Re: [PATCH V2 2/2] module: Fix performance regression on modules with large symbol tables

Rusty Russell
2011-11-22T00:59:59Z

On Sun, 20 Nov 2011 22:29:55 -0800, Kevin Cernekee <cernekee@gmail.com> wrote:
> On Sun, Nov 20, 2011 at 9:06 PM, Rusty Russell <rusty@rustcorp.com.au> wrote:
> > On Thu, 17 Nov 2011 19:15:02 -0800, Kevin Cernekee <cernekee@gmail.com> wrote:
> > I'm deeply tempted.  It's very simple, 46 lines shorter, preserves exact
> > matches, and doesn't have any strange slowdowns on corner cases.
>
> Unfortunately, the last patch I posted still makes duplicate copies of
> the exact matches. And the copy of nvidia.ko I'm looking at right now
> does reuse strtab entries for the duplicated symbols.

Ah, I read it too fast.

Still, simplicity wins. I like Jan's point, I'll put in a comment and
reference to this thread for future adventurers. Result below.

Thanks!
Rusty.

From: Kevin Cernekee <cernekee@gmail.com>
Subject: 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: Kevin Cernekee <cernekee@gmail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
LKML-Reference: <73defb5e4bca04a6431392cc341112b1@localhost>
---
kernel/module.c | 65 ++++++++++++++++++--------------------------------------
1 file changed, 21 insertions(+), 44 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -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
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
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->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_allocat
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 _
if (err < 0)
goto unlink;

- /* Get rid of temporary copy and strmap. */
- kfree(info.strmap);
+ /* Get rid of temporary copy. */
free_copy(&info);

/* Done! */