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https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
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![Stephen Rothwell](/assets/img/avatar_default.png)
ld -r is an incremental link used to create built-in.o files in build subdirectories. It produces relocatable object files containing all its input files, and these are are then pulled together and relocated in the final link. Aside from the bloat, this constrains the final link relocations, which has bitten large powerpc builds with unresolvable relocations in the final link. Alan Modra has recommended the kernel use thin archives for linking. This is an alternative and means that the linker has more information available to it when it links the kernel. This patch enables a config option architectures can select, which causes all built-in.o files to be built as thin archives. built-in.o files in subdirectories do not get symbol table or index attached, which improves speed and size. The final link pass creates a built-in.o archive in the root output directory which includes the symbol table and index. The linker then uses takes this file to link. The --whole-archive linker option is required, because the linker now has visibility to every individual object file, and it will otherwise just completely avoid including those without external references (consider a file with EXPORT_SYMBOL or initcall or hardware exceptions as its only entry points). The traditional built works "by luck" as built-in.o files are large enough that they're going to get external references. However this optimisation is unpredictable for the kernel (due to above external references), ineffective at culling unused, and costly because the .o files have to be searched for references. Superior alternatives for link-time culling should be used instead. Build characteristics for inclink vs thinarc, on a small powerpc64le pseries VM with a modest .config: inclink thinarc sizes vmlinux 15 618 680 15 625 028 sum of all built-in.o 56 091 808 1 054 334 sum excluding root built-in.o 151 430 find -name built-in.o | xargs rm ; time make vmlinux real 22.772s 21.143s user 13.280s 13.430s sys 4.310s 2.750s - Final kernel pulled in only about 6K more, which shows how ineffective the object file culling is. - Build performance looks improved due to less pagecache activity. On IO constrained systems it could be a bigger win. - Build size saving is significant. Side note, the toochain understands archives, so there's some tricks, $ ar t built-in.o # list all files you linked with $ size built-in.o # and their sizes $ objdump -d built-in.o # disassembly (unrelocated) with filenames Implementation by sfr, minor tweaks by npiggin. Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au> Signed-off-by: Nicholas Piggin <npiggin@gmail.com> Signed-off-by: Michal Marek <mmarek@suse.com>
302 lines
6.6 KiB
Bash
Executable file
302 lines
6.6 KiB
Bash
Executable file
#!/bin/sh
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#
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# link vmlinux
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#
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# vmlinux is linked from the objects selected by $(KBUILD_VMLINUX_INIT) and
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# $(KBUILD_VMLINUX_MAIN). Most are built-in.o files from top-level directories
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# in the kernel tree, others are specified in arch/$(ARCH)/Makefile.
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# Ordering when linking is important, and $(KBUILD_VMLINUX_INIT) must be first.
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#
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# vmlinux
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# ^
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# |
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# +-< $(KBUILD_VMLINUX_INIT)
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# | +--< init/version.o + more
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# |
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# +--< $(KBUILD_VMLINUX_MAIN)
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# | +--< drivers/built-in.o mm/built-in.o + more
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# |
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# +-< ${kallsymso} (see description in KALLSYMS section)
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#
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# vmlinux version (uname -v) cannot be updated during normal
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# descending-into-subdirs phase since we do not yet know if we need to
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# update vmlinux.
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# Therefore this step is delayed until just before final link of vmlinux.
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#
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# System.map is generated to document addresses of all kernel symbols
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# Error out on error
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set -e
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# Nice output in kbuild format
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# Will be supressed by "make -s"
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info()
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{
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if [ "${quiet}" != "silent_" ]; then
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printf " %-7s %s\n" ${1} ${2}
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fi
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}
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# Thin archive build here makes a final archive with
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# symbol table and indexes from vmlinux objects, which can be
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# used as input to linker.
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#
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# Traditional incremental style of link does not require this step
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#
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# built-in.o output file
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#
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archive_builtin()
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{
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if [ -n "${CONFIG_THIN_ARCHIVES}" ]; then
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info AR built-in.o
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rm -f built-in.o;
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${AR} rcsT${KBUILD_ARFLAGS} built-in.o \
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${KBUILD_VMLINUX_INIT} \
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${KBUILD_VMLINUX_MAIN}
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fi
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}
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# Link of vmlinux.o used for section mismatch analysis
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# ${1} output file
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modpost_link()
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{
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local objects
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if [ -n "${CONFIG_THIN_ARCHIVES}" ]; then
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objects="--whole-archive built-in.o"
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else
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objects="${KBUILD_VMLINUX_INIT} \
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--start-group \
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${KBUILD_VMLINUX_MAIN} \
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--end-group"
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fi
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${LD} ${LDFLAGS} -r -o ${1} ${objects}
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}
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# Link of vmlinux
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# ${1} - optional extra .o files
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# ${2} - output file
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vmlinux_link()
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{
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local lds="${objtree}/${KBUILD_LDS}"
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local objects
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if [ "${SRCARCH}" != "um" ]; then
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if [ -n "${CONFIG_THIN_ARCHIVES}" ]; then
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objects="--whole-archive built-in.o ${1}"
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else
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objects="${KBUILD_VMLINUX_INIT} \
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--start-group \
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${KBUILD_VMLINUX_MAIN} \
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--end-group \
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${1}"
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fi
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${LD} ${LDFLAGS} ${LDFLAGS_vmlinux} -o ${2} \
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-T ${lds} ${objects}
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else
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if [ -n "${CONFIG_THIN_ARCHIVES}" ]; then
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objects="-Wl,--whole-archive built-in.o ${1}"
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else
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objects="${KBUILD_VMLINUX_INIT} \
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-Wl,--start-group \
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${KBUILD_VMLINUX_MAIN} \
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-Wl,--end-group \
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${1}"
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fi
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${CC} ${CFLAGS_vmlinux} -o ${2} \
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-Wl,-T,${lds} \
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${objects} \
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-lutil -lrt -lpthread
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rm -f linux
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fi
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}
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# Create ${2} .o file with all symbols from the ${1} object file
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kallsyms()
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{
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info KSYM ${2}
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local kallsymopt;
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if [ -n "${CONFIG_HAVE_UNDERSCORE_SYMBOL_PREFIX}" ]; then
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kallsymopt="${kallsymopt} --symbol-prefix=_"
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fi
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if [ -n "${CONFIG_KALLSYMS_ALL}" ]; then
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kallsymopt="${kallsymopt} --all-symbols"
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fi
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if [ -n "${CONFIG_KALLSYMS_ABSOLUTE_PERCPU}" ]; then
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kallsymopt="${kallsymopt} --absolute-percpu"
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fi
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if [ -n "${CONFIG_KALLSYMS_BASE_RELATIVE}" ]; then
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kallsymopt="${kallsymopt} --base-relative"
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fi
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local aflags="${KBUILD_AFLAGS} ${KBUILD_AFLAGS_KERNEL} \
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${NOSTDINC_FLAGS} ${LINUXINCLUDE} ${KBUILD_CPPFLAGS}"
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local afile="`basename ${2} .o`.S"
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${NM} -n ${1} | scripts/kallsyms ${kallsymopt} > ${afile}
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${CC} ${aflags} -c -o ${2} ${afile}
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}
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# Create map file with all symbols from ${1}
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# See mksymap for additional details
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mksysmap()
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{
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${CONFIG_SHELL} "${srctree}/scripts/mksysmap" ${1} ${2}
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}
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sortextable()
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{
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${objtree}/scripts/sortextable ${1}
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}
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# Delete output files in case of error
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cleanup()
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{
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rm -f .old_version
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rm -f .tmp_System.map
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rm -f .tmp_kallsyms*
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rm -f .tmp_version
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rm -f .tmp_vmlinux*
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rm -f built-in.o
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rm -f System.map
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rm -f vmlinux
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rm -f vmlinux.o
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}
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on_exit()
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{
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if [ $? -ne 0 ]; then
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cleanup
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fi
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}
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trap on_exit EXIT
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on_signals()
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{
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exit 1
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}
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trap on_signals HUP INT QUIT TERM
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#
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#
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# Use "make V=1" to debug this script
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case "${KBUILD_VERBOSE}" in
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*1*)
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set -x
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;;
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esac
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if [ "$1" = "clean" ]; then
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cleanup
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exit 0
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fi
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# We need access to CONFIG_ symbols
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case "${KCONFIG_CONFIG}" in
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*/*)
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. "${KCONFIG_CONFIG}"
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;;
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*)
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# Force using a file from the current directory
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. "./${KCONFIG_CONFIG}"
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esac
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archive_builtin
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#link vmlinux.o
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info LD vmlinux.o
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modpost_link vmlinux.o
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# modpost vmlinux.o to check for section mismatches
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${MAKE} -f "${srctree}/scripts/Makefile.modpost" vmlinux.o
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# Update version
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info GEN .version
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if [ ! -r .version ]; then
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rm -f .version;
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echo 1 >.version;
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else
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mv .version .old_version;
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expr 0$(cat .old_version) + 1 >.version;
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fi;
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# final build of init/
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${MAKE} -f "${srctree}/scripts/Makefile.build" obj=init GCC_PLUGINS_CFLAGS="${GCC_PLUGINS_CFLAGS}"
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kallsymso=""
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kallsyms_vmlinux=""
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if [ -n "${CONFIG_KALLSYMS}" ]; then
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# kallsyms support
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# Generate section listing all symbols and add it into vmlinux
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# It's a three step process:
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# 1) Link .tmp_vmlinux1 so it has all symbols and sections,
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# but __kallsyms is empty.
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# Running kallsyms on that gives us .tmp_kallsyms1.o with
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# the right size
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# 2) Link .tmp_vmlinux2 so it now has a __kallsyms section of
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# the right size, but due to the added section, some
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# addresses have shifted.
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# From here, we generate a correct .tmp_kallsyms2.o
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# 2a) We may use an extra pass as this has been necessary to
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# woraround some alignment related bugs.
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# KALLSYMS_EXTRA_PASS=1 is used to trigger this.
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# 3) The correct ${kallsymso} is linked into the final vmlinux.
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#
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# a) Verify that the System.map from vmlinux matches the map from
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# ${kallsymso}.
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kallsymso=.tmp_kallsyms2.o
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kallsyms_vmlinux=.tmp_vmlinux2
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# step 1
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vmlinux_link "" .tmp_vmlinux1
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kallsyms .tmp_vmlinux1 .tmp_kallsyms1.o
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# step 2
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vmlinux_link .tmp_kallsyms1.o .tmp_vmlinux2
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kallsyms .tmp_vmlinux2 .tmp_kallsyms2.o
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# step 2a
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if [ -n "${KALLSYMS_EXTRA_PASS}" ]; then
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kallsymso=.tmp_kallsyms3.o
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kallsyms_vmlinux=.tmp_vmlinux3
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vmlinux_link .tmp_kallsyms2.o .tmp_vmlinux3
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kallsyms .tmp_vmlinux3 .tmp_kallsyms3.o
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fi
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fi
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info LD vmlinux
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vmlinux_link "${kallsymso}" vmlinux
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if [ -n "${CONFIG_BUILDTIME_EXTABLE_SORT}" ]; then
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info SORTEX vmlinux
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sortextable vmlinux
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fi
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info SYSMAP System.map
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mksysmap vmlinux System.map
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# step a (see comment above)
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if [ -n "${CONFIG_KALLSYMS}" ]; then
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mksysmap ${kallsyms_vmlinux} .tmp_System.map
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if ! cmp -s System.map .tmp_System.map; then
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echo >&2 Inconsistent kallsyms data
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echo >&2 Try "make KALLSYMS_EXTRA_PASS=1" as a workaround
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exit 1
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fi
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fi
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# We made a new kernel - delete old version file
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rm -f .old_version
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