Messages in this thread Patches in this message | | | Date | Wed, 11 May 2011 18:03:29 +0200 | From | Stephan Bärwolf <> | Subject | [PATCH] sched: fix/optimise calculation of weight-inverse |
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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. (This could explain some aritmetical issues for small shares...)
The correct inverse would be 1/lw->weight multiplied by "WMULT_CONST" for fixcomma-scaling it into integers. (So WMULT_CONST/lw->weight ...)
For safety it is also important to check if division by zero could happen...
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> --- kernel/sched.c | 12 +++++++++--- 1 files changed, 9 insertions(+), 3 deletions(-)
diff --git a/kernel/sched.c b/kernel/sched.c index 312f8b9..bb55996 100644 --- a/kernel/sched.c +++ b/kernel/sched.c @@ -1307,15 +1307,21 @@ calc_delta_mine(unsigned long delta_exec, unsigned long weight, { u64 tmp; + tmp = (u64)delta_exec * weight; + + // actually we would have to trap - division by zero - but we stay and pretend the limit of the operation... + if (unlikely(lw->weight == 0)) { + if (unlikely(tmp == ((u64)0))) return (unsigned long)0; + else return (unsigned long)LONG_MAX; + } + 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: */ -- 1.7.3.4
-- Dipl.-Inf. Stephan Bärwolf Ilmenau University of Technology, Integrated Communication Systems Group Phone: +49 (0)3677 69 2821, FAX: +49 (0)3677 69 1614 Email: stephan.baerwolf@tu-ilmenau.de, Web: http://www.tu-ilmenau.de/iks
From 46d3b89632aee75643b2ef9ea9ccd3085c631211 Mon Sep 17 00:00:00 2001 From: Stephan Baerwolf <stephan.baerwolf@tu-ilmenau.de> Date: Wed, 11 May 2011 17:36:26 +0200 Subject: [PATCH] sched: fix/optimise calculation of 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. (This could explain some aritmetical issues for small shares...)
The correct inverse would be 1/lw->weight multiplied by "WMULT_CONST" for fixcomma-scaling it into integers. (So WMULT_CONST/lw->weight ...)
For safety it is also important to check if division by zero could happen...
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> --- kernel/sched.c | 12 +++++++++--- 1 files changed, 9 insertions(+), 3 deletions(-)
diff --git a/kernel/sched.c b/kernel/sched.c index 312f8b9..bb55996 100644 --- a/kernel/sched.c +++ b/kernel/sched.c @@ -1307,15 +1307,21 @@ calc_delta_mine(unsigned long delta_exec, unsigned long weight, { u64 tmp; + tmp = (u64)delta_exec * weight; + + // actually we would have to trap - division by zero - but we stay and pretend the limit of the operation... + if (unlikely(lw->weight == 0)) { + if (unlikely(tmp == ((u64)0))) return (unsigned long)0; + else return (unsigned long)LONG_MAX; + } + 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: */ -- 1.7.3.4
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