d/freebsd-skq
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

Speed up symbol lookup for the amd64 kernel modules.

Browse Source 
Amd64 uses relocatable object files as the modules format.  It is good
WRT not having unneeded overhead for PIC code, in particular, due to
absence of useless GOT and PLT.  But the cost is that the module
linking process cannot use hash to speed up the symbol lookup, and
that each reference to the symbol requiring a relocation, instead of
single-place relocation in GOT.

Cache the successfull symbol lookup results in the module symbol
table, using the newly allocated SHN_FBSD_CACHED value from
SHN_LOOS-HIOS range as an indicator.  The SHN_FBSD_CACHED together
with the non-existent definition of the found symbol are reverted
after successfull relocations, which is done under kld_sx lock, so it
should not be visible to other consumers of the symbol table.

Submitted by:	Conrad Meyer
Differential Revision:  https://reviews.freebsd.org/D1718
MFC after:	3 weeks
This commit is contained in:
Konstantin Belousov 2015-04-02 20:14:51 +00:00
parent bf64b1d71c
commit 2832cd544f
2 changed files with 43 additions and 2 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

42
sys/kern/link_elf_obj.c
View File

@ -173,6 +173,7 @@ static struct linker_class link_elf_class = {
};
static int relocate_file(elf_file_t ef);
static void elf_obj_cleanup_globals_cache(elf_file_t);
static void
link_elf_error(const char *filename, const char *s)
@ -1075,6 +1076,13 @@ relocate_file(elf_file_t ef)
}
}
/*
* Only clean SHN_FBSD_CACHED for successfull return. If we
* modified symbol table for the object but found an
* unresolved symbol, there is no reason to roll back.
*/
elf_obj_cleanup_globals_cache(ef);
return 0;
}
@ -1223,6 +1231,21 @@ link_elf_each_function_nameval(linker_file_t file,
return (0);
}
static void
elf_obj_cleanup_globals_cache(elf_file_t ef)
{
Elf_Sym *sym;
Elf_Size i;
for (i = 0; i < ef->ddbsymcnt; i++) {
sym = ef->ddbsymtab + i;
if (sym->st_shndx == SHN_FBSD_CACHED) {
sym->st_shndx = SHN_UNDEF;
sym->st_value = 0;
}
}
}
/*
* Symbol lookup function that can be used when the symbol index is known (ie
* in relocations). It uses the symbol index instead of doing a fully fledged
@ -1234,7 +1257,7 @@ static Elf_Addr
elf_obj_lookup(linker_file_t lf, Elf_Size symidx, int deps)
{
elf_file_t ef = (elf_file_t)lf;
const Elf_Sym *sym;
Elf_Sym *sym;
const char *symbol;
Elf_Addr ret;
@ -1262,7 +1285,22 @@ elf_obj_lookup(linker_file_t lf, Elf_Size symidx, int deps)
if (*symbol == 0)
return (0);
ret = ((Elf_Addr)linker_file_lookup_symbol(lf, symbol, deps));
return ret;
/*
* Cache global lookups during module relocation. The failure
* case is particularly expensive for callers, who must scan
* through the entire globals table doing strcmp(). Cache to
* avoid doing such work repeatedly.
*
* After relocation is complete, undefined globals will be
* restored to SHN_UNDEF in elf_obj_cleanup_globals_cache(),
* above.
*/
if (ret != 0) {
sym->st_shndx = SHN_FBSD_CACHED;
sym->st_value = ret;
}
return (ret);
case STB_WEAK:
printf("link_elf_obj: Weak symbols not supported\n");

3
sys/sys/elf_common.h
View File

@ -363,6 +363,9 @@ typedef struct {
#define SHN_LOPROC 0xff00 /* First processor-specific. */
#define SHN_HIPROC 0xff1f /* Last processor-specific. */
#define SHN_LOOS 0xff20 /* First operating system-specific. */
#define SHN_FBSD_CACHED SHN_LOOS /* Transient, for sys/kern/link_elf_obj
linker only: Cached global in local
symtab. */
#define SHN_HIOS 0xff3f /* Last operating system-specific. */
#define SHN_ABS 0xfff1 /* Absolute values. */
#define SHN_COMMON 0xfff2 /* Common data. */