On Tue, Nov 16, 2021 at 01:00:18PM +0100, Ævar Arnfjörð Bjarmason wrote: > Add a template to do the "mkdir -p" of $(@D) (the parent dir of $@) > for us, and use it for the "make lint-docs" targets I added in > 8650c6298c1 (doc lint: make "lint-docs" non-.PHONY, 2021-10-15). > > As seen in 4c64fb5aad9 (Documentation/Makefile: fix lint-docs mkdir > dependency, 2021-10-26) maintaining these manual lists of parent > directory dependencies is fragile, in addition to being obviously > verbose. > > I used this pattern at the time because I couldn't find another method > than "order-only" prerequisites to avoid doing a "mkdir -p $(@D)" for > every file being created, which as noted in [1] would be significantly > slower. > > But as it turns out we can use this neat trick of only doing a "mkdir > -p" if the $(wildcard) macro tells us the path doesn't exist. A re-run > of a performance test similar to thatnoted downthread of [1] in [2] > shows that this is faster, in addition to being less verbose and more > reliable (this uses my "git-hyperfine" thin wrapper for "hyperfine"[3]): > > $ git hyperfine -L rev HEAD~0,HEAD~1 -b 'make -C Documentation lint-docs' -p 'rm -rf Documentation/.build' 'make -C Documentation lint-docs' > Benchmark 1: make -C Documentation lint-docs' in 'HEAD~0 > Time (mean ± σ): 2.129 s ± 0.011 s [User: 1.840 s, System: 0.321 s] > Range (min … max): 2.121 s … 2.158 s 10 runs > > Benchmark 2: make -C Documentation lint-docs' in 'HEAD~1 > Time (mean ± σ): 2.659 s ± 0.002 s [User: 2.306 s, System: 0.397 s] > Range (min … max): 2.657 s … 2.662 s 10 runs > > Summary > 'make -C Documentation lint-docs' in 'HEAD~0' ran > 1.25 ± 0.01 times faster than 'make -C Documentation lint-docs' in 'HEAD~1' > > So let's use that pattern both for the "lint-docs" target, and a few > miscellaneous other targets. > > This method of creating parent directories is explicitly racy in that > we don't know if we're going to say always create a "foo" followed by > a "foo/bar" under parallelism, or skip the "foo" because we created > "foo/bar" first. In this case it doesn't matter for anything except > that we aren't guaranteed to get the same number of rules firing when > running make in parallel. Something else that is racy is that $(wildcard) might be saying the directory doesn't exist while there's another make subprocess that has already started spawning `mkdir -p` for that directory. That doesn't make a huge difference, but you can probably still end up with multiple `mkdir -p` runs for the same directory. I think something like the following could work while avoiding those races: define create_parent_dir_RULE $(1): | $(dir $(1)). ALL_DIRS += $(dir $(1)) endef define create_parent_dir_TARGET $(1)/.: $(dir $(1)). echo mkdir $$(@D) endef $(eval $(call create_parent_dir_RULE, first/path/file)) $(eval $(call create_parent_dir_RULE, second/path/file)) # ... $(foreach dir,$(sort $(ALL_DIRS)),$(eval $(call create_parent_dir_TARGET,$(dir:%/=%)))) Mike
