Hi Douglas, 2017-10-05 7:37 GMT+09:00 Douglas Anderson <dianders@xxxxxxxxxxxx>: > 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. > > Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx> I reviewed and tested this patch more closely. V2 is almost good, but I see some problem and things that should be improved. (including bike-shed) [1] If you apply this patch and run "make clean" on a machine without "sphinx-build" installed, you will see a mysterious error message like follows: $ make clean Documentation/Makefile:24: The 'sphinx-build' command was not found. Make sure you have Sphinx installed and in PATH, or set the SPHINXBUILD make variable to point to the full path of the 'sphinx-build' executable. Detected OS: Ubuntu 16.04.2 LTS. Warning: better to also install "dot". Warning: better to also install "rsvg-convert". ERROR: please install "virtualenv", otherwise, build won't work. You should run: sudo apt-get install graphviz librsvg2-bin virtualenv virtualenv sphinx_1.4 . sphinx_1.4/bin/activate pip install -r Documentation/sphinx/requirements.txt Can't build as 2 mandatory dependencies are missing at ./scripts/sphinx-pre-install line 566. This comes from the ".DEFAULT" target when "make clean" descends into Documentation/ directory. You can fix it by adding $(make-cache): ; to scripts/Kbuild.include This will prevent Make from searching a target that would generate $(make-cache). (Of course, we can fix Documentation/Makefile to not use '.DEFAULT', but canceling $(make-cache) rule is a good thing.) You will need this https://patchwork.kernel.org/patch/9998651/ before adding the target to Kbuild.include [2] Please clean up .cache.mk Adding .cache.mk pattern around line 1540 will be good. A few more comments below. > +--- 3.14 $(LD) support function cache > + > +One thing to realize about all the calls to the above support functions > +is that each use of them requires a full invocation of an external tool, like > +the C compiler, assembler, or linker. If nothing else that invocation will > +cause a fork/exec/shared library link. In some build environments, however, it > +could also involve traversing thorough one or more wrappers. To put some > +numbers on it, I've measured compiler invocations of as fast as 4ms or > +as slow as 150ms. > + > +Many of the above support functions are used in places where they are > +evaluated on each invocation of Make before anything else can run. Even on > +a simple command like "make help" we need to evaluate every single one of the > +variables. > + > +To avoid this slow overhead we can cache the result of these invocations. > +If we store this cache in a way that's easy for Make to use (like in Makefile > +format) then it will be very quick and we'll save a lot of time with each > +invocation of make. > + > + The section number should be 3.13 instead of 3.14 but is this explanation necessary? If you read the "2 Who does what" section of Documentation/kbuild/makefile.txt this file is mostly written for "normal developers" and "arch developers". It focuses on how to write Linux makefiles. It does not explain how Kbuild works. Knowing what cool things happening behind the scene is great. But, it is probably out of scope of this file. You added very nice explanation in scripts/Kbuild.include and git-log. Those who are interested in this feature will be able to find enough information. > === 4 Host Program support > > Kbuild supports building executables on the host for use during the > diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include > index 9ffd3dda3889..c539c709a00c 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 > @@ -69,6 +71,56 @@ endef > # gcc support functions > # See documentation in Documentation/kbuild/makefiles.txt > > +# 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 > +-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) := $$($(1))' >> $(make-cache)) > +endif > +endef > +__shell-cached = $(eval $(call __run-and-store,$(1)))$($(1)) > +shell-cached = $(call __shell-cached,__cached_$(call __sanitize-opt,$(1)),$(1)) > + Bike shed: Could you insert this hunk below cc-cross-prefix? cc-cross-prefix is unrelated thing here. So, I want to shell-cache and try-run things come closer. > # cc-cross-prefix > # Usage: CROSS_COMPILE := $(call cc-cross-prefix, m68k-linux-gnu- m68k-linux-) > # Return first prefix where a prefix$(CC) is found in PATH. > @@ -87,30 +139,40 @@ TMPOUT := $(if $(KBUILD_EXTMOD),$(firstword $(KBUILD_EXTMOD))/) > # Usage: option = $(call try-run, $(CC)...-o "$$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" "$$TMPO" > + > +# try-run > +# Usage: option = $(call try-run, $(CC)...-o "$$TMP",option-ok,otherwise) > +# Exit code chooses option. "$$TMP" serves as a temporary file and is > +# automatically cleaned up. > +try-run = $(shell $(__try-run)) This is unnecessary. It is completely the same as a few lines above. -- Best Regards Masahiro Yamada -- 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
