Commit 48fd1188 authored by Kevin Cernekee's avatar Kevin Cernekee Committed by Rusty Russell
Browse files

module: Fix performance regression on modules with large symbol tables 



Looking at /proc/kallsyms, one starts to ponder whether all of the extra
strtab-related complexity in module.c is worth the memory savings.

Instead of making the add_kallsyms() loop even more complex, I tried the
other route of deleting the strmap logic and naively copying each string
into core_strtab with no consideration for consolidating duplicates.

Performance on an "already exists" insmod of nvidia.ko (runs
add_kallsyms() but does not actually initialize the module):

	Original scheme: 1.230s
	With naive copying: 0.058s

Extra space used: 35k (of a 408k module).

Signed-off-by: default avatarKevin Cernekee <cernekee@gmail.com>
Signed-off-by: default avatarRusty Russell <rusty@rustcorp.com.au>
LKML-Reference: <73defb5e4bca04a6431392cc341112b1@localhost>
parent 70b1e916

kernel/module.c

+21 −44
Original line number Diff line number Diff line
@@ -138,7 +138,6 @@ struct load_info { 
	unsigned long len;

	Elf_Shdr *sechdrs;

	char *secstrings, *strtab;

	unsigned long *strmap;

	unsigned long symoffs, stroffs;

	struct _ddebug *debug;

	unsigned int num_debug;
@@ -2178,12 +2177,19 @@ static bool is_core_symbol(const Elf_Sym *src, const Elf_Shdr *sechdrs, 
	return true;

}



/*

 * We only allocate and copy the strings needed by the parts of symtab

 * we keep.  This is simple, but has the effect of making multiple

 * copies of duplicates.  We could be more sophisticated, see

 * linux-kernel thread starting with

 * <73defb5e4bca04a6431392cc341112b1@localhost>.

 */

static void layout_symtab(struct module *mod, struct load_info *info)

{

	Elf_Shdr *symsect = info->sechdrs + info->index.sym;

	Elf_Shdr *strsect = info->sechdrs + info->index.str;

	const Elf_Sym *src;

	unsigned int i, nsrc, ndst;

	unsigned int i, nsrc, ndst, strtab_size;



	/* Put symbol section at end of init part of module. */

	symsect->sh_flags |= SHF_ALLOC;
@@ -2194,38 +2200,23 @@ static void layout_symtab(struct module *mod, struct load_info *info) 
	src = (void *)info->hdr + symsect->sh_offset;

	nsrc = symsect->sh_size / sizeof(*src);



	/*

	 * info->strmap has a '1' bit for each byte of .strtab we want to

	 * keep resident in mod->core_strtab.  Everything else in .strtab

	 * is unreferenced by the symbols in mod->core_symtab, and will be

	 * discarded when add_kallsyms() compacts the string table.

	 */

	for (ndst = i = 1; i < nsrc; ++i, ++src)

	/* Compute total space required for the core symbols' strtab. */

	for (ndst = i = strtab_size = 1; i < nsrc; ++i, ++src)

		if (is_core_symbol(src, info->sechdrs, info->hdr->e_shnum)) {

			unsigned int j = src->st_name;



			while (!__test_and_set_bit(j, info->strmap)

			       && info->strtab[j])

				++j;

			++ndst;

			strtab_size += strlen(&info->strtab[src->st_name]) + 1;

			ndst++;

		}



	/* Append room for core symbols at end of core part. */

	info->symoffs = ALIGN(mod->core_size, symsect->sh_addralign ?: 1);

	mod->core_size = info->symoffs + ndst * sizeof(Elf_Sym);

	info->stroffs = mod->core_size = info->symoffs + ndst * sizeof(Elf_Sym);

	mod->core_size += strtab_size;



	/* Put string table section at end of init part of module. */

	strsect->sh_flags |= SHF_ALLOC;

	strsect->sh_entsize = get_offset(mod, &mod->init_size, strsect,

					 info->index.str) | INIT_OFFSET_MASK;

	DEBUGP("\t%s\n", info->secstrings + strsect->sh_name);



	/* Append room for core symbols' strings at end of core part. */

	info->stroffs = mod->core_size;



	/* First strtab byte (and first symtab entry) are zeroes. */

	__set_bit(0, info->strmap);

	mod->core_size += bitmap_weight(info->strmap, strsect->sh_size);

}



static void add_kallsyms(struct module *mod, const struct load_info *info)
@@ -2246,22 +2237,19 @@ static void add_kallsyms(struct module *mod, const struct load_info *info) 
		mod->symtab[i].st_info = elf_type(&mod->symtab[i], info);



	mod->core_symtab = dst = mod->module_core + info->symoffs;

	mod->core_strtab = s = mod->module_core + info->stroffs;

	src = mod->symtab;

	*dst = *src;

	*s++ = 0;

	for (ndst = i = 1; i < mod->num_symtab; ++i, ++src) {

		if (!is_core_symbol(src, info->sechdrs, info->hdr->e_shnum))

			continue;



		dst[ndst] = *src;

		dst[ndst].st_name = bitmap_weight(info->strmap,

						  dst[ndst].st_name);

		++ndst;

		dst[ndst++].st_name = s - mod->core_strtab;

		s += strlcpy(s, &mod->strtab[src->st_name], KSYM_NAME_LEN) + 1;

	}

	mod->core_num_syms = ndst;



	mod->core_strtab = s = mod->module_core + info->stroffs;

	for (*s = 0, i = 1; i < info->sechdrs[info->index.str].sh_size; ++i)

		if (test_bit(i, info->strmap))

			*++s = mod->strtab[i];

}

#else

static inline void layout_symtab(struct module *mod, struct load_info *info)
@@ -2751,27 +2739,18 @@ static struct module *layout_and_allocate(struct load_info *info) 
	   this is done generically; there doesn't appear to be any

	   special cases for the architectures. */

	layout_sections(mod, info);



	info->strmap = kzalloc(BITS_TO_LONGS(info->sechdrs[info->index.str].sh_size)

			 * sizeof(long), GFP_KERNEL);

	if (!info->strmap) {

		err = -ENOMEM;

		goto free_percpu;

	}

	layout_symtab(mod, info);



	/* Allocate and move to the final place */

	err = move_module(mod, info);

	if (err)

		goto free_strmap;

		goto free_percpu;



	/* Module has been copied to its final place now: return it. */

	mod = (void *)info->sechdrs[info->index.mod].sh_addr;

	kmemleak_load_module(mod, info);

	return mod;



free_strmap:

	kfree(info->strmap);

free_percpu:

	percpu_modfree(mod);

out:
@@ -2781,7 +2760,6 @@ out: 
/* mod is no longer valid after this! */

static void module_deallocate(struct module *mod, struct load_info *info)

{

	kfree(info->strmap);

	percpu_modfree(mod);

	module_free(mod, mod->module_init);

	module_free(mod, mod->module_core);
@@ -2911,8 +2889,7 @@ static struct module *load_module(void __user *umod, 
	if (err < 0)

		goto unlink;



	/* Get rid of temporary copy and strmap. */

	kfree(info.strmap);

	/* Get rid of temporary copy. */

	free_copy(&info);



	/* Done! */

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