While timing a "no-op" build of the kernel (incrementally building the kernel even though nothing changed) in the Chrome OS build system I found that it was much slower than I expected. Digging into things a bit, I found that quite a bit of the time was spent invoking the C compiler even though we weren't actually building anything. Currently in the Chrome OS build system the C compiler is called through a number of wrappers (one of which is written in python!) and can take upwards of 100 ms to invoke even if we're not doing anything difficult, so these invocations of the compiler were taking a lot of time. Worse the invocations couldn't seem to take advantage of the multiple cores on my system. Certainly it seems like we could make the compiler invocations in the Chrome OS build system faster, but only to a point. Inherently invoking a program as big as a C compiler is a fairly heavy operation. Thus even if we can speed the compiler calls it made sense to track down what was happening. It turned out that all the compiler invocations were coming from usages like this in the kernel's Makefile: KBUILD_CFLAGS += $(call cc-option,-fno-delete-null-pointer-checks,) Due to the way cc-option and similar statements work the above contains an implicit call to the C compiler. ...and due to the fact that we're storing the result in KBUILD_CFLAGS, a simply expanded variable, the call will happen every time the Makefile is parsed, even if there are no users of KBUILD_CFLAGS. Rather than redoing this computation every time, it makes a lot of sense to cache the result of all of the Makefile's compiler calls just like we do when we compile a ".c" file to a ".o" file. Conceptually this is quite a simple idea. ...and since the calls to invoke the compiler and similar tools are centrally located in the Kbuild.include file this doesn't even need to be super invasive. Implementing the cache in a simple-to-use and efficient way is not quite as simple as it first sounds, though. To get maximum speed we really want the cache in a format that make can natively understand and make doesn't really have an ability to load/parse files. ...but make _can_ import other Makefiles, so the solution is to store the cache in Makefile format. This requires coming up with a valid/unique Makefile variable name for each value to be cached, but that's solvable with some cleverness. After this change, we'll automatically create a ".cache.mk" file that will contain our cached variables. We'll load this on each invocation of make and will avoid recomputing anything that's already in our cache. The cache is stored in a format that it shouldn't need any invalidation since anything that might change should affect the "key" and any old cached value won't be used. NOTE: This change requires <https://patchwork.kernel.org/patch/9998651/> ("kbuild: add forward declaration of default target to Makefile.asm-generic") for correctness. Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx> Tested-by: Ingo Molnar <mingo@xxxxxxxxxx> Tested-by: Guenter Roeck <linux@xxxxxxxxxxxx> --- Changes in v3: - Rule to prevent make from trying to generate the cache - Rule to clean .cache.mk - No more doc changes - Moved cache stuff below cc-cross-prefix - Removed duplicate documentation of try-run (oops) - Add Tested-by for Ingo and Guenter since v2 and v3 are very similar Changes in v2: - Abstract at a different level (like shell-cached) per Masahiro Yamada - Include ld-version, which I missed the first time Makefile | 1 + scripts/Kbuild.include | 87 ++++++++++++++++++++++++++++++++++++++++++-------- 2 files changed, 74 insertions(+), 14 deletions(-) diff --git a/Makefile b/Makefile index 2835863bdd5a..29c06eacf491 100644 --- a/Makefile +++ b/Makefile @@ -1550,6 +1550,7 @@ clean: $(clean-dirs) -o -name '.*.d' -o -name '.*.tmp' -o -name '*.mod.c' \ -o -name '*.symtypes' -o -name 'modules.order' \ -o -name modules.builtin -o -name '.tmp_*.o.*' \ + -o -name .cache.mk \ -o -name '*.c.[012]*.*' \ -o -name '*.ll' \ -o -name '*.gcno' \) -type f -print | xargs rm -f diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include index 9ffd3dda3889..4203fff8d4c3 100644 --- a/scripts/Kbuild.include +++ b/scripts/Kbuild.include @@ -8,6 +8,8 @@ squote := ' empty := space := $(empty) $(empty) space_escape := _-_SPACE_-_ +right_paren := ) +left_paren := ( ### # Name of target with a '.' as filename prefix. foo/bar.o => foo/.bar.o @@ -80,6 +82,57 @@ cc-cross-prefix = \ echo $(c); \ fi))) +# Tools for caching Makefile variables that are "expensive" to compute. +# +# Here we want to help deal with variables that take a long time to compute +# by making it easy to store these variables in a cache. +# +# The canonical example here is testing for compiler flags. On a simple system +# each call to the compiler takes 10 ms, but on a system with a compiler that's +# called through various wrappers it can take upwards of 100 ms. If we have +# 100 calls to the compiler this can take 1 second (on a simple system) or 10 +# seconds (on a complicated system). +# +# The "cache" will be in Makefile syntax and can be directly included. +# Any time we try to reference a variable that's not in the cache we'll +# calculate it and store it in the cache for next time. + +# Include values from last time +make-cache := $(if $(KBUILD_EXTMOD),$(KBUILD_EXTMOD)/,$(if $(obj),$(obj)/)).cache.mk +$(make-cache): ; +-include $(make-cache) + +# Usage: $(call __sanitize-opt,Hello=Hola$(comma)Goodbye Adios) +# +# Convert all '$', ')', '(', '\', '=', ' ', ',', ':' to '_' +__sanitize-opt = $(subst $$,_,$(subst $(right_paren),_,$(subst $(left_paren),_,$(subst \,_,$(subst =,_,$(subst $(space),_,$(subst $(comma),_,$(subst :,_,$(1))))))))) + +# Usage: $(call shell-cached,shell_command) +# Example: $(call shell-cached,md5sum /usr/bin/gcc) +# +# If we've already seen a call to this exact shell command (even in a +# previous invocation of make!) we'll return the value. If not, we'll +# compute it and store the result for future runs. +# +# This is a bit of voodoo, but basic explanation is that if the variable +# was undefined then we'll evaluate the shell command and store the result +# into the variable. We'll then store that value in the cache and finally +# output the value. +# +# NOTE: The $$(2) here isn't actually a parameter to __run-and-store. We +# happen to know that the caller will have their shell command in $(2) so the +# result of "call"ing this will produce a reference to that $(2). The reason +# for this strangeness is to avoid an extra level of eval (and escaping) of +# $(2). +define __run-and-store +ifeq ($(origin $(1)),undefined) + $$(eval $(1) := $$(shell $$(2))) + $$(shell echo '$(1) := $$($(1))' >> $(make-cache)) +endif +endef +__shell-cached = $(eval $(call __run-and-store,$(1)))$($(1)) +shell-cached = $(call __shell-cached,__cached_$(call __sanitize-opt,$(1)),$(1)) + # output directory for tests below TMPOUT := $(if $(KBUILD_EXTMOD),$(firstword $(KBUILD_EXTMOD))/) @@ -87,30 +140,36 @@ TMPOUT := $(if $(KBUILD_EXTMOD),$(firstword $(KBUILD_EXTMOD))/) # Usage: option = $(call try-run, $(CC)...-o "$$TMP",option-ok,otherwise) # Exit code chooses option. "$$TMP" serves as a temporary file and is # automatically cleaned up. -try-run = $(shell set -e; \ +__try-run = set -e; \ TMP="$(TMPOUT).$$$$.tmp"; \ TMPO="$(TMPOUT).$$$$.o"; \ if ($(1)) >/dev/null 2>&1; \ then echo "$(2)"; \ else echo "$(3)"; \ fi; \ - rm -f "$$TMP" "$$TMPO") + rm -f "$$TMP" "$$TMPO" + +try-run = $(shell $(__try-run)) + +# try-run-cached +# This works like try-run, but the result is cached. +try-run-cached = $(call shell-cached,$(__try-run)) # as-option # Usage: cflags-y += $(call as-option,-Wa$(comma)-isa=foo,) -as-option = $(call try-run,\ +as-option = $(call try-run-cached,\ $(CC) $(KBUILD_CFLAGS) $(1) -c -x assembler /dev/null -o "$$TMP",$(1),$(2)) # as-instr # Usage: cflags-y += $(call as-instr,instr,option1,option2) -as-instr = $(call try-run,\ +as-instr = $(call try-run-cached,\ printf "%b\n" "$(1)" | $(CC) $(KBUILD_AFLAGS) -c -x assembler -o "$$TMP" -,$(2),$(3)) # __cc-option # Usage: MY_CFLAGS += $(call __cc-option,$(CC),$(MY_CFLAGS),-march=winchip-c6,-march=i586) -__cc-option = $(call try-run,\ +__cc-option = $(call try-run-cached,\ $(1) -Werror $(2) $(3) -c -x c /dev/null -o "$$TMP",$(3),$(4)) # Do not attempt to build with gcc plugins during cc-option tests. @@ -130,23 +189,23 @@ hostcc-option = $(call __cc-option, $(HOSTCC),\ # cc-option-yn # Usage: flag := $(call cc-option-yn,-march=winchip-c6) -cc-option-yn = $(call try-run,\ +cc-option-yn = $(call try-run-cached,\ $(CC) -Werror $(KBUILD_CPPFLAGS) $(CC_OPTION_CFLAGS) $(1) -c -x c /dev/null -o "$$TMP",y,n) # cc-disable-warning # Usage: cflags-y += $(call cc-disable-warning,unused-but-set-variable) -cc-disable-warning = $(call try-run,\ +cc-disable-warning = $(call try-run-cached,\ $(CC) -Werror $(KBUILD_CPPFLAGS) $(CC_OPTION_CFLAGS) -W$(strip $(1)) -c -x c /dev/null -o "$$TMP",-Wno-$(strip $(1))) # cc-name # Expands to either gcc or clang -cc-name = $(shell $(CC) -v 2>&1 | grep -q "clang version" && echo clang || echo gcc) +cc-name = $(call shell-cached,$(CC) -v 2>&1 | grep -q "clang version" && echo clang || echo gcc) # cc-version -cc-version = $(shell $(CONFIG_SHELL) $(srctree)/scripts/gcc-version.sh $(CC)) +cc-version = $(call shell-cached,$(CONFIG_SHELL) $(srctree)/scripts/gcc-version.sh $(CC)) # cc-fullversion -cc-fullversion = $(shell $(CONFIG_SHELL) \ +cc-fullversion = $(call shell-cached,$(CONFIG_SHELL) \ $(srctree)/scripts/gcc-version.sh -p $(CC)) # cc-ifversion @@ -159,22 +218,22 @@ cc-if-fullversion = $(shell [ $(cc-fullversion) $(1) $(2) ] && echo $(3) || echo # cc-ldoption # Usage: ldflags += $(call cc-ldoption, -Wl$(comma)--hash-style=both) -cc-ldoption = $(call try-run,\ +cc-ldoption = $(call try-run-cached,\ $(CC) $(1) -nostdlib -x c /dev/null -o "$$TMP",$(1),$(2)) # ld-option # Usage: LDFLAGS += $(call ld-option, -X) -ld-option = $(call try-run,\ +ld-option = $(call try-run-cached,\ $(CC) -x c /dev/null -c -o "$$TMPO" ; $(LD) $(1) "$$TMPO" -o "$$TMP",$(1),$(2)) # ar-option # Usage: KBUILD_ARFLAGS := $(call ar-option,D) # Important: no spaces around options -ar-option = $(call try-run, $(AR) rc$(1) "$$TMP",$(1),$(2)) +ar-option = $(call try-run-cached, $(AR) rc$(1) "$$TMP",$(1),$(2)) # ld-version # Note this is mainly for HJ Lu's 3 number binutil versions -ld-version = $(shell $(LD) --version | $(srctree)/scripts/ld-version.sh) +ld-version = $(call shell-cached,$(LD) --version | $(srctree)/scripts/ld-version.sh) # ld-ifversion # Usage: $(call ld-ifversion, -ge, 22252, y) -- 2.15.0.rc0.271.g36b669edcc-goog -- To unsubscribe from this list: send the line "unsubscribe linux-kbuild" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
