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Date:   Wed,  3 Mar 2021 14:46:53 -0800
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Subject: [PATCH v2] sched: Optimize __calc_delta.
From:   Josh Don <joshdon@google.com>
To:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>
Cc:     Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com,
        Clement Courbet <courbet@google.com>,
        Oleg Rombakh <olegrom@google.com>,
        Josh Don <joshdon@google.com>
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Precedence: bulk

From: Clement Courbet <courbet@google.com>

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=3D49406.

```
name                                   cpu/op
BM_Calc<__calc_delta_loop>             9.57ms =C2=B112%
BM_Calc<__calc_delta_generic_fls>      2.36ms =C2=B113%
BM_Calc<__calc_delta_asm_fls>          2.45ms =C2=B113%
BM_Calc<__calc_delta_asm_fls_nomem>    1.66ms =C2=B112%
BM_Calc<__calc_delta_asm_fls64>        2.46ms =C2=B113%
BM_Calc<__calc_delta_asm_fls64_nomem>  1.34ms =C2=B115%
BM_Calc<__calc_delta_builtin>          1.32ms =C2=B111%
```

Signed-off-by: Clement Courbet <courbet@google.com>
Signed-off-by: Josh Don <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 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 *l=
w)
 static u64 __calc_delta(u64 delta_exec, unsigned long weight, struct load_=
weight *lw)
 {
 	u64 fact =3D scale_load_down(weight);
+	u32 fact_hi =3D (u32)(fact >> 32);
 	int shift =3D WMULT_SHIFT;
+	int fs;
=20
 	__update_inv_weight(lw);
=20
-	if (unlikely(fact >> 32)) {
-		while (fact >> 32) {
-			fact >>=3D 1;
-			shift--;
-		}
+	if (unlikely(fact_hi)) {
+		fs =3D fls(fact_hi);
+		shift -=3D fs;
+		fact >>=3D fs;
 	}
=20
 	fact =3D mul_u32_u32(fact, lw->inv_weight);
=20
-	while (fact >> 32) {
-		fact >>=3D 1;
-		shift--;
+	fact_hi =3D (u32)(fact >> 32);
+	if (fact_hi) {
+		fs =3D fls(fact_hi);
+		shift -=3D fs;
+		fact >>=3D fs;
 	}
=20
 	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>
=20
 #include <linux/binfmts.h>
+#include <linux/bitops.h>
 #include <linux/blkdev.h>
 #include <linux/compat.h>
 #include <linux/context_tracking.h>
--=20
2.30.1.766.gb4fecdf3b7-goog