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2d3b1b8f0d
The incremental build of Linux kernel is pretty slow when lots of
objects are compiled. The rebuild of allmodconfig may take a few
minutes even when none of the objects needs to be rebuilt.
The time-consuming part in the incremental build is the evaluation of
if_changed* macros since they are used in the recipes to compile C and
assembly source files into objects.
I notice the following code in if_changed* is expensive:
$(filter-out $(PHONY) $(wildcard $^),$^)
In the incremental build, every object has its .*.cmd file, which
contains the auto-generated list of included headers. So, $^ are
expanded into the long list of the source file + included headers,
and $(wildcard $^) checks whether they exist.
It may not be clear why this check exists there.
Here is the record of my research.
[1] The first code addition into Kbuild
This code dates back to 2002. It is the pre-git era. So, I copy-pasted
it from the historical git tree.
| commit 4a6db0791528c220655b063cf13fefc8470dbfee (HEAD)
| Author: Kai Germaschewski <kai@tp1.ruhr-uni-bochum.de>
| Date: Mon Jun 17 00:22:37 2002 -0500
|
| kbuild: Handle removed headers
|
| New and old way to handle dependencies would choke when a file
| #include'd by other files was removed, since the dependency on it was
| still recorded, but since it was gone, make has no idea what to do about
| it (and would complain with "No rule to make <file> ...")
|
| We now add targets for all the previously included files, so make will
| just ignore them if they disappear.
|
| diff --git a/Rules.make b/Rules.make
| index 6ef827d3df39..7db5301ea7db 100644
| --- a/Rules.make
| +++ b/Rules.make
| @@ -446,7 +446,7 @@ if_changed = $(if $(strip $? \
| # execute the command and also postprocess generated .d dependencies
| # file
|
| -if_changed_dep = $(if $(strip $? \
| +if_changed_dep = $(if $(strip $? $(filter-out FORCE $(wildcard $^),$^)\
| $(filter-out $(cmd_$(1)),$(cmd_$@))\
| $(filter-out $(cmd_$@),$(cmd_$(1)))),\
| @set -e; \
| diff --git a/scripts/fixdep.c b/scripts/fixdep.c
| index b5d7bee8efc7..db45bd1888c0 100644
| --- a/scripts/fixdep.c
| +++ b/scripts/fixdep.c
| @@ -292,7 +292,7 @@ void parse_dep_file(void *map, size_t len)
| exit(1);
| }
| memcpy(s, m, p-m); s[p-m] = 0;
| - printf("%s: \\\n", target);
| + printf("deps_%s := \\\n", target);
| m = p+1;
|
| clear_config();
| @@ -314,7 +314,8 @@ void parse_dep_file(void *map, size_t len)
| }
| m = p + 1;
| }
| - printf("\n");
| + printf("\n%s: $(deps_%s)\n\n", target, target);
| + printf("$(deps_%s):\n", target);
| }
|
| void print_deps(void)
The "No rule to make <file> ..." error can be solved by passing -MP to
the compiler, but I think the detection of header removal is a good
feature. When a header is removed, all source files that previously
included it should be re-compiled. This makes sure we has correctly
got rid of #include directives of it.
This is also related with the behavior of $?. The GNU Make manual says:
$?
The names of all the prerequisites that are newer than the target,
with spaces between them.
This does not explain whether a non-existent prerequisite is considered
to be newer than the target.
At this point of time, GNU Make 3.7x was used, where the $? did not
include non-existent prerequisites. Therefore,
$(filter-out FORCE $(wildcard $^),$^)
was useful to detect the header removal, and to rebuild the related
objects if it is the case.
[2] Change of $? behavior
Later, the behavior of $? was changed (fixed) to include prerequisites
that did not exist.
First, GNU Make commit 64e16d6c00a5 ("Various changes getting ready for
the release of 3.81.") changed it, but in the release test of 3.81, it
turned out to break the kernel build.
See these:
- http://lists.gnu.org/archive/html/bug-make/2006-03/msg00003.html
- https://savannah.gnu.org/bugs/?16002
- https://savannah.gnu.org/bugs/?16051
Then, GNU Make commit 6d8d9b74d9c5 ("Numerous updates to tests for
issues found on Cygwin and Windows.") reverted it for the 3.81 release
to give Linux kernel time to adjust to the new behavior.
After the 3.81 release, GNU Make commit 7595f38f62af ("Fixed a number
of documentation bugs, plus some build/install issues:") re-added it.
[3] Adjustment to the new $? behavior on Kbuild side
Meanwhile, the kernel build was changed by commit
|
||
|---|---|---|
| .. | ||
| atomic | ||
| basic | ||
| coccinelle | ||
| dtc | ||
| gcc-plugins | ||
| gdb | ||
| genksyms | ||
| kconfig | ||
| ksymoops | ||
| mod | ||
| package | ||
| selinux | ||
| tracing | ||
| .gitignore | ||
| adjust_autoksyms.sh | ||
| asn1_compiler.c | ||
| bin2c.c | ||
| bloat-o-meter | ||
| bootgraph.pl | ||
| bpf_helpers_doc.py | ||
| cc-can-link.sh | ||
| check_extable.sh | ||
| checkincludes.pl | ||
| checkkconfigsymbols.py | ||
| checkpatch.pl | ||
| checkstack.pl | ||
| checksyscalls.sh | ||
| checkversion.pl | ||
| clang-version.sh | ||
| cleanfile | ||
| cleanpatch | ||
| coccicheck | ||
| config | ||
| conmakehash.c | ||
| const_structs.checkpatch | ||
| decode_stacktrace.sh | ||
| decodecode | ||
| depmod.sh | ||
| diffconfig | ||
| documentation-file-ref-check | ||
| export_report.pl | ||
| extract_xc3028.pl | ||
| extract-cert.c | ||
| extract-ikconfig | ||
| extract-module-sig.pl | ||
| extract-sys-certs.pl | ||
| extract-vmlinux | ||
| faddr2line | ||
| file-size.sh | ||
| find-unused-docs.sh | ||
| gcc-goto.sh | ||
| gcc-ld | ||
| gcc-plugin.sh | ||
| gcc-version.sh | ||
| gcc-x86_32-has-stack-protector.sh | ||
| gcc-x86_64-has-stack-protector.sh | ||
| gen_compile_commands.py | ||
| gen_ksymdeps.sh | ||
| get_abi.pl | ||
| get_dvb_firmware | ||
| get_maintainer.pl | ||
| gfp-translate | ||
| headerdep.pl | ||
| headers_check.pl | ||
| headers_install.sh | ||
| insert-sys-cert.c | ||
| kallsyms.c | ||
| Kbuild.include | ||
| Kconfig.include | ||
| kernel-doc | ||
| ld-version.sh | ||
| leaking_addresses.pl | ||
| Lindent | ||
| link-vmlinux.sh | ||
| Makefile | ||
| Makefile.asm-generic | ||
| Makefile.build | ||
| Makefile.clean | ||
| Makefile.dtbinst | ||
| Makefile.extrawarn | ||
| Makefile.gcc-plugins | ||
| Makefile.headersinst | ||
| Makefile.host | ||
| Makefile.kasan | ||
| Makefile.kcov | ||
| Makefile.lib | ||
| Makefile.modbuiltin | ||
| Makefile.modfinal | ||
| Makefile.modinst | ||
| Makefile.modpost | ||
| Makefile.modsign | ||
| Makefile.package | ||
| Makefile.ubsan | ||
| makelst | ||
| markup_oops.pl | ||
| mkcompile_h | ||
| mkmakefile | ||
| mksysmap | ||
| mkuboot.sh | ||
| module-common.lds | ||
| modules-check.sh | ||
| namespace.pl | ||
| nsdeps | ||
| objdiff | ||
| parse-maintainers.pl | ||
| patch-kernel | ||
| profile2linkerlist.pl | ||
| prune-kernel | ||
| recordmcount.c | ||
| recordmcount.h | ||
| recordmcount.pl | ||
| setlocalversion | ||
| show_delta | ||
| sign-file.c | ||
| sortextable.c | ||
| sortextable.h | ||
| spdxcheck-test.sh | ||
| spdxcheck.py | ||
| spelling.txt | ||
| sphinx-pre-install | ||
| split-man.pl | ||
| stackdelta | ||
| stackusage | ||
| subarch.include | ||
| tags.sh | ||
| tools-support-relr.sh | ||
| unifdef.c | ||
| ver_linux | ||
| xen-hypercalls.sh | ||
| xz_wrap.sh | ||