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

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f5d6541..2e2ab1e 100644 (file)
--- 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 (file)
--- 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>