3.16.54-rc1 review patch. If anyone has any objections, please let me know.
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From: Peter Zijlstra peterz@infradead.org
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@infradead.org Fixes: 30493cc9dddb ("lib/int_sqrt.c: optimize square root algorithm") Signed-off-by: Peter Zijlstra (Intel) peterz@infradead.org Suggested-by: Anshul Garg aksgarg1989@gmail.com Acked-by: Linus Torvalds torvalds@linux-foundation.org Cc: Davidlohr Bueso dave@stgolabs.net Cc: Thomas Gleixner tglx@linutronix.de Cc: Ingo Molnar mingo@kernel.org Cc: Will Deacon will.deacon@arm.com Cc: Joe Perches joe@perches.com Cc: David Miller davem@davemloft.net Cc: Matthew Wilcox mawilcox@microsoft.com Cc: Kees Cook keescook@chromium.org Cc: Michael Davidson md@google.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Ben Hutchings ben@decadent.org.uk --- 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;