The drm subsystem has *lots* of debug statements, in 11 categories:
$> ack '\w*_dbg' drivers/gpu/drm/ | wc
5532 29318 553806
$> ack 'DRM_DEBUG\w*' drivers/gpu/drm/ | wc
2208 12856 212035
All of these are bit-tests on __drm_debug, exposed to users as
/sys/module/drm/parameters/debug. Many of these are done often;
vblank is done ~100/sec for some displays. Over the uptime of many
boxes, this is a lot of cpu cycles on bits that are almost always off.
Dynamic-debug excels at replacing such tests with NOOPs (via static
keys). Classmaps was devised to bring that 0-off-cost to drm's
categories.
Classmaps-v1 went into the kernel in Sept 2022, in 2 chunks:
b7b4eebdba7b..6ea3bf466ac6 # core dyndbg changes
0406faf25fb1..ee7d633f2dfb # drm adoption
Sadly DRM-CI found a regression during init with drm.debug=<initval>;
the static-keys underneath the drm-dbgs in drm.ko got enabled, but
those in drivers & helpers did not.
So in Feb 2023, it got pulled:
commit bb2ff6c27bc9 ("drm: Disable dynamic debug as broken")
Root Problem:
DECLARE_DYNDBG_CLASSMAP defined the classmap, but its repeated use in
both core and drivers violated a K&R rule "define once, refer
afterwards". This flaw resulted in a regression; with drm.debug=0xFF
boot arg, drm-core got enabled, but drivers/helpers did not.
This patchset replaces DECLARE_DYNDBG_CLASSMAP with:
- DYNAMIC_DEBUG_CLASSMAP_DEFINE (invoked once in the exporting module)
- DYNAMIC_DEBUG_CLASSMAP_USE (invoked repeatedly in drivers & helpers)
_DEFINE exports the classmap it creates (in drm.ko), and other modules
_USE the classmap. The _USE adds a record referencing the _DEFINEd (&
exported) classmap in a 2nd __dyndbg_class_users section.
At modprobe, dyndbg scans the new section after __dyndbg_class_maps,
follows the linkage to the _DEFINEr module, finds the (optional)
kernel-param controlling the classmap, examines its drm.debug=<initval>,
and applies it to the module being initialized.
To recapitulate the multi-module problem wo DRM involvement, we add:
- tools/testing/selftests/dynamic_debug/dyndbg_selftest.sh:
Alters pr_debugs in builtins and test modules,
checks results against checksums of expected results
- lib/test_dynamic_debug.c & test_dynamic_debug_submod.c:
Builds parent & _submod modules with _DEFINE and _USE inside #if/#else
blocks, reproducing the 2-module scenario under selftests.
Series Breakdown (38 Patches):
0. Subsystem separation: Drop DRM opt-in patches to route separately via
dri-devel / drm-misc, keeping core dyndbg infrastructure focused for
-mm.
1. Selftest up front: Introduce dyndbg_selftest.sh early (commit 2) to
verify syntax and query baseline; subsequent testpoints are phased in
alongside the exact features they test. Strict mode (K=0) validates
exact golden output, while "I know" mode (K=1) tracks drift.
2. Linker script 32-bit fixes & cleanup: Refactor BOUNDED_SECTION* into
include/asm-generic/bounded_sections.lds.h. Add dyndbg sections to
scripts/module.lds.S to prevent lost sections in loadable modules.
Fix 8-byte alignment omission causing i386 NULL pointer derefs.
3. Built-in module naming hook: Introduce DDEBUG_MODNAME in
include/linux/dynamic_debug.h, defaulting cleanly to KBUILD_MODNAME.
Bind callsites and classmaps through this hook without altering
dynamic_debug/control output or userspace ABI.
4. Validation hardening & API: Add compile-time validation for classmap
parameters and offset bounds (__DYNAMIC_DEBUG_CLASSMAP_CHECK). Harden
modprobe error detection for unknown classes and class ID reservation
conflicts. Promote DYNAMIC_DEBUG_CLASSMAP_PARAM to API. Reduce class
parameter storage to u32.
5. Parser & grammar extensions: Treat comma as a token separator. Add
multi-query splitting via '@'. Bump max tokens per command from 9 to
15. Drop class "protection" special-casing per maintainer feedback.
What's Unchanged:
0. /proc/dynamic_debug/control format and output: Module names remain
strictly identical to KBUILD_MODNAME; no userspace breakage.
1. struct _ddebug footprint: Callsites maintain zero pointer overhead;
no per-callsite module pointers or classmap references are added.
2. Query grammar backwards compatibility: Existing queries using
module, file, format, line, and flags continue to operate unchanged.
Testing:
- Tested locally using virtme-ng on x86_64 across ~10 configs.
- dyndbg_selftest.sh passes all checksum tests through the series.
Signed-off-by: Jim Cromie <[email protected]>
---
Changes in v10:
- Builtin module naming across revisions:
Recognized that v8, v9 were misadventures: both altered modname in
dynamic_debug/control to avoid a hypothetical collision that was not
going to happen (and could be addressed if it ever did). While
improved built-in module names (non-fragmented names amongst them)
might have merit someday, that is not for now; existing ambiguity is
better than new novel bugs.
. v8 used KBUILD_MODFILE to disambiguate built-ins sharing
KBUILD_MODNAME="main" (init/main.c, drivers/.../main.c).
. sashiko review noted the hyphen/underscore mismatch problem across
module names.
. v9 tried further changes to "main" via KBUILD_DD_MODNAME in
scripts/Makefile.lib to synthesize subsystem prefixes (e.g. [init],
[power]), mutating column 2 in dynamic_debug/control.
. Dropped KBUILD_DD_MODNAME from scripts/Makefile.lib and dropped
hyphen-underscore matching (match_wildcard_hyphen).
. Introduced DDEBUG_MODNAME defaulting cleanly to KBUILD_MODNAME as
a lightweight hook preserving dynamic_debug/control invariants.
- Reorganize dyndbg_selftest.sh so tests are phased in alongside the
features they verify rather than defined in advance.
- Address review findings from sashiko-9 across core dyndbg:
. Elevate NULL dp->format check to top of ddebug_match_desc() to guard
format-less queries.
. Add const qualifier to struct ddebug_class_user.mod_name.
. Pass init_bits computed from _DPRINTK_CLASSBITS_INIT in
ddebug_sync_classbits() to properly disable -DDEBUG callsites.
. Define _DPRINTK_CLASSBITS_INIT, initialize static bitvectors, and
add module_param_named fallback for DYNAMIC_DEBUG_CLASSMAP_PARAM*
under !CONFIG_DYNAMIC_DEBUG.
. Add "V8" and Vu8 to classmaps and do_levels().
. Replace strpbrk() with quote-aware ddebug_find_delimiter() to ignore
'@' inside quotes.
. Drop dead #if 0 ddebug_apply_class_maps() block.
. Fix pr_warn_ratelimited() line wrap in ddebug_match_desc().
. Pass raw fmt to DEFINE_DYNAMIC_DEBUG_METADATA in
pr_debug_ratelimited() to avoid mangled control file strings.
. Fix ddebug_proc_start() epilogue token boundary check (n <= 1).
. Handle "reset_stats" inside ddebug_exec_queries() to preserve
batched multi-queries.
. Add __DYNAMIC_DEBUG_BRANCH() checking _ENABLED while
DYNAMIC_DEBUG_BRANCH() checks _ACTIVE; add early return guards in
__dynamic_*_dbg() helpers.
- Link to v9:
https://lore.kernel.org/r/[email protected]
Changes in v9:
- Decouple DRM core setup and driver opt-in commits to route separately
through dri-devel / drm-misc, keeping this foundation series 100%
focused on core dynamic_debug infrastructure for -mm.
- Incorporate review findings and automated test fixes prompted by
sashiko:
. Rework Kbuild naming heuristic in scripts/Makefile.lib to assign
clean subsystem-scoped names to built-ins and shared helpers
(e.g. [init], [power], [coco/sev], [mmc/host]), eliminating
multi-token whitespace in column 2 of dynamic_debug/control.
. Fix missing DYNAMIC_DEBUG_CLASSMAP_USE_() stub under
!CONFIG_DYNAMIC_DEBUG.
. Make ddebug_add_module() non-fatal on failure during
dynamic_debug_init().
. Fix off-by-one assertions in __DYNAMIC_DEBUG_CLASSMAP_CHECK and
ddebug_add_module().
. Prevent loop wrapping lockup in test_dynamic_debug:do_bulk() on
UINT_MAX and add cond_resched() to bulk print loops.
. Fix 4-argument signature on DYNAMIC_DEBUG_CLASSMAP_PARAM_REF
fallback macro stub.
. Assign map->controlling_param to enforce classmap protection on
parameterized classes.
. Update ddebug_proc_start() seeking to return EPILOGUE_TOKEN
when n == 0.
. Free temporary buffer in reset_stats handler and iterate across
possible CPUs for dynamic debug call counter.
. Selftests: sanitize "$K", drop duplicate declarations, fix
unquoted error output, align LACK_DD_BUILTIN filters to labels,
add K=2 silent mode, and prime params with +p in
FT_comma_terminators.
- Link to v8:
https://lore.kernel.org/r/[email protected]
Changes in v8:
- Unified 44-patch series (incorporating follow-on compile-time checks,
comma-token delimiters, '@' multi-query separator, and inheritance
tests).
- Rebased onto upstream v7.3-rc1.
- Passing on dyndbg_selftest.sh under KASAN + KMEMLEAK.
- Link to v7:
https://lore.kernel.org/r/[email protected]
Changes in v6..v7:
- Add compile-time validation for classmap parameters and offset
bounds (__DYNAMIC_DEBUG_CLASSMAP_CHECK).
- Harden modprobe-time error detection for unknown class names and
class ID reservation conflicts.
- Promote DYNAMIC_DEBUG_CLASSMAP_PARAM to public API.
- Shrink class parameter storage to u32.
- Hoist classmap filtering up to ddebug_add_module().
Changes in v4..v5:
- Tighten function signatures (ddebug_apply_class_bitmap,
param_set_dyndbg_classes).
- Replace classmap linked-list with vector / array-slice.
- Add for_subvec() loop helper and restructure _ddebug_info substructs.
- Move mod_name down from struct ddebug_table to _ddebug_info.
Changes in v2..v3:
- Refactor BOUNDED_SECTION* macros from
include/asm-generic/vmlinux.lds.h into
include/asm-generic/bounded_sections.lds.h.
- Add dyndbg output sections to scripts/module.lds.S to fix lost
sections in loadable modules.
- Fix 8-byte section alignment omission causing i386 NULL pointer deref.
- Move dyndbg_selftest.sh to the front of the series for bisectability.
Changes in v12 (DRM combined series):
- Refactor vmlinux.lds.h and add bounded_sections.lds.h and
dyndbg.lds.h.
- Refine DYNAMIC_DEBUG_CLASSMAP_USE*() with extern'd classmap linkages
and compile-time offset checks.
- Include patch from Philipp Hahn <[email protected]> (Ignore additional
arguments from pr_fmt).
- Link to v12:
https://lore.kernel.org/lkml/[email protected]/
Changes in v11 (DRM combined series):
- Rebase on drm-misc-next for DRM-CI testing.
- Fix 32-bit truncation error in drm_buddy.
- Fix drm_printer_debug_fn message spew causing test timeouts.
- Verify on DRM-CI Pipeline #1622778 (621 KUnit tests, 370 i915-CML
tests passed, 0 failures).
- Link to v11:
https://lore.kernel.org/lkml/[email protected]/
Changes in v10 (DRM combined series):
- Initial replacement of DECLARE_DYNDBG_CLASSMAP with
DYNAMIC_DEBUG_CLASSMAP_DEFINE and DYNAMIC_DEBUG_CLASSMAP_USE.
- Add tools/testing/selftests/dynamic_debug/ and
test_dynamic_debug_submod.ko.
- Drop class "protection" special-casing per Jason Baron's feedback.
- Link to v10:
https://lore.kernel.org/lkml/[email protected]/
---
Jim Cromie (37):
selftests/dyndbg: Add kselftest script to verify dynamic-debug
vmlinux.lds.h: refactor BOUNDED_SECTION_* macros into
bounded_sections.lds.h
vmlinux.lds.h: drop unused HEADERED_SECTION* macros
vmlinux.lds.h: Fix ALIGN(8) omission causing NULL ptr on i386
vmlinux.lds.h: remove redundant ALIGN(8) directives
dyndbg.lds.S: fix lost dyndbg sections in modules
dyndbg: factor ddebug_match_desc out from ddebug_change
dyndbg: add stub macro for DECLARE_DYNDBG_CLASSMAP
dyndbg: reword "class unknown," to "class:_UNKNOWN_"
dyndbg-API: remove DD_CLASS_TYPE_(DISJOINT|LEVEL)_NAMES and code
dyndbg: drop NUM_TYPE_ARGS
dyndbg: bump num-tokens in a query-cmd from 9 to 15
dyndbg: reduce verbose/debug clutter
dyndbg: Bind callsites and classmaps to DDEBUG_MODNAME
dyndbg: refactor param_set_dyndbg_classes and below
dyndbg: tighten fn-sig of ddebug_apply_class_bitmap
dyndbg: replace classmap list with an array-slice
dyndbg: macrofy a 2-index for-loop pattern
dyndbg: reduce class param storage to u32
dyndbg,module: make proper substructs in _ddebug_info
dyndbg: move mod_name down from struct ddebug_table to _ddebug_info
dyndbg: hoist classmap-filter-by-modname up to ddebug_add_module
dyndbg-API: replace DECLARE_DYNDBG_CLASSMAP
selftests/dyndbg: Enable FT_classmap_inheritance
dyndbg: detect class_id reservation conflicts
dyndbg: check DYNAMIC_DEBUG_CLASSMAP_{DEFINE,USE_} args at compile-time
dyndbg-test: add do_bulk testpoint, rename do_prints to do_classes
dyndbg-API: promote DYNAMIC_DEBUG_CLASSMAP_PARAM to API
dyndbg: control-parser: treat comma as a token separator
selftests: enable comma-terminator tests
dyndbg: split multi-query strings with @
dyndbg: resolve "protection" of class'd pr_debug
dyndbg: harden classmap and descriptor validation
docs/dyndbg: add classmap info to howto
dyndbg: add epilogue to dynamic_debug/control file
dyndbg: add +c flag to count advantage of classmaps for DRM
dyndbg: add DEBUG-biased fallback stubs for _dynamic_func_call_cls
Philipp Hahn (1):
dyndbg: Ignore additional arguments from pr_fmt
Documentation/admin-guide/dynamic-debug-howto.rst | 157 +++-
MAINTAINERS | 2 +
drivers/gpu/drm/drm_print.c | 4 +-
include/asm-generic/bounded_sections.lds.h | 32 +
include/asm-generic/dyndbg.lds.h | 22 +
include/asm-generic/vmlinux.lds.h | 68 +-
include/drm/drm_print.h | 2 +-
include/linux/dynamic_debug.h | 397 +++++++--
include/linux/printk.h | 2 +-
kernel/module/main.c | 15 +-
lib/Kconfig.debug | 24 +-
lib/Makefile | 3 +
lib/dynamic_debug.c | 971 +++++++++++++++------
lib/test_dynamic_debug.c | 277 ++++--
lib/test_dynamic_debug_submod.c | 21 +
scripts/module.lds.S | 2 +
tools/testing/selftests/dynamic_debug/Makefile | 10 +
tools/testing/selftests/dynamic_debug/config | 8 +
.../selftests/dynamic_debug/dyndbg_selftest.sh | 735 ++++++++++++++++
.../dynamic_debug/syslog_hash_validation.sh | 393 +++++++++
20 files changed, 2620 insertions(+), 525 deletions(-)
---
base-commit: 136ebbeb1c6040f2739ac4a9e0f704395faaf64f
change-id: 20260901-dd-cmap-part2-clean-369ec194e4af
Best regards,
--
Jim Cromie <[email protected]>
