Re: [PATCH 17/23] modpost: perform srcversion hashing in parallel

[David Laight <david.laight.linux@gmail.com>]

On Tue, 08 Sep 2026 21:55:17 +0100
"Lorenzo Stoakes (ARM)" <ljs@kernel.org> wrote:

> modpost does a lot of single-threaded work hashing files from each object's
> .cmd file.
> 
> This makes the build slower than it needs to be, so do this work in
> parallel.
> 
> This is egregious for allmodconfig builds - for instance x86-64 can end up
> opening 200,000 files individually and hashing them all serially.
> 
> Parallelise this operation by maintaining a thread pool for the hashing
> work.
> 
> Combined with the per-file hashing commit this cuts modpost's run time
> nearly in half for an allmodconfig build.
> 
> Module.symvers and every *.mod.S are byte for byte the same.
> 
> modpost is on the serial tail of every allmodconfig build, however
> defconfig does not set CONFIG_MODULE_SRCVERSION_ALL and is unchanged.
> 
> Whole build, 128-thread Threadripper 9980X, best of N runs:
> 
>                                          before   after     delta
>                                          -------------------------------
>   x86 allmodconfig, touch mm/vma.c, gcc    33.4s    30.2s     -3.2s (-10%)
>   x86 allmodconfig, touch mm/vma.c, clang  31.1s    28.1s     -3.0s (-10%)
> 
> Assisted-by: LLM
> Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> ---
>  scripts/mod/Makefile     |  1 +
>  scripts/mod/modpost.c    | 72 ++++++++++++++++++++++++++++++++++++++++++++++--
>  scripts/mod/modpost.h    |  2 ++
>  scripts/mod/sumversion.c |  3 +-
>  4 files changed, 74 insertions(+), 4 deletions(-)
> 
> diff --git a/scripts/mod/Makefile b/scripts/mod/Makefile
> index fbd5099e0441..fdd486184f9c 100644
> --- a/scripts/mod/Makefile
> +++ b/scripts/mod/Makefile
> @@ -5,6 +5,7 @@ hostprogs-always-y	+= modpost mk_elfconfig
>  always-y		+= empty.o
>  
>  modpost-objs	:= modpost.o file2alias.o sumversion.o symsearch.o
> +HOSTLDLIBS_modpost := -lpthread
>  
>  devicetable-offsets-file := devicetable-offsets.h
>  
> diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
> index 550ccd753ed8..882169e51851 100644
> --- a/scripts/mod/modpost.c
> +++ b/scripts/mod/modpost.c
> @@ -16,6 +16,7 @@
>  #include <fnmatch.h>
>  #include <stdio.h>
>  #include <ctype.h>
> +#include <pthread.h>
>  #include <string.h>
>  #include <limits.h>
>  #include <stdbool.h>
> @@ -1717,8 +1718,7 @@ static void read_symbols(const char *modname)
>  	if (!mod->is_vmlinux) {
>  		version = get_modinfo(&info, "version");
>  		if (version || all_versions)
> -			get_src_version(mod->name, mod->srcversion,
> -					sizeof(mod->srcversion) - 1);
> +			mod->need_srcversion = true;
>  	}
>  
>  	parse_elf_finish(&info);
> @@ -1736,6 +1736,72 @@ static void read_symbols(const char *modname)
>  	}
>  }
>  
> +static struct module **srcversion_mods;
> +static unsigned int nr_srcversion_mods, next_srcversion_mod;
> +
> +static bool get_next_src_version(void)
> +{
> +	struct module *mod;
> +	unsigned int idx;
> +
> +	idx = __sync_fetch_and_add(&next_srcversion_mod, 1);

When I've done this is the past, adding 'a few' items was faster
because it reduced the contention on the counter.
(Although there were probably some very cheap 'actions'.)

> +	if (idx >= nr_srcversion_mods)
> +		return false;
> +	mod = srcversion_mods[idx];
> +
> +	get_src_version(mod->name, mod->srcversion,
> +			sizeof(mod->srcversion) - 1);
> +	return true;
> +}
> +
> +static void *srcversion_worker(void *arg)
> +{
> +	while (get_next_src_version())
> +		;

I don't think the extra function call helps.

> +
> +	return NULL;
> +}
> +
> +static void hash_srcversions(void)
> +{
> +	unsigned int i = 0;
> +	struct module *mod;
> +	pthread_t *threads;
> +	long nr_threads;
> +
> +	list_for_each_entry(mod, &modules, list)
> +		if (mod->need_srcversion)
> +			nr_srcversion_mods++;
> +
> +	if (!nr_srcversion_mods)
> +		return;
> +
> +	srcversion_mods = xmalloc(nr_srcversion_mods * sizeof(*srcversion_mods));
> +
> +	list_for_each_entry(mod, &modules, list)
> +		if (mod->need_srcversion)
> +			srcversion_mods[i++] = mod;
> +
> +	nr_threads = sysconf(_SC_NPROCESSORS_ONLN);

One thread per cpu is probably a few to many.
It might be worth adding a command line parameter for the 'max threads'.
Possibly with -n meaning 'all but n'.

> +	nr_threads = nr_threads < 1 ? 1 : nr_threads; /* On error assume 1. */
> +	if (nr_threads > nr_srcversion_mods)
> +		nr_threads = nr_srcversion_mods;
> +
> +	sumversion_init();
> +	threads = xmalloc(nr_threads * sizeof(*threads));
> +	for (i = 0; i < nr_threads; i++) {
> +		if (pthread_create(&threads[i], NULL, srcversion_worker, NULL)) {
> +			perror("pthread_create");
> +			exit(1);
> +		}
> +	}

The main code can call srcversion_worker() here.

David

> +	for (i = 0; i < nr_threads; i++)
> +		pthread_join(threads[i], NULL);
> +
> +	free(threads);
> +	free(srcversion_mods);
> +}
> +
>  static void read_symbols_from_files(const char *filename)
>  {
>  	FILE *in = stdin;
> @@ -2729,6 +2795,8 @@ int main(int argc, char **argv)
>  	if (files_source)
>  		read_symbols_from_files(files_source);
>  
> +	hash_srcversions();
> +
>  	list_for_each_entry(mod, &modules, list) {
>  		keep_no_trim_symbols(mod);
>  
> diff --git a/scripts/mod/modpost.h b/scripts/mod/modpost.h
> index d5f6d82837d5..10d5f8f2f293 100644
> --- a/scripts/mod/modpost.h
> +++ b/scripts/mod/modpost.h
> @@ -127,6 +127,7 @@ struct module {
>  	bool has_init;
>  	bool has_cleanup;
>  	char	     srcversion[25];
> +	bool need_srcversion;
>  	// Missing namespace dependencies
>  	struct list_head missing_namespaces;
>  	// Actual imported namespaces
> @@ -213,6 +214,7 @@ void handle_moddevtable(struct module *mod, struct elf_info *info,
>  			Elf_Sym *sym, const char *symname);
>  
>  /* sumversion.c */
> +void sumversion_init(void);
>  void get_src_version(const char *modname, char sum[], unsigned sumlen);
>  
>  /* from modpost.c */
> diff --git a/scripts/mod/sumversion.c b/scripts/mod/sumversion.c
> index 5501d6aa0bea..4521b92ef868 100644
> --- a/scripts/mod/sumversion.c
> +++ b/scripts/mod/sumversion.c
> @@ -249,7 +249,7 @@ static int parse_comment(const char *file, unsigned long len)
>  /* FIXME: Handle .s files differently (eg. # starts comments) --RR */
>  static bool stop_char[256];
>  
> -static void sumversion_init(void)
> +void sumversion_init(void)
>  {
>  	static bool done;
>  	int chr;
> @@ -402,7 +402,6 @@ static int parse_source_files(const char *objfile, struct md4_ctx *md)
>  				     line, strerror(errno));
>  				goto out_file;
>  			}
> -
>  		}
>  
>  	}
>