Messages in this thread Patch in this message | | | Date | Mon, 16 May 2011 10:38:20 GMT | From | tip-bot for Stephan Baerwolf <> | Subject | [tip:sched/core] sched: Fix and optimise calculation of the weight-inverse |
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Commit-ID: db670dac49b5423b39b5e523d28fe32045d71b10 Gitweb: http://git.kernel.org/tip/db670dac49b5423b39b5e523d28fe32045d71b10 Author: Stephan Baerwolf <stephan.baerwolf@tu-ilmenau.de> AuthorDate: Wed, 11 May 2011 18:03:29 +0200 Committer: Ingo Molnar <mingo@elte.hu> CommitDate: Mon, 16 May 2011 11:01:18 +0200
sched: Fix and optimise calculation of the weight-inverse
If the inverse loadweight should be zero, function "calc_delta_mine" calculates the inverse of "lw->weight" (in 32bit integer ops).
This calculation is actually a little bit impure (because it is inverting something around "lw-weight"+1), especially when "lw->weight" becomes smaller.
The correct inverse would be 1/lw->weight multiplied by "WMULT_CONST" for fixcomma-scaling it into integers. (So WMULT_CONST/lw->weight ...)
The old, impure algorithm took two divisions for inverting lw->weight, the new, more exact one only takes one and an additional unlikely-if.
Signed-off-by: Stephan Baerwolf <stephan.baerwolf@tu-ilmenau.de> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Linus Torvalds <torvalds@linux-foundation.org> Link: http://lkml.kernel.org/n/tip-0pz0wnyalr4tk4ln11xwumdx@git.kernel.org [ This could explain some aritmetical issues for small shares but nothing concrete has been reported yet so we are not confident enough to queue this up in sched/urgent and for -stable backport. But if anyone finds this commit and sees it to fix some badness then we can certainly change our mind! ] Signed-off-by: Ingo Molnar <mingo@elte.hu> --- kernel/sched.c | 6 +++--- 1 files changed, 3 insertions(+), 3 deletions(-)
diff --git a/kernel/sched.c b/kernel/sched.c index 70bec4f..c62acf4 100644 --- a/kernel/sched.c +++ b/kernel/sched.c @@ -1330,15 +1330,15 @@ calc_delta_mine(unsigned long delta_exec, unsigned long weight, { u64 tmp; + tmp = (u64)delta_exec * weight; + if (!lw->inv_weight) { if (BITS_PER_LONG > 32 && unlikely(lw->weight >= WMULT_CONST)) lw->inv_weight = 1; else - lw->inv_weight = 1 + (WMULT_CONST-lw->weight/2) - / (lw->weight+1); + lw->inv_weight = WMULT_CONST / lw->weight; } - tmp = (u64)delta_exec * weight; /* * Check whether we'd overflow the 64-bit multiplication: */
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