linux-kernel - [PATCH] sched: fix/optimise calculation of weight-inverse

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Date:	Wed, 11 May 2011 18:03:29 +0200
From:	Stephan Bärwolf <stephan.baerwolf@...ilmenau.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Nikhil Rao <ncrao@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <efault@....de>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org
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@...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@...ilmenau.de,
Web: http://www.tu-ilmenau.de/iks



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