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Date: Wed, 11 Mar 2015 11:01:01 +0100 From: Michal Marek <mmarek@...e.cz> To: Linus Torvalds <torvalds@...ux-foundation.org> Cc: Valdis Kletnieks <Valdis.Kletnieks@...edu>, Andrew Morton <akpm@...ux-foundation.org>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Linux Kbuild mailing list <linux-kbuild@...r.kernel.org> Subject: Re: [RFC PATCH] Don't reset timestamps in include/generated if not needed On Sun, Mar 08, 2015 at 04:08:06PM -0700, Linus Torvalds wrote: > On Tue, Mar 3, 2015 at 5:07 PM, Valdis Kletnieks > <Valdis.Kletnieks@...edu> wrote: > > > > Kbuild regenerates bounds.h and asm-offsets.h, resetting the timestamps > > and forcing rebuilds even if the contents haven't changed. Add a bit of > > shell magic to only replace the file if the contents have in fact changed, > > which should speed up git bisects and similar. > > ... > > RFC because I can't wrap my head around why this wasn't done ages ago. > > If I'm missing something something obvious, please apply a cluestick. :) We do not regenerate the files *always*, but only if bounds.c / asm-offsets.c or one of the headers they include change. So the rule is not any worse than the rule for compiling a regular .c file into an .o file. You can use ccache to avoid repeated rebuilds. However, I can see that especially bounds.h ends up being included in many places, so cutting its dependencies helps in some cases. > > Lightly tested - if I rm one of those two files, it gets rebuilt. If I > > insert some whitespace, it gets replaced. If I don't touch it, the datestamp > > doesn't change. > > I don't think this is wrong, but I'd really prefer to do the whole > move-if-changed thing as a separate rule, and a separate command. > > IOW, could we please have something that generates the "build.h" file > by separately creating a new temporary file, and then having the > traditional kind of move-if-changed definition > > define move-if-changed > if ! cmp -s $(1) $(2); then mv -f $(1) $(2); else rm -f $(1); fi > endef We already have it and it is called "filechk." Valdis, can you check if the below patch works equally well for you? diff --git a/Kbuild b/Kbuild index ab8ded9..47d0630 100644 --- a/Kbuild +++ b/Kbuild @@ -13,8 +13,9 @@ define sed-y s:->::; p;}" endef -quiet_cmd_offsets = GEN $@ -define cmd_offsets +# Use filechk to avoid rebuilds when a header changes, but the resulting file +# does not +define filechk_offsets (set -e; \ echo "#ifndef $2"; \ echo "#define $2"; \ @@ -24,9 +25,9 @@ define cmd_offsets echo " * This file was generated by Kbuild"; \ echo " */"; \ echo ""; \ - sed -ne $(sed-y) $<; \ + sed -ne $(sed-y); \ echo ""; \ - echo "#endif" ) > $@ + echo "#endif" ) endef ##### @@ -44,7 +45,7 @@ kernel/bounds.s: kernel/bounds.c FORCE $(obj)/$(bounds-file): kernel/bounds.s Kbuild $(Q)mkdir -p $(dir $@) - $(call cmd,offsets,__LINUX_BOUNDS_H__) + $(call filechk,offsets,__LINUX_BOUNDS_H__) ##### # 2) Generate asm-offsets.h @@ -63,7 +64,7 @@ arch/$(SRCARCH)/kernel/asm-offsets.s: arch/$(SRCARCH)/kernel/asm-offsets.c \ $(call if_changed_dep,cc_s_c) $(obj)/$(offsets-file): arch/$(SRCARCH)/kernel/asm-offsets.s Kbuild - $(call cmd,offsets,__ASM_OFFSETS_H__) + $(call filechk,offsets,__ASM_OFFSETS_H__) ##### # 3) Check for missing system calls -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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