lib/raid6: add option to skip algo benchmarking

This is helpful for systems where fast startup time is important.
It is especially nice to avoid benchmarking RAID functions that are
never used (for example, BTRFS selects RAID6_PQ even if the parity RAID
mode is not in use).

This saves 250+ milliseconds of boot time on modern x86 and ARM systems
with a dozen or more available implementations.

The new option is defaulted to 'y' to match the previous behavior of
always benchmarking on init.
Signed-off-by: NDaniel Verkamp <dverkamp@chromium.org>
Signed-off-by: NShaohua Li <shli@fb.com>

include/linux/raid/pq.h

@@ -70,6 +70,9 @@ extern const char raid6_empty_zero_page[PAGE_SIZE];
 #define MODULE_DESCRIPTION(desc)
 #define subsys_initcall(x)
 #define module_exit(x)
+
+#define IS_ENABLED(x) (x)
+#define CONFIG_RAID6_PQ_BENCHMARK 1
 #endif /* __KERNEL__ */
 
 /* Routine choices */

lib/Kconfig

@@ -10,6 +10,14 @@ menu "Library routines"
 config RAID6_PQ
 	tristate
 
+config RAID6_PQ_BENCHMARK
+	bool "Automatically choose fastest RAID6 PQ functions"
+	depends on RAID6_PQ
+	default y
+	help
+	  Benchmark all available RAID6 PQ functions on init and choose the
+	  fastest one.
+
 config BITREVERSE
 	tristate
 

lib/raid6/algos.c

@@ -163,6 +163,11 @@ static inline const struct raid6_calls *raid6_choose_gen(
 			if ((*algo)->valid && !(*algo)->valid())
 				continue;
 
+			if (!IS_ENABLED(CONFIG_RAID6_PQ_BENCHMARK)) {
+				best = *algo;
+				break;
+			}
+
 			perf = 0;
 
 			preempt_disable();