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Date: Wed, 10 Mar 2021 11:26:20 -0000
From: "tip-bot2 for Clement Courbet" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Clement Courbet <courbet@...gle.com>,
Josh Don <joshdon@...gle.com>,
"Peter Zijlstra (Intel)" <peterz@...radead.org>, x86@...nel.org,
linux-kernel@...r.kernel.org
Subject: [tip: sched/core] sched: Optimize __calc_delta()
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@...gle.com>
AuthorDate: Wed, 03 Mar 2021 14:46:53 -08:00
Committer: Peter Zijlstra <peterz@...radead.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@...gle.com>
Signed-off-by: Josh Don <joshdon@...gle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.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>
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