[RFC PATCH v6 00/34] DYNAMIC_DEBUG diet progress, dropped 30kb

Jim Cromie
2021-05-29T20:00:46Z

DYNAMIC_DEBUG plants a vector/table of struct _ddebug's in kernel
data; its (module,filename,function) columns have ordered, repeating
data (90%,55%,45%), which is readily compressible.

This patchset restructures the table to isolate that redundant data,
then packs out the duplicates, achieving 40% reduction in the memory
needed for that data, about 30kb on my laptop. No de/compression used.

On my i7 laptop kernel:

dyndbg: 62 44 2899 1728 mptcp.check_fully_established.895 <== 1728/2899 compresion
dyndbg: 63 debug prints in module mptcp (in 44 functions)
dyndbg: 2900 prdebugs in 277 modules, \
12 KiB in ddebug tables, \
90 KiB in __dyndbg section, \
67 KiB in __dyndbg_sites section <== on this data

It should be practical to actually reclaim that memory; RFC on how
that could be done. On a hail-mary, I kreallocd sites on a module,
it panic'd.

To isolate the compressible part, we split the table/struct in 2. The
redundant columns (modname, filename, function) are moved to struct
_ddebug_site ("__dyndbg_sites" elf-section), and refd by a
new/temporary _ddebug.site pointer. The site data can then be made
optional (6 patches), managed separately, and "compressed".

The pointer adds 8/56 to the memory footprint, we want to replace it
with indexing; the _index only needs to span ~10k callsites, and comes
"free" from unused bits in struct _ddebug.

The difficulty with this, and the reason for the multi-commit
adaptation, is that we have only a struct _ddebug pointer.
We know its in __dyndbgs[], but we dont know &__dyndbg_sites[],
which we need to finally get __dyndbg_sites[_index].

We get it with an "up-over-and-down" maneuver;

1. __dyndbgs[._index] ==> __dyndbgs[0]
pretty trivial with ._index
2. __dyndbgs[0].site
indirectly use __dyndbgs[0].site instead of [._index].site
3. insert "header" record - just _ddebug, renamed
uses .gnu.linkonce to create headers, linker script to insert it - RFC
becomes __dyndbgs[0]
provides .site for [2]
4. __dyndbgs[0].site->__dyndbg_sites[_index]
trivial

With "up-over-and-down" working, we can get rid of .site pointer

Next is specialization and unionization:

We create a header union, containing a struct _ddebug to make it "fit"
in __dyndbgs[], and an anonymous struct with .site and a several other
struct _ddebug fields checked to validate headers.

This allows the header to keep its .site pointer, while struct _ddebug
drops it, shrinking memory usage 8/56 back to parity.

It is working, but Im certain it can be restructured more tightly.
In particular, [header,__dyndbgs[]] fits well in a flex-array.
My anonymous struct/union fu is insufficient, hints or examples
are solicited.

Finally comes "consolidation" (not compression). To do this, we
split/clone ._index to its two tasks (1,4 above); ._back (to n=0), then
._map (to soon to be repacked site). When adding each module, we scan
__dyndbg_sites[], repack it with its unique entries, and ._map it and
the function's remaining _ddebugs to use that unique entry in the
repacked vector.

Version-5 is here:
https://lore.kernel.org/lkml/CAJfuBxyjfopQMzMyFrrZK7YppsL8kh0VVWySrJDXeUB15uwFag@mail.gmail.com/#t

Version-6 differs from -v5 by:

It works so far..
it's been surviving 0day tests:

It compresses, can save 30kb of 70kb table, of 200kb total data overhead.

Ive resolved most of my previous QsRFCs, but Im using .gnu.linkonce
sections and linker script tweaks for the 1st time and figured it
needs more scrutiny than usual. Several of the commit messages say
"RFC" where I have most uncertainty.

The "most broken" part of the patchset is the specialization &
unionization, Im certain theres a tighter way to construct it. ISTM
theres a good chance that [header,__dyndbgs[]] fits well in a
flex-array, and container_of could help. Hints, tips, pointers to
examples are welcome.

Patch 34 - prototype pr_debug_once(), pr_debug_ratelimited()

= print-once.

$> echo function foo lineno 1234 +po > control

Doable with just a new flag, and a new "did_print" data bit, both
which will fit in struct _ddebug, no separate storage needed. Its
just one more flag check for dynamic_pr_debug, AFTER its +p enabled.

Unsetting flag and reseting it would clear the "did_print" bit.
Maybe just resetting "+o" too.

= Rate-Limiting

We don't need this often, so allocate RLs only when enabled & needed,
and store in hash on [ &builtin|&module, ._back|._map ]

$> echo module serio_raw +rp > control

By default, we could allocate a rate-limiting (RL) structure for every
pr-debug flagged, store them in a RLs-hash (on ._back + module), and
retrieve and evaluate them only when the pr-debug is enabled. We
could even defer RL alloc/attach until the pr-debug is actually
called.

If we also (optionally) hash RLs on (._map + module), then they are
shared across a function. This could be useful behavior, and would
save lots of memory where it is appropriate.

$> echo function foo +rg > control
- on function alone, RL-group capable
- flag modifier. use _map not _back.

Next ? More Dieting

_ddebug as 3 vectors of components.

We have split _ddebug in 2, maybe also split out static-keys to a 3rd
vector ? This would smoothe the path towards a single struct _ddebug
per function, which controls all N pr_debugs in that function.

We could recapitulate the repack, with some changes in details;
- we're packing the referring table, not the referred table.
ie: we'd condense __dyndbgs[], keep the vector of static-keys.
- we'd lose meaning of lineno, could reclaim bits for something else.

New API elements ?

. pr_debug_once() etal
. pr_debug_ratelimited() etal
. pr_debug_rl_grouped() etal ??

This is good for signaling programmer intent. A tweak to DD_METADATA
could set the corresponding bits at compile-time, causing its
provisioning when prdebug'd.

RL contol - Some speculation:

$> echo module serio_raw +r rate 10 burst 3 > control
- after required flags.
- only legal with +r
- suggests sharing across group (by default)
- should rates scale per callsite count ? or only by user ?

$> echo module serio_raw lineno 333 +r rate 100 burst 3 > control
- individual RLs are possible

Jim Cromie (33):

Jim Cromie (34):

3 trivial patches, 2 are in -rc3:
dyndbg: avoid calling dyndbg_emit_prefix when it has no work
dyndbg: drop uninformative vpr_info
dyndbg: display KiB of data memory used.

split struct _ddebug in 2:

dyndbg: split struct _ddebug's display fields to new _ddebug_site
dyndbg: __init iterate over __dyndbg & __dyndbg_site in parallel
dyndbg+module: expose ddebug_sites to modules

make site info optional:
Even though !.site is impossible, by linker init + DD_METADATA,
lets allow it, to shorten codepaths and work towards dropping site data.

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

decoupling:
dyndbg: add ddebug_site(_get|_put) abstraction

cleanup, tidy:
dyndbg: ddebug_add_module avoid adding empty modules

do the "up-over-down" maneuver:
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 (almost)
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: RFC drop _ddebug.site pointer

condense unique site data:
dyndbg: split/copy ._index into 2 new fields: ._back, ._map
dyndbg: detect repeated site recs in add_module
dyndbg: pack module pr_debug sites
dyndbg: pack pr-debug site-recs in builtin modules

new features - _once, _ratelimited:
dyndbg: prototype print-once and print-ratelimited RFC

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 | 9 +-
include/asm-generic/vmlinux.lds.h | 24 +-
include/linux/dynamic_debug.h | 143 +++++++--
kernel/module-internal.h | 1 +
kernel/module.c | 11 +-
lib/dynamic_debug.c | 421 ++++++++++++++++++++------
12 files changed, 499 insertions(+), 122 deletions(-)

--
2.31.1