On Thu, 2016-01-28 at 13:42 -0800, Andi Kleen wrote:
> From: Andi Kleen <a...@linux.intel.com>
>
> The menu cpuidle governor does at least two int_sqrt() each time
> we go into idle in get_typical_interval to compute stddev
>
> int_sqrts take 100-120 cycles each. Short idle latency is important
> for many workloads.
>
> I instrumented the function on my workstation and most values are
> 16bit only and most others 32bit (50% percentile is 122094,
> 75% is 3699533).
>
> sqrt is implemented by starting with an initial estimation,
> and then iterating. int_sqrt currently only uses a fixed
> estimating which is good for 64bits worth of input.
>
> This patch adds some checks at the beginning to start with
> a better estimate for values fitting in 8, 16bit and 32bit.
> This makes int_sqrt between 60+% faster for values in 16bit,
> and still somewhat faster (between 10 and 30%) for larger values
> upto 32bit. Full 64bit is slightly slower.
>
> This optimizes the short idle calls and does not hurt the
> long sleep (which probably do not care) much.
>
> An alternative would be a full table drive approach, or
> trying some inverted sqrt optimization, but this simple change
> already seems to have a good payoff.
>
> Signed-off-by: Andi Kleen <a...@linux.intel.com>
> ---
> lib/int_sqrt.c | 10 +++++++++-
> 1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/lib/int_sqrt.c b/lib/int_sqrt.c
> index 1ef4cc3..2479ccf 100644
> --- a/lib/int_sqrt.c
> +++ b/lib/int_sqrt.c
> @@ -21,7 +21,15 @@ unsigned long int_sqrt(unsigned long x)
> if (x <= 1)
> return x;
The above test (x <= 1) should also be moved
>
> - m = 1UL << (BITS_PER_LONG - 2);
> + if (x <= 0xffff) {
> + if (m <= 0xff)
m or x ? if (x <= 0xff) looks more correct.
> + m = 1UL << (8 - 2);
> + else
> + m = 1UL << (16 - 2);
> + } else if (x <= 0xffffffff)
> + m = 1UL << (32 - 2);
> + else
> + m = 1UL << (BITS_PER_LONG - 2);
> while (m != 0) {
> b = y + m;
> y >>= 1;
