Re: [PATCH 4.4 034/131] lib/int_sqrt: optimize small argument

[Joe Perches]

On Mon, 2019-04-01 at 19:01 +0200, Greg Kroah-Hartman wrote:
> 4.4-stable review patch.  If anyone has any objections, please let me know.

If this is to be ported just for an optimization
(which I think is dubious as it's not a bug fix),
why not port the __fls optimization too?

__fls has a bigger effect on performance.

------------------
> 
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> 
> commit 3f3295709edea6268ff1609855f498035286af73 upstream.
> 
> The current int_sqrt() computation is sub-optimal for the case of small
> @x.  Which is the interesting case when we're going to do cumulative
> distribution functions on idle times, which we assume to be a random
> variable, where the target residency of the deepest idle state gives an
> upper bound on the variable (5e6ns on recent Intel chips).
> 
> In the case of small @x, the compute loop:
> 
> 	while (m != 0) {
> 		b = y + m;
> 		y >>= 1;
> 
> 		if (x >= b) {
> 			x -= b;
> 			y += m;
> 		}
> 		m >>= 2;
> 	}
> 
> can be reduced to:
> 
> 	while (m > x)
> 		m >>= 2;
> 
> Because y==0, b==m and until x>=m y will remain 0.
> 
> And while this is computationally equivalent, it runs much faster
> because there's less code, in particular less branches.
> 
>       cycles:                 branches:              branch-misses:
> 
> OLD:
> 
> hot:   45.109444 +- 0.044117  44.333392 +- 0.002254  0.018723 +- 0.000593
> cold: 187.737379 +- 0.156678  44.333407 +- 0.002254  6.272844 +- 0.004305
> 
> PRE:
> 
> hot:   67.937492 +- 0.064124  66.999535 +- 0.000488  0.066720 +- 0.001113
> cold: 232.004379 +- 0.332811  66.999527 +- 0.000488  6.914634 +- 0.006568
> 
> POST:
> 
> 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
> 
> 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/20171020164644.876503355@xxxxxxxxxxxxx
> Fixes: 30493cc9dddb ("lib/int_sqrt.c: optimize square root algorithm")
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Suggested-by: Anshul Garg <aksgarg1989@xxxxxxxxx>
> Acked-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Cc: Joe Perches <joe@xxxxxxxxxxx>
> Cc: David Miller <davem@xxxxxxxxxxxxx>
> Cc: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Michael Davidson <md@xxxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> 
> ---
>  lib/int_sqrt.c |    3 +++
>  1 file changed, 3 insertions(+)
> 
> --- a/lib/int_sqrt.c
> +++ b/lib/int_sqrt.c
> @@ -22,6 +22,9 @@ unsigned long int_sqrt(unsigned long x)
>  		return x;
>  
>  	m = 1UL << (BITS_PER_LONG - 2);
> +	while (m > x)
> +		m >>= 2;
> +
>  	while (m != 0) {
>  		b = y + m;
>  		y >>= 1;
> 
>