[PATCH v2] sched: Optimize __calc_delta.

[Josh Don]

From: Clement Courbet <courbet@xxxxxxxxxx>

A significant portion of __calc_delta time is spent in the loop
shifting a u64 by 32 bits. Use `fls` instead of iterating.

This is ~7x faster on benchmarks.

The generic `fls` implementation (`generic_fls`) is still ~4x faster
than the loop.
Architectures that have a better implementation will make use of it. For
example, on X86 we get an additional factor 2 in speed without dedicated
implementation.

On gcc, the asm versions of `fls` are about the same speed as the
builtin. On clang, the versions that use fls are more than twice as
slow as the builtin. This is because the way the `fls` function is
written, clang puts the value in memory:
https://godbolt.org/z/EfMbYe. This bug is filed at
https://bugs.llvm.org/show_bug.cgi?id=49406.

```
name                                   cpu/op
BM_Calc<__calc_delta_loop>             9.57ms ±12%
BM_Calc<__calc_delta_generic_fls>      2.36ms ±13%
BM_Calc<__calc_delta_asm_fls>          2.45ms ±13%
BM_Calc<__calc_delta_asm_fls_nomem>    1.66ms ±12%
BM_Calc<__calc_delta_asm_fls64>        2.46ms ±13%
BM_Calc<__calc_delta_asm_fls64_nomem>  1.34ms ±15%
BM_Calc<__calc_delta_builtin>          1.32ms ±11%
```

Signed-off-by: Clement Courbet <courbet@xxxxxxxxxx>
Signed-off-by: Josh Don <joshdon@xxxxxxxxxx>
---
 kernel/sched/fair.c  | 19 +++++++++++--------
 kernel/sched/sched.h |  1 +
 2 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 8a8bd7b13634..a691371960ae 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -229,22 +229,25 @@ static void __update_inv_weight(struct load_weight *lw)
 static u64 __calc_delta(u64 delta_exec, unsigned long weight, struct load_weight *lw)
 {
 	u64 fact = scale_load_down(weight);
+	u32 fact_hi = (u32)(fact >> 32);
 	int shift = WMULT_SHIFT;
+	int fs;
 
 	__update_inv_weight(lw);
 
-	if (unlikely(fact >> 32)) {
-		while (fact >> 32) {
-			fact >>= 1;
-			shift--;
-		}
+	if (unlikely(fact_hi)) {
+		fs = fls(fact_hi);
+		shift -= fs;
+		fact >>= fs;
 	}
 
 	fact = mul_u32_u32(fact, lw->inv_weight);
 
-	while (fact >> 32) {
-		fact >>= 1;
-		shift--;
+	fact_hi = (u32)(fact >> 32);
+	if (fact_hi) {
+		fs = fls(fact_hi);
+		shift -= fs;
+		fact >>= fs;
 	}
 
 	return mul_u64_u32_shr(delta_exec, fact, shift);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 10a1522b1e30..714af71cf983 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -36,6 +36,7 @@
 #include <uapi/linux/sched/types.h>
 
 #include <linux/binfmts.h>
+#include <linux/bitops.h>
 #include <linux/blkdev.h>
 #include <linux/compat.h>
 #include <linux/context_tracking.h>
-- 
2.30.1.766.gb4fecdf3b7-goog