4.14-stable review patch. If anyone has any objections, please let me know.
------------------ 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 @@ -8,6 +8,7 @@ #include <linux/kernel.h> #include <linux/export.h> +#include <linux/bitops.h> /** * int_sqrt - rough approximation to sqrt @@ -22,10 +23,7 @@ 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;
