[PATCH v2 next 09/15] kallsyms: Add a kernel parameter for the marker shift

[David Laight <david.laight.linux@gmail.com>]

Set the default to 6 (one marker every 64 symbols).
Improves symbol lookup by name by about 35% (over an interval of 256).
Net improvement for the patch series is about 18 times.

Static data increases by a little less than 1 bit for every 2 symbols.

Signed-off-by: David Laight <david.laight.linux@gmail.com>
---
 init/Kconfig            | 19 +++++++++++++++++++
 scripts/link-vmlinux.sh |  6 ++++++
 2 files changed, 25 insertions(+)

diff --git a/init/Kconfig b/init/Kconfig
index 8583d9f06c52..2d0716662c67 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -2101,6 +2101,25 @@ config KALLSYMS
 	  symbolic stack backtraces. This increases the size of the kernel
 	  somewhat, as all symbols have to be loaded into the kernel image.
 
+config KALLSYMS_MARKER_SHIFT
+	int "Interval between kallsyms markers (6 => 64, 8 => 256)"
+	depends on KALLSYMS
+	default 6
+	range 0 24
+	help
+	  Set the interval between the kallsyms marker entries as a power of 2.
+
+	  This bounds the linear search when locating the name of a symbol
+	  from its index in the array of values (and v.v.).
+
+	  Smaller values speed up the search but increase the memory footprint.
+	  Values below 5 (one 32 bit marker every 32 symbols) are unlikely to
+	  make a significant difference because other parts of the code
+	  dominate.
+
+	  The default of 6 is a reasonable compromise. It makes looking up a
+	  symbol by name around 35% faster than setting it to 8.
+
 config KALLSYMS_SELFTEST
 	bool "Test the basic functions and performance of kallsyms"
 	depends on KALLSYMS
diff --git a/scripts/link-vmlinux.sh b/scripts/link-vmlinux.sh
index ab0b8125c8cb..12b119d024b4 100755
--- a/scripts/link-vmlinux.sh
+++ b/scripts/link-vmlinux.sh
@@ -37,6 +37,10 @@ is_enabled() {
 	grep -q "^$1=y" include/config/auto.conf
 }
 
+value() {
+	sed -n "/^$1=/{s///;p;q}" include/config/auto.conf
+}
+
 # Nice output in kbuild format
 # Will be suppressed by "make -s"
 info()
@@ -131,6 +135,8 @@ kallsyms()
 		kallsymopt="${kallsymopt} --pc-relative"
 	fi
 
+	kallsymopt="${kallsymopt} --marker-shift=$(value CONFIG_KALLSYMS_MARKER_SHIFT)"
+
 	info KSYMS "${2}.S"
 	scripts/kallsyms ${kallsymopt} "${1}" > "${2}.S"
 
-- 
2.39.5