Similar to CONFIG_GENERIC_BUG_RELATIVE_POINTERS that replaces (8 byte)
const char* members by (4 byte) signed offsets from the bug_entry,
this implements the similar thing for struct _ddebug, the descriptors
underlying pr_debug() and friends in a CONFIG_DYNAMIC_DEBUG kernel.
Since struct _ddebug has four string members, we save 16 byte per
instance. The total savings seem to be comparable to what is saved by
an architecture selecting GENERIC_BUG_RELATIVE_POINTERS (see patch 8
for some numbers for a common distro config).
While refreshing these patches, which were orignally just targeted at
x86-64, it occured to me that despite the implementation relying on
inline asm, there's nothing x86 specific about it, and indeed it seems
to work out-of-the-box for ppc64 and arm64 as well, but those have
only been compile-tested.
The first 6 patches are rather pedestrian preparations. The fun stuff
is in patch 7, and the last patch is just the minimal boilerplate to
hook up the asm-generic header and selecting
HAVE_DYNAMIC_DEBUG_RELATIVE_POINTERS on x86-64. Doing arm64 and ppc64
would be similar 2-line patches.
Unfortunately, the "fun stuff" includes some magic that breaks the
build in some corner cases with older gcc (and some not-so-magic that
clang only supports since version 9 for non-x86 targets). So in this
v6, the config option DYNAMIC_DEBUG_RELATIVE_POINTERS has been made
opt-in, depending on the arch-selected
HAVE_DYNAMIC_DEBUG_RELATIVE_POINTERS, instead of the arch selecting it
directly. Also, I've set a lower bound for the gcc and clang versions,
so that hopefully nobody should be able to select the option and have
the build break.
Rasmus Villemoes (8):
linux/device.h: use unique identifier for each struct _ddebug
linux/net.h: use unique identifier for each struct _ddebug
linux/printk.h: use unique identifier for each struct _ddebug
dynamic_debug: introduce accessors for string members of struct
_ddebug
dynamic_debug: drop use of bitfields in struct _ddebug
dynamic_debug: introduce CONFIG_DYNAMIC_DEBUG_RELATIVE_POINTERS
dynamic_debug: add asm-generic implementation for
DYNAMIC_DEBUG_RELATIVE_POINTERS
x86-64: select HAVE_DYNAMIC_DEBUG_RELATIVE_POINTERS
arch/x86/Kconfig | 1 +
arch/x86/entry/vdso/vdso32/vclock_gettime.c | 1 +
arch/x86/include/asm/Kbuild | 1 +
include/asm-generic/dynamic_debug.h | 116 ++++++++++++++++++++
include/linux/device.h | 4 +-
include/linux/dynamic_debug.h | 26 +++--
include/linux/jump_label.h | 2 +
include/linux/net.h | 4 +-
include/linux/printk.h | 4 +-
lib/Kconfig.debug | 16 +++
lib/dynamic_debug.c | 111 ++++++++++++++-----
11 files changed, 246 insertions(+), 40 deletions(-)
create mode 100644 include/asm-generic/dynamic_debug.h
--
2.20.1
