[tip: sched/core] sched: Optimize __calc_delta()

["tip-bot2 for Clement Courbet" <tip-bot2@linutronix.de>]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     1e17fb8edc5ad6587e9303ccdebce853bc8cf30c
Gitweb:        https://git.kernel.org/tip/1e17fb8edc5ad6587e9303ccdebce853bc8cf30c
Author:        Clement Courbet <courbet@google.com>
AuthorDate:    Wed, 03 Mar 2021 14:46:53 -08:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Wed, 10 Mar 2021 09:51:49 +01:00

sched: Optimize __calc_delta()

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?idI406.

```
name                                   cpu/op
BM_Calc<__calc_delta_loop>             9.57ms Â=B112%
BM_Calc<__calc_delta_generic_fls>      2.36ms Â=B113%
BM_Calc<__calc_delta_asm_fls>          2.45ms Â=B113%
BM_Calc<__calc_delta_asm_fls_nomem>    1.66ms Â=B112%
BM_Calc<__calc_delta_asm_fls64>        2.46ms Â=B113%
BM_Calc<__calc_delta_asm_fls64_nomem>  1.34ms Â=B115%
BM_Calc<__calc_delta_builtin>          1.32ms Â=B111%
```

Signed-off-by: Clement Courbet <courbet@google.com>
Signed-off-by: Josh Don <joshdon@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210303224653.2579656-1-joshdon@google.com
---
 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 f5d6541..2e2ab1e 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 bb8bb06..d2e09a6 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>