sched: 64-bit: fix arithmetics overflow

(overflow means weight >= 2^32 here, because inv_weigh = 2^32/weight) A weight of a cfs_rq is the sum of weights of which entities are queued on this cfs_rq, so it will overflow when there are too many entities. Although, overflow occurs very rarely, but it break fairness when it occurs. 64-bits systems have more memory than 32-bit systems and 64-bit systems can create more process usually, so overflow may occur more frequently. This patch guarantees fairness when overflow happens on 64-bit systems. Thanks to the optimization of compiler, it changes nothing on 32-bit. Signed-off-by: Lai Jiangshan <[email protected]> Acked-by: Peter Zijlstra <[email protected]> Signed-off-by: Ingo Molnar <[email protected]>
yshchan · Jun 12, 2008 · 7a232e0 · 7a232e0
1 parent 2e08478
commit 7a232e0
Showing 1 changed file with 7 additions and 2 deletions.
diff --git a/kernel/sched.c b/kernel/sched.c
@@ -1340,8 +1340,13 @@ calc_delta_mine(unsigned long delta_exec, unsigned long weight,
 {
 	u64 tmp;
 
-	if (!lw->inv_weight)
-		lw->inv_weight = 1 + (WMULT_CONST-lw->weight/2)/(lw->weight+1);
+	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);
+	}
 
 	tmp = (u64)delta_exec * weight;
 	/*