804455 2816544 290577 11911576 b5c198 before
> >>> > 8692295 2779872 288977 11761144 b375f8 after
> >>> >
> >>> > defconfig:
> >>> > text data bss dec hex
> >>> > 9438267 3391332 485333 13314932 cb2b74 before
> >>> > 9285914 3350052 483349 13119315 c82f53 after
> >>> >
> >>> > patch1 and patch2 are clean ups.
> >>> > patch3 fixes a typo.
> >>> > patch4 finally enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION for riscv.
> >>> >
> >>> > NOTE: Zhangjin Wu firstly sent out a patch to enable dead code
> >>> > elimination for riscv several months ago, I didn't notice it until
> >>> > yesterday. Although it missed some preparations and some sections's
> >>> > keeping, he is the first person to enable this feature for riscv. To
> >>> > ease merging, this series take his patch into my entire series and
> >>> > makes patch4 authored by him after getting his ack to reflect
> >>> > the above fact.
> >>> >
> >>> > Since v1:
> >>> > - collect Reviewed-by, Tested-by tag
> >>> > - Make patch4 authored by Zhangjin Wu, add my co-developed-by tag
> >>> >
> >>> > Jisheng Zhang (3):
> >>> > riscv: move options to keep entries sorted
> >>> > riscv: vmlinux-xip.lds.S: remove .alternative section
> >>> > vmlinux.lds.h: use correct .init.data.* section name
> >>> >
> >>> > Zhangjin Wu (1):
> >>> > riscv: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION
> >>> >
> >>> > arch/riscv/Kconfig | 13 +-
> >>> > arch/riscv/kernel/vmlinux-xip.lds.S | 6 -
> >>> > arch/riscv/kernel/vmlinux.lds.S | 6 +-
> >>> > include/asm-generic/vmlinux.lds.h | 2 +-
> >>> > 4 files changed, 11 insertions(+), 16 deletions(-)
> >>>
> >>> Do you have a base commit for this? It's not applying to 6.4-rc1 and the
> >>> patchwork bot couldn't find one either.
> >>
> >> Hi Palmer,
> >>
> >> Commit 3b90b09af5be ("riscv: Fix orphan section warnings caused by
> >> kernel/pi") touches vmlinux.lds.S, so to make the merge easy, this
> >> series is based on 6.4-rc2.
> >
> > Thanks.
>
> Sorry to be so slow here, but I think this is causing LLD to hang on
> allmodconfig. I'm still getting to the bottom of it, there's a few
> other things I have in flight still.

Confirmed with v3 on mainline (linux-next is pretty red at the moment).
https://lore.kernel.org/linux-riscv/20230517082936.37563-1-falcon@tinylab.org/

I was able to dump a backtrace of all of LLD's threads and all threads
seemed parked in a futex wait except for one thread with a more
interesting trace.

0x0000555557ea01ce in
lld::elf::LinkerScript::addOrphanSections()::$_0::operator()(lld::elf::InputSectionBase*)
const ()
(gdb) bt
#0 0x0000555557ea01ce in
lld::elf::LinkerScript::addOrphanSections()::$_0::operator()(lld::elf::InputSectionBase*)
const ()
#1 0x0000555557e9fc3f in lld::elf::LinkerScript::addOrphanSections() ()
#2 0x0000555557dd0ca1 in
lld::elf::LinkerDriver::link(llvm::opt::InputArgList&) ()
#3 0x0000555557dc19a8 in
lld::elf::LinkerDriver::linkerMain(llvm::ArrayRef<char const*>) ()
#4 0x0000555557dbfff9 in lld::elf::link(llvm::ArrayRef<char const*>,
llvm::raw_ostream&, llvm::raw_ostream&, bool, bool) ()
#5 0x0000555557c3ffcf in lldMain(int, char const**,
llvm::raw_ostream&, llvm::raw_ostream&, bool) ()
#6 0x0000555557c3f7aa in lld_main(int, char**, llvm::ToolContext const&) ()
#7 0x0000555557c41ee1 in main ()

Makes me wonder if there's some kind of loop adding orphan sections
that aren't referenced, so they're cleaned up.

Though I don't think it's a hang; IIRC dead code elimination adds a
measurable amount of time to the build. As code is unreferenced and
removed, I think the linker is reshuffling layout and thus recomputing
relocations.

Though triple checking mainline without this patch vs mainline with
this patch, twice now I just got an error from LLD (in 2 minutes on my
system):

ld.lld: error: ./drivers/firmware/efi/libstub/lib.a(efi-stub-entry.stub.o):(.init.bss.screen_info_offset)
is being placed in '.init.bss.screen_info_offset'
ld.lld: error: ./drivers/firmware/efi/libstub/lib.a(efi-stub-helper.stub.o):(.init.data.efi_nokaslr)
is being placed in '.init.data.efi_nokaslr'
ld.lld: error: ./drivers/firmware/efi/libstub/lib.a(efi-stub-helper.stub.o):(.init.bss.efi_noinitrd)
is being placed in '.init.bss.efi_noinitrd'
ld.lld: error: ./drivers/firmware/efi/libstub/lib.a(efi-stub-helper.stub.o):(.init.bss.efi_nochunk)
is being placed in '.init.bss.efi_nochunk'
ld.lld: error: ./drivers/firmware/efi/libstub/lib.a(efi-stub-helper.stub.o):(.init.bss.efi_novamap)
is being placed in '.init.bss.efi_novamap'
ld.lld: error: ./drivers/firmware/efi/libstub/lib.a(efi-stub-helper.stub.o):(.init.bss.efi_disable_pci_dma)
is being placed in '.init.bss.efi_disable_pci_dma'
ld.lld: error: ./drivers/firmware/efi/libstub/lib.a(file.stub.o):(.init.bss.efi_open_device_path.text_to_dp)
is being placed in '.init.bss.efi_open_device_path.text_to_dp'
ld.lld: error: ./drivers/firmware/efi/libstub/lib.a(gop.stub.o):(.init.bss.cmdline.0)
is being placed in '.init.bss.cmdline.0'
ld.lld: error: ./drivers/firmware/efi/libstub/lib.a(gop.stub.o):(.init.bss.cmdline.1)
is being placed in '.init.bss.cmdline.1'
ld.lld: error: ./drivers/firmware/efi/libstub/lib.a(gop.stub.o):(.init.bss.cmdline.2)
is being placed in '.init.bss.cmdline.2'
ld.lld: error: ./drivers/firmware/efi/libstub/lib.a(gop.stub.o):(.init.bss.cmdline.3)
is being placed in '.init.bss.cmdline.3'
ld.lld: error: ./drivers/firmware/efi/libstub/lib.a(gop.stub.o):(.init.bss.cmdline.4)
is being placed in '.init.bss.cmdline.4'
ld.lld: error: ./drivers/firmware/efi/libstub/lib.a(printk.stub.o):(.init.data.efi_loglevel)
is being placed in '.init.data.efi_loglevel'
ld.lld: error: ./drivers/firmware/efi/libstub/lib.a(riscv.stub.o):(.init.bss.hartid)
is being placed in '.init.bss.hartid'
ld.lld: error: ./drivers/firmware/efi/libstub/lib.a(systable.stub.o):(.init.bss.efi_system_table)
is being placed in '.init.bss.efi_system_table'

is it perhaps that these sections need placement in the linker script?
This is from the orphan section warn linker command line flag.

Does the EFI stub have one linker script, or one per arch? (Or am I
mistaken and the EFI stub is part of vmlinux)?

>
> >
> >>
> >> Thanks
>

--
Thanks,
~Nick Desaulniers