diff options
| author | Clement Courbet <courbet@google.com> | 2021-03-03 14:46:53 -0800 |
|---|---|---|
| committer | Peter Zijlstra <peterz@infradead.org> | 2021-03-10 09:51:49 +0100 |
| commit | 1e17fb8edc5ad6587e9303ccdebce853bc8cf30c (patch) | |
| tree | 65052ced18c491a4a3b229f5dc780b94efa3f063 /kernel/sched | |
| parent | 4117cebf1a9fcbf35b9aabf0e37b6c5eea296798 (diff) | |
| download | linux-1e17fb8edc5ad6587e9303ccdebce853bc8cf30c.tar.bz2 | |
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
Diffstat (limited to 'kernel/sched')
| -rw-r--r-- | kernel/sched/fair.c | 19 | ||||
| -rw-r--r-- | 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 f5d6541334b3..2e2ab1e00ef9 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 bb8bb06582c4..d2e09a647c4f 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> |