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| author | Stephan Baerwolf <stephan.baerwolf@tu-ilmenau.de> | 2011-05-11 18:03:29 +0200 |
|---|---|---|
| committer | Ingo Molnar <mingo@elte.hu> | 2011-05-16 11:01:18 +0200 |
| commit | db670dac49b5423b39b5e523d28fe32045d71b10 (patch) | |
| tree | 194220c83cb097504ddf7bcccc7ce93d6a23eac3 | |
| parent | db44fc017d5989302713ab4e7f9e922b648f4b59 (diff) | |
| download | linux-stable-db670dac49b5423b39b5e523d28fe32045d71b10.tar.gz linux-stable-db670dac49b5423b39b5e523d28fe32045d71b10.tar.bz2 linux-stable-db670dac49b5423b39b5e523d28fe32045d71b10.zip | |
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>
| -rw-r--r-- | kernel/sched.c | 6 |
1 files changed, 3 insertions, 3 deletions
diff --git a/kernel/sched.c b/kernel/sched.c index 70bec4f1edbb..c62acf45d3b9 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: */ |