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: Daniel Verkamp <dverkamp@chromium.org>
Signed-off-by: Shaohua Li <shli@fb.com>

3 files changed, 16 insertions, 0 deletions

diff --git a/include/linux/raid/pq.h b/include/linux/raid/pq.h
index d7c99161bba2..605cf46c17bd 100644
--- a/include/linux/raid/pq.h
+++ b/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 */
diff --git a/lib/Kconfig b/lib/Kconfig
index a9965f4af4dd..fcb05305a5a2 100644
--- a/lib/Kconfig
+++ b/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
 
diff --git a/lib/raid6/algos.c b/lib/raid6/algos.c
index a753ff56670f..7e4f7a8ffa8e 100644
--- a/lib/raid6/algos.c
+++ b/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();