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authorPeter Zijlstra <peterz@infradead.org>2017-11-17 15:28:04 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2017-11-17 16:10:01 -0800
commit3f3295709edea6268ff1609855f498035286af73 (patch)
tree9cfeb38c4610b5992b8212de7482f6ac32336d4c /lib
parentdc2bf000a2848cf1dee373db14ce2cd1fe3ee394 (diff)
download96b-common-3f3295709edea6268ff1609855f498035286af73.tar.gz
lib/int_sqrt: optimize small argument
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>
Diffstat (limited to 'lib')
-rw-r--r--lib/int_sqrt.c3
1 files changed, 3 insertions, 0 deletions
diff --git a/lib/int_sqrt.c b/lib/int_sqrt.c
index db0b5aa071fc..036c96781ea8 100644
--- a/lib/int_sqrt.c
+++ b/lib/int_sqrt.c
@@ -23,6 +23,9 @@ 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;