DYNAMIC_DEBUG has extra weight/memory in its data table/section; its
(module,filename,function) columns have ordered, repeating data
(90%,55%,45%), and is highly compressible (~110/300) with simple RLE.
On my i7 laptop, the compressible chunk is ~70kb, This patchset
prepares for compression, but doesn't try it; RFC. Using that RLE
compression estimate, we could save ~45kb; Id hope for better with ZSTD.
Series Overview:
1st, split the struct _ddebug & __dyndbg elf section into 2, moving
compressible fields into their own section/array/block. A temporary
site pointer connects the 2 halves of the composite callsite, it will
be obsoleted later, after we verify our replacement against it, we can
drop the .site pointer.
table change:
from: [ key,mod,func,filenm,fmt,ln,flg ]
into: [ key,fmt,ln,flg,site-> ] [ mod,func,filenm ]
2nd, make the 'site' data optional. The "module:function:line"
log-message prefix is fundamentally an optional decoration, if we can
do without it, we simplify codepath, and we could drop the storage in
those cases, possibly reducing memory footprint.
into: [ key,fmt,ln,flg,site->? ] 0
Then things get more interesting:
3rd, make kernel/module handle "__dyndbg_sites" ELF section (which was
added in 1) as is already done for "__dyndbg".
Hide/replace ->site with site_get/_put api
this will let us retrieve site info from compressed storage.
Add _index.
gives us the fixed offset from __dyndbg[N] to __dyndbg[0].
into: [ key,fmt,ln,flg,_index,site->? ] 0
Add table/header record-pair.
These 2 records are created into ".gnu.linkonce.dyndbg*" sections.
The linker script places these 2 sections in front of their respective
"__dyndbg*" sections.
So this header-pair becomes __dyndbg[0], __dyndbg_sites[0].
we can use __dyndbg[0]->site[N] to get the site info.
module.lds.h - insert ".gnu.linkonce.dyndbg*" sections into "__dyndbg*".
Without this, headers arent part of the sections that kernel/module loads.
validate ddebug_site_get() invariants.
prove that our indirection to ->site[N] is correct.
BUG_ON uses survived 0-day tests.
they go away with !SITE_CHK
into:
0: [ ...,_index,site-> ] ---> 0:[ ... ]
^
..N: [ key,fmt,ln,flg,_index ] N:[ ... ]
specialize header from _ddebug, keep header.site, drop _ddebug.site
RFC: needs refinement, better type choices?
Status:
recent revs passed 0day compile grind, including BUG_ONs
I got one KASAN report, quick reproduction failed, none more recently
memory footprint is same as master.
Whats Next ? Compression: RFC !!!
POC: could compress during __init, decompress on-demand. Doing this
would give something to decompress, to build out that code.
ISTM Compression during build is best, but targets are unclear:
initrd, initramfs, cpio, FW-blob all seem suitable at some level, but
so does (ro) zram & zswap. How should I think about this ?
Also, I understand that support exists for compressed elf sections, I
dunno if/how kernel uses it currently, which would considerably ease
this. Builtin __dyndbg_site data is in .data, probably needs to be
moved to a separate elf section to get generic tool support for
compressed sections.
It would be nice if a compressed "__dyndbg_sites" elf section could be
loaded by unchanged kernel/module.c. A magic number in the attached
memory block could distinguish it from the current elf block. There
is room in the 2 header records for new private data; we could store
decompression contexts there while iterating thru a block.
Decompression - a few use-cases:
= for builtins, each loadable module:
1- stream, for `cat control` etc.
these are 1 visit and forget
2- `module foo +mfp > control` - enable contiguous blocks w decorations
3- pr_debug - semi-random access - by (ptr - _index)
[0-5k] for builtin pr-debugs, [0-20] for most loadable modules
_index is 16 bit currently
Simplest, minimum-memory strategy is late-fetch; to wait until a
pr_debug is actually called, then check flags before fetching the
site-rec, and save it to a hash on modname+_index (and delete on -p).
Without the save, it works for (1) too.
The only downside with late-fetch is that during pr_debug, a 1st fetch
of site-info maybe cant come from io/zswap without locking/latency
concerns. A forced-prefetch ala (2) with a new flag '!' could work
around this, but might look like a wart.
So thats it. I hope its clear and concise.
Jim Cromie (28):
trivial cleanup:
dyndbg: avoid calling dyndbg_emit_prefix when it has no work
dyndbg: drop uninformative vpr_info
split to 2 vectors of callsite:
dyndbg: split struct _ddebug's display fields to new _ddebug_site
dyndbg: __init iterate over __dyndbg & __dyndbg_site in parallel
make site info optional:
dyndbg: refactor part of ddebug_change to ddebug_match_site
dyndbg: accept null site in ddebug_match_site
dyndbg: hoist ->site out of ddebug_match_site
dyndbg: accept null site in ddebug_change
dyndbg: accept null site in dynamic_emit_prefix
dyndbg: accept null site in ddebug_proc_show
dyndbg: refactor ddebug_alter_site out of ddebug_change
dyndbg: allow deleting site info via control interface
interesting:
dyndbg+module: expose ddebug_sites to modules
dyndbg: add ddebug_site(_get|_put) abstraction
dyndbg: ddebug_add_module avoid adding empty modules
dyndbg: add _index to struct _ddebug
dyndbg: prevent build bugs via -DNO_DYNAMIC_DEBUG_TABLE
dyndbg: RFC - DEFINE_DYNAMIC_DEBUG_TABLE
dyndbg: RFC handle __dyndbg* sections in module.lds.h
dyndbg: ddebug_add_module() handle headers.
dyndbg: validate ddebug_site_get invariants
dyndbg: fix NULL deref after deleting sites
dyndbg: dont show header records in control
dyndbg: make site pointer and checks on it optional (not quite)
dyndbg: swap WARN_ON for BUG_ON see what 0-day says
dyndbg: fixup protect header when deleting site
dyndbg: unionize _ddebug*_headers with struct _ddebug*
dyndbg: drop _ddebug.site pointer
arch/arm/boot/compressed/Makefile | 2 +
arch/sparc/vdso/Makefile | 2 +
arch/x86/boot/compressed/Makefile | 1 +
arch/x86/entry/vdso/Makefile | 3 +
arch/x86/purgatory/Makefile | 1 +
drivers/firmware/efi/libstub/Makefile | 3 +-
include/asm-generic/module.lds.h | 12 +-
include/asm-generic/vmlinux.lds.h | 24 +-
include/linux/dynamic_debug.h | 135 ++++++++--
kernel/module-internal.h | 1 +
kernel/module.c | 9 +-
lib/dynamic_debug.c | 363 +++++++++++++++++++-------
12 files changed, 437 insertions(+), 119 deletions(-)
--
2.31.1
