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libdrgn: linux_kernel: optimize reading module list

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An upcoming fix requires us to always use the module list from the core
dump rather than /proc/modules. However, with the existing code, this
would cause a major startup time regression for the live kernel, mainly
because reading from /proc/kcore is stupidly slow. We currently do 3 +
strlen(module->name) reads for every module. We can reduce this to 1
read per module by reading the entire struct module at once. The size of
struct module is ~700-900 bytes depending on the kernel configuration,
which is still much faster to read than only reading what we need.

In some benchmarks that I did with DRGN_USE_PROC_AND_SYS_MODULES=0, this
reduced the time spent in the kernel module iterator from ~2.5ms per
module to ~0.4ms per module.

Signed-off-by: Omar Sandoval <osandov@osandov.com>
This commit is contained in:
Omar Sandoval 2022-09-12 11:07:20 -07:00
parent a2db11ebae
commit 94036f6daf
1 changed files with 39 additions and 24 deletions
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63
libdrgn/linux_kernel.c
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@ -351,8 +351,15 @@ struct kernel_module_iterator {
size_t name_capacity;
/* If not using /proc/modules. */
struct {
/* `struct module` type. */
struct drgn_qualified_type module_type;
struct drgn_object mod, node, tmp1, tmp2, tmp3;
/* Current `struct module` (not a pointer). */
struct drgn_object mod;
/* `struct list_head *` in next module to return. */
struct drgn_object node;
/* Temporary objects reused for various purposes. */
struct drgn_object tmp1, tmp2, tmp3;
/* Address of `struct list_head modules`. */
uint64_t head;
};
};
@ -408,15 +415,18 @@ kernel_module_iterator_init(struct kernel_module_iterator *it,
&it->node);
if (err)
goto err;
err = drgn_object_address_of(&it->node, &it->node);
if (it->node.kind != DRGN_OBJECT_REFERENCE) {
err = drgn_error_create(DRGN_ERROR_OTHER,
"can't get address of modules list");
goto err;
}
it->head = it->node.address;
err = drgn_object_member(&it->node, &it->node, "next");
if (err)
goto err;
err = drgn_object_read(&it->node, &it->node);
if (err)
goto err;
err = drgn_object_read_unsigned(&it->node, &it->head);
if (err)
goto err;
}
return NULL;
@ -471,12 +481,6 @@ kernel_module_iterator_next(struct kernel_module_iterator *it)
struct drgn_error *err;
err = drgn_object_member_dereference(&it->node, &it->node, "next");
if (err)
return err;
err = drgn_object_read(&it->node, &it->node);
if (err)
return err;
uint64_t addr;
err = drgn_object_read_unsigned(&it->node, &addr);
if (err)
@ -488,10 +492,25 @@ kernel_module_iterator_next(struct kernel_module_iterator *it)
"list");
if (err)
return err;
err = drgn_object_dereference(&it->mod, &it->mod);
if (err)
return err;
// We need several fields from the `struct module`. Especially for
// /proc/kcore, it is faster to read the entire structure (which is <1kB
// as of Linux 6.0) from the core dump all at once than it is to read
// each field individually.
err = drgn_object_read(&it->mod, &it->mod);
if (err)
return err;
err = drgn_object_member(&it->node, &it->mod, "list");
if (err)
return err;
err = drgn_object_member(&it->node, &it->node, "next");
if (err)
return err;
// Set tmp1 to the module base address and tmp2 to the size.
err = drgn_object_member_dereference(&it->tmp1, &it->mod,
"core_layout");
err = drgn_object_member(&it->tmp1, &it->mod, "core_layout");
if (!err) {
// Since Linux kernel commit 7523e4dc5057 ("module: use a
// structure to encapsulate layout.") (in v4.5), the base and
@ -507,12 +526,10 @@ kernel_module_iterator_next(struct kernel_module_iterator *it)
// Before that, they are directly in the `struct module`.
drgn_error_destroy(err);
err = drgn_object_member_dereference(&it->tmp2, &it->mod,
"core_size");
err = drgn_object_member(&it->tmp2, &it->mod, "core_size");
if (err)
return err;
err = drgn_object_member_dereference(&it->tmp1, &it->mod,
"module_core");
err = drgn_object_member(&it->tmp1, &it->mod, "module_core");
if (err)
return err;
} else {
@ -526,7 +543,7 @@ kernel_module_iterator_next(struct kernel_module_iterator *it)
return err;
it->end += it->start;
err = drgn_object_member_dereference(&it->tmp2, &it->mod, "name");
err = drgn_object_member(&it->tmp2, &it->mod, "name");
if (err)
return err;
char *name;
@ -675,8 +692,7 @@ kernel_module_iterator_gnu_build_id(struct kernel_module_iterator *it,
struct drgn_error *err;
struct drgn_program *prog = drgn_object_program(&it->mod);
const bool bswap =
drgn_type_little_endian(it->mod.type) != HOST_LITTLE_ENDIAN;
const bool bswap = drgn_platform_bswap(&prog->platform);
struct drgn_object attrs, attr, tmp;
drgn_object_init(&attrs, prog);
@ -685,7 +701,7 @@ kernel_module_iterator_gnu_build_id(struct kernel_module_iterator *it,
// n = mod->notes_attrs->notes
uint64_t n;
err = drgn_object_member_dereference(&attrs, &it->mod, "notes_attrs");
err = drgn_object_member(&attrs, &it->mod, "notes_attrs");
if (err)
goto out;
err = drgn_object_member_dereference(&tmp, &attrs, "notes");
@ -790,9 +806,8 @@ kernel_module_section_iterator_init(struct kernel_module_section_iterator *it,
it->i = 0;
it->name = NULL;
/* it->nsections = mod->sect_attrs->nsections */
err = drgn_object_member_dereference(&kmod_it->tmp1,
&kmod_it->mod,
"sect_attrs");
err = drgn_object_member(&kmod_it->tmp1, &kmod_it->mod,
"sect_attrs");
if (err)
return err;
err = drgn_object_member_dereference(&kmod_it->tmp2,