kallsyms: increase marker density to 16:1 to accelerate lookups

kallsyms stores symbols with remarkably efficient packing, and simple
streaming unpacking, laid out sequentially in address order.  That said,
variable-length records make arbitrary access inherently linear. 
kallsyms_markers[] addressed this by marking stream offsets every 256
symbols, reducing the scan distance by 256x down to an average of 127.5
sequential steps.

While 127.5 hops was negligible for rare, single-shot oops backtraces,
both table size (~184k symbols) and lookup traffic have expanded
substantially.  In alphabetical binary search (kallsyms_lookup_names),
each of the ~17 comparison probes must locate candidate symbols via
get_symbol_offset(), compounding into ~2,170 sequential symbol hops per
lookup.  In bulk tracing workloads (such as BPF multi-kprobe attach), this
penalty compounds into multi-second latency.

Without altering the underlying storage layout, we can retune this
trade-off directly by increasing marker density from 256:1 down to 16:1
(KALLSYMS_MARKER_SHIFT 4) in kernel/kallsyms_internal.h, shared between
scripts/kallsyms.c and kernel/kallsyms.c.

This caps the remainder scan at 15 symbols and cuts average scan distance
from 127.5 down to 7.5 hops (a 17x reduction).  Across a 17-step binary
search, total hops collapse from ~2,170 down to ~127.  For a kernel with
~184,000 symbols, this adds ~10,800 u32 marker entries (+42 KiB) to
write-protected .rodata.

In-tree CONFIG_KALLSYMS_SELFTEST measurements across all ~184k symbols
show average lookup latency dropping from 6,102 ns down to 866 ns (a 7.0x
speedup).

Link: https://lore.kernel.org/20260929-ksyms-tune-v7-2-be568ceef41e@gmail.com
Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Kees Cook <kees@kernel.org>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Zhen Lei <thunder.leizhen@huawei.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Andrey Grodzovsky <andrey.grodzovsky@crowdstrike.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: David Laight <david.laight.linux@gmail.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Jiri Olsa <olsajiri@gmail.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>

1 files changed, 9 insertions, 5 deletions

diff --git a/scripts/kallsyms.c b/scripts/kallsyms.c
index 494852ade6d8..be42a9111350 100644
--- a/scripts/kallsyms.c
+++ b/scripts/kallsyms.c
@@ -29,6 +29,8 @@
 
 #include <xalloc.h>
 
+#include "../kernel/kallsyms_internal.h"
+
 #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof(arr[0]))
 
 #define KSYM_NAME_LEN		512
@@ -349,16 +351,18 @@ static void write_src(void)
 	printf("\t.long\t%u\n", table_cnt);
 	printf("\n");
 
-	/* table of offset markers, that give the offset in the compressed stream
-	 * every 256 symbols */
-	markers_cnt = (table_cnt + 255) / 256;
+	/*
+	 * Table of offset markers, giving the offset in the compressed stream
+	 * every (1 << KALLSYMS_MARKER_SHIFT) symbols.
+	 */
+	markers_cnt = (table_cnt + KALLSYMS_MARKER_MASK) >> KALLSYMS_MARKER_SHIFT;
 	markers = xmalloc(sizeof(*markers) * markers_cnt);
 
 	output_label("kallsyms_names");
 	off = 0;
 	for (i = 0; i < table_cnt; i++) {
-		if ((i & 0xFF) == 0)
-			markers[i >> 8] = off;
+		if ((i & KALLSYMS_MARKER_MASK) == 0)
+			markers[i >> KALLSYMS_MARKER_SHIFT] = off;
 		table[i]->seq = i;
 
 		/* There cannot be any symbol of length zero. */