[PATCH v2 0/3] kallsyms: Accelerate symbol name lookups by ~19x

[Jim Cromie <jim.cromie@gmail.com>]

kallsyms_lookup_names() resolves symbol names to addresses using a
17-step binary search over kallsyms_names[] (~184k symbols on x86_64).
At each step of the search, two bottlenecks compound to create
substantial lookup latency:

0. Marker scanning: get_symbol_offset() scans sequentially from the
   nearest 256-symbol marker, decoding an average of ~128 ULEB128 record
   headers per probe (~2,176 header decodes per lookup).

1. Redundant string expansion: kallsyms_expand_symbol() decompresses
   the entire candidate symbol into a 512-byte stack buffer (namebuf)
   before calling strcmp(), even though ~94% of binary search probes
   mismatch on the first 1-2 characters.

Together, these bottlenecks impose a ~3.8 us latency penalty per hit and
~3.6 us per miss.

This 3-patch series eliminates both overheads while keeping the
symbol table strictly in sequential address order and adding 0 bytes to
.rodata:

0. Patch 1 adds lib/test_kallsyms_perf.ko, a microbenchmark module to
   measure unindexed vs dynamic indexed name searches, address
   resolution, and table iteration latency, with built-in correctness
   validation and a sysfs trigger.

1. Patch 2 introduces a dynamic u32 lookup index bracketed by
   kallsyms_lookup_batch_start() and kallsyms_lookup_batch_end().
   It allocates ~736 KiB in transient RAM via kvmalloc_array() only
   while bulk workloads (BPF attach, module loading) run, resolves
   each probe in O(1) with 0 hops, and leaves .rodata bloat at exactly
   0 bytes while retaining kallsyms_markers[] as fallback.

2. Patch 3 introduces kallsyms_strcmp_symbol() to compare ASCII queries
   against compressed tokens incrementally on the fly, bailing out on
   the first mismatched character without expanding subsequent tokens.
   This drops the 512-byte namebuf buffer from the kernel stack.

Live Microbenchmark Results (via test_kallsyms_perf, 100k iters):

Metric                    Baseline      Patched       Speedup
-----------------------------------------------------------------
Name Search Hit           3,811 ns      246 ns        15.5x
Name Search Miss          3,625 ns      196 ns        18.5x
sprint_symbol               412 ns      412 ns        parity
sprint_symbol_no_offset     300 ns      300 ns        parity
Table Full Walk           13,626 us   13,626 us       parity

Address-to-name resolution (sprint_symbol) and sequential table walks
(/proc/kallsyms) remain completely unaffected, maintaining full L1/L2
hardware prefetching.

Memory footprint: +0 KiB .rodata added to kernel image.  Transient RAM
is ~736 KiB (184k * 4 bytes) allocated only during active batch
lookup sessions.

Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
---
Changes in v2:
- Replaced static build-time 3-byte offset table with a dynamic u32
  index bracketed by kallsyms_lookup_batch_start() and
  kallsyms_lookup_batch_end().
- Dropped .rodata image footprint addition from +573 KiB to 0 KiB,
  addressing Kees Cook's memory footprint objection.
- Native u32 loads in transient RAM eliminate 24-bit big-endian shifts
  and unaligned loads, addressing David Laight's endianness critique.
- Direct O(1) table indexing provides 0 hops for all symbol lookups
  without remainder logic or odd/even branching.
- Restored scripts/kallsyms.c and kernel/kallsyms_internal.h to pristine
  state, leaving legacy kallsyms_markers[] as safety fallback.
- Rebased out Lorenzo Stoakes' kbuild series; this series is now
  completely decoupled and applies cleanly directly onto mainline.
- Updated test_kallsyms_perf to benchmark unindexed marker scans and
  dynamic index side by side in a single run.
- Link to v1: https://lore.kernel.org/r/20260919-ksyms-tune-v1-0-d85c97da1a32@gmail.com

---
Jim Cromie (3):
      kallsyms: Add test_kallsyms_perf module to benchmark lookup latency
      kallsyms: Add dynamic lookup index for batch resolution
      kallsyms: Match compressed tokens on the fly during binary search

 include/linux/kallsyms.h |  13 +++
 kernel/kallsyms.c        | 210 ++++++++++++++++++++++++++++--------
 lib/Kconfig.debug        |  10 ++
 lib/Makefile             |   1 +
 lib/test_kallsyms_perf.c | 269 +++++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 457 insertions(+), 46 deletions(-)
---
base-commit: 93f51579e7df248780214094418f205253383cc5
change-id: 20260919-ksyms-tune-e22a42d8a31a

Best regards,
-- 
Jim Cromie <jim.cromie@gmail.com>