+cc Petr FYI that were some discussion on this in the v2 thread at [0], which will result in some changes for v4.
[0]:https://lore.kernel.org/all/arEZrFmXeCO4hZpP@gremlin On Thu, Sep 17, 2026 at 05:06:24PM +0100, Lorenzo Stoakes (ARM) wrote: > With the .mod.S change in place, module finalisation on allmodconfig builds > consists of a large number of very short-lived jobs. > > For allmodconfig x86-64 this can be on the order of ~22,000 jobs of a few > milliseconds in duration each. > > Each job entails processing ~22k .cmd files, so the combination of heavy > overhead and small individual job results in a lot of unnecessary and > repeated work even with all cores being utilised. > > The solution is to batch by a number of jobs. Determining which value makes > sense was done empirically. > > On a 128-thread threadripper box doing an allmodconfig build, best of > 2, *.ko, *.mod.o deleted each time: > > modules per chunk instances wall > ----------------- --------- ------ > 1 11171 10.22s > 2 5586 6.72s > 4 2793 4.97s > 8 1397 4.26s > 16 699 4.05s > 32 350 4.05s > 64 175 4.07s > 128 88 4.05s > 256 44 4.03s > 512 22 4.18s > > Wall time flattens for 16-256 module batches. > > A slower/lower core machine will do better with fewer modules-per-batch, a > faster/higher core machine will do better with more modules-per-batch. > > Therefore, take the midpoint which works in the most margin in either > direction - 128 modules per batch. > > This naturally scales with module count too as the optimum gains are > obtained with higher module count, so fewer batches in this case costs less > overhead. > > Each instance holds only its own modules' variables and the top-level one > reads no per-module .cmd files at all, the same rules serve both levels, > and an instance is told its chunk with modfinal-first=<index>. > > Only the top-level instance builds .module-common.o, to the chunks it is a > plain prerequisite, so no two instances ever write it. > > "make modules" with every *.mod.o and *.ko deleted goes from 28.9s to 15.9s > with clang 22. No-op "make modules" goes from 5.6s to 4.8s, as checking the > 22,000 targets is spread over the chunks too. > > Whole build, 128-thread Threadripper 9980X, best of N runs: > > before after delta > ------------------------------- > x86 allmodconfig, no-op make, gcc 2.3s 1.4s -0.85s (-38%) > x86 allmodconfig, no-op make, clang 2.8s 1.8s -0.94s (-34%) > x86 allmodconfig, clean, gcc 306.6s 294.1s -12.6s (-4%) > x86 allmodconfig, clean, clang 290.2s 283.9s -6.4s (-2%) > > Assisted-by: LLM > Signed-off-by: Lorenzo Stoakes (ARM) <[email protected]> > --- > scripts/Makefile.modfinal | 30 +++++++++++++++++++++++++++++- > 1 file changed, 29 insertions(+), 1 deletion(-) > > diff --git a/scripts/Makefile.modfinal b/scripts/Makefile.modfinal > index 75e9effdf02c..4d5e6e1a0ff9 100644 > --- a/scripts/Makefile.modfinal > +++ b/scripts/Makefile.modfinal > @@ -13,9 +13,30 @@ include $(srctree)/scripts/Makefile.lib > # find all modules listed in modules.order > modules := $(call read-file, modules.order) > > +modfinal-chunk-size := 128 > + > +ifdef modfinal-first > + > +# this instance handles the chunk of modules.order starting at > $(modfinal-first) > +modules := $(wordlist $(modfinal-first), $(words $(modules)), $(modules)) > +modules := $(wordlist 1, $(modfinal-chunk-size), $(modules)) > + > __modfinal: $(modules:%.o=%.ko) > @: > > +else > + > +modfinal-chunks := $(addprefix chunk-, $(shell seq 1 $(modfinal-chunk-size) > $(words $(modules)))) > + > +PHONY += $(modfinal-chunks) > +$(modfinal-chunks): .module-common.o > + $(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modfinal > modfinal-first=$(@:chunk-%=%) > + > +__modfinal: $(modfinal-chunks) > + @: > + > +endif > + > # modname and part-of-module are set to make c_flags define proper module > flags > modname = $(notdir $(@:.mod.o=)) > part-of-module = y > @@ -29,8 +50,11 @@ quiet_cmd_as_mod_o = AS [M] $@ > %.mod.o: %.mod.S FORCE > $(call if_changed,as_mod_o) > > +# Built by the top-level instance alone, the chunks take it as a plain file. > +ifndef modfinal-first > .module-common.o: $(srctree)/scripts/module-common.c FORCE > $(call if_changed_rule,cc_o_c) > +endif > > ifneq ($(WARN_ON_UNUSED_TRACEPOINTS),) > cmd_check_tracepoint = $(objtree)/scripts/tracepoint-update --module $<; > @@ -58,7 +82,11 @@ ifdef CONFIG_DEBUG_INFO_BTF_MODULES > endif > +$(call cmd,check_tracepoint) > > -targets += $(modules:%.o=%.ko) $(modules:%.o=%.mod.o) .module-common.o > +ifdef modfinal-first > +targets += $(modules:%.o=%.ko) $(modules:%.o=%.mod.o) > +else > +targets += .module-common.o > +endif > > # Add FORCE to the prerequisites of a target to force it to be always > rebuilt. > # --------------------------------------------------------------------------- > > -- > 2.55.0 > -- Cheers, Lorenzo
