sched: round a bit better

round a tiny bit better in high-frequency rescheduling scenarios,
by rounding around zero instead of rounding down.

(this is pretty theoretical though)

Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 11 insertions, 7 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index 5470ab0258a8..b0afd8db1396 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -638,6 +638,11 @@ static u64 div64_likely32(u64 divident, unsigned long divisor)
 
 #define WMULT_SHIFT     32
 
+/*
+ * Shift right and round:
+ */
+#define RSR(x, y) (((x) + (1UL << ((y) - 1))) >> (y))
+
 static unsigned long
 calc_delta_mine(unsigned long delta_exec, unsigned long weight,
                 struct load_weight *lw)
@@ -645,18 +650,17 @@ calc_delta_mine(unsigned long delta_exec, unsigned long weight,
         u64 tmp;
 
         if (unlikely(!lw->inv_weight))
-                lw->inv_weight = WMULT_CONST / lw->weight;
+                lw->inv_weight = (WMULT_CONST - lw->weight/2) / lw->weight + 1;
 
         tmp = (u64)delta_exec * weight;
         /*
          * Check whether we'd overflow the 64-bit multiplication:
          */
-        if (unlikely(tmp > WMULT_CONST)) {
-                tmp = ((tmp >> WMULT_SHIFT/2) * lw->inv_weight)
-                                >> (WMULT_SHIFT/2);
-        } else {
-                tmp = (tmp * lw->inv_weight) >> WMULT_SHIFT;
-        }
+        if (unlikely(tmp > WMULT_CONST))
+                tmp = RSR(RSR(tmp, WMULT_SHIFT/2) * lw->inv_weight,
+                        WMULT_SHIFT/2);
+        else
+                tmp = RSR(tmp * lw->inv_weight, WMULT_SHIFT);
 
         return (unsigned long)min(tmp, (u64)(unsigned long)LONG_MAX);
 }