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>

diff --git a/kernel/sched.c b/kernel/sched.c
index 5470ab0..b0afd8d 100644 (file)
--- 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);
 }