From: Peter Zijlstra <[email protected]>
commit f8ae107eef209bff29a5816bc1aad40d5cd69a80 upstream.
The initial value (@m) compute is:
m = 1UL << (BITS_PER_LONG - 2);
while (m > x)
m >>= 2;
Which is a linear search for the highest even bit smaller or equal to @x
We can implement this using a binary search using __fls() (or better when
its hardware implemented).
m = 1UL << (__fls(x) & ~1UL);
Especially for small values of @x; which are the more common arguments
when doing a CDF on idle times; the linear search is near to worst case,
while the binary search of __fls() is a constant 6 (or 5 on 32bit)
branches.
cycles: branches: branch-misses:
PRE:
hot: 43.633557 +- 0.034373 45.333132 +- 0.002277 0.023529 +- 0.000681
cold: 207.438411 +- 0.125840 45.333132 +- 0.002277 6.976486 +- 0.004219
SOFTWARE FLS:
hot: 29.576176 +- 0.028850 26.666730 +- 0.004511 0.019463 +- 0.000663
cold: 165.947136 +- 0.188406 26.666746 +- 0.004511 6.133897 +- 0.004386
HARDWARE FLS:
hot: 24.720922 +- 0.025161 20.666784 +- 0.004509 0.020836 +- 0.000677
cold: 132.777197 +- 0.127471 20.666776 +- 0.004509 5.080285 +- 0.003874
Averages computed over all values <128k using a LFSR to generate order.
Cold numbers have a LFSR based branch trace buffer 'confuser' ran between
each int_sqrt() invocation.
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Suggested-by: Joe Perches <[email protected]>
Acked-by: Will Deacon <[email protected]>
Acked-by: Linus Torvalds <[email protected]>
Cc: Anshul Garg <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: David Miller <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Michael Davidson <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
Cc: Joe Perches <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
lib/int_sqrt.c | 6 ++----
1 file changed, 2 insertions(+), 4 deletions(-)
--- a/lib/int_sqrt.c
+++ b/lib/int_sqrt.c
@@ -7,6 +7,7 @@
#include <linux/kernel.h>
#include <linux/export.h>
+#include <linux/bitops.h>
/**
* int_sqrt - rough approximation to sqrt
@@ -21,10 +22,7 @@ unsigned long int_sqrt(unsigned long x)
if (x <= 1)
return x;
- m = 1UL << (BITS_PER_LONG - 2);
- while (m > x)
- m >>= 2;
-
+ m = 1UL << (__fls(x) & ~1UL);
while (m != 0) {
b = y + m;
y >>= 1;
