| From 3f3295709edea6268ff1609855f498035286af73 Mon Sep 17 00:00:00 2001 |
| From: Peter Zijlstra <peterz@infradead.org> |
| Date: Fri, 17 Nov 2017 15:28:04 -0800 |
| Subject: lib/int_sqrt: optimize small argument |
| |
| 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: Arnd Bergmann <arnd@arndb.de> |
| Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> |
| |
| --- |
| 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; |