4.9-stable review patch. If anyone has any objections, please let me know.
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From: Peter Zijlstra peterz@infradead.org
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/20171020164644.936577234@infradead.org Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Suggested-by: Joe Perches joe@perches.com Acked-by: Will Deacon will.deacon@arm.com Acked-by: Linus Torvalds torvalds@linux-foundation.org Cc: Anshul Garg aksgarg1989@gmail.com Cc: Davidlohr Bueso dave@stgolabs.net Cc: David Miller davem@davemloft.net Cc: Ingo Molnar mingo@kernel.org Cc: Kees Cook keescook@chromium.org Cc: Matthew Wilcox mawilcox@microsoft.com Cc: Michael Davidson md@google.com Cc: Thomas Gleixner tglx@linutronix.de Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Cc: Joe Perches joe@perches.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- 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;