diff --git a/tools/sched_ext/scx_flatcg.bpf.c b/tools/sched_ext/scx_flatcg.bpf.c
index 2d178c2ecacc..64cf4dd964d6 100644
--- a/tools/sched_ext/scx_flatcg.bpf.c
+++ b/tools/sched_ext/scx_flatcg.bpf.c
@@ -772,10 +772,18 @@ void BPF_STRUCT_OPS(fcg_dispatch, s32 cpu, struct task_struct *prev)
 		 * cgroup to execute but the latter needs to be done in a loop
 		 * and we can't keep the lock held. Oh well...
 		 */
+		s64 delta = now - cpuc->cur_at - cgrp_slice_ns;
+
 		bpf_spin_lock(&cgv_tree_lock);
-		__sync_fetch_and_add(&cgc->cvtime_delta,
-				     (cpuc->cur_at + cgrp_slice_ns - now) *
-				     FCG_HWEIGHT_ONE / (cgc->hweight ?: 1));
+		/* keep the dividends positive, BPF division is unsigned */
+		if (delta >= 0)
+			__sync_fetch_and_add(&cgc->cvtime_delta,
+					     (u64)delta * FCG_HWEIGHT_ONE /
+					     (cgc->hweight ?: 1));
+		else
+			__sync_fetch_and_sub(&cgc->cvtime_delta,
+					     (u64)-delta * FCG_HWEIGHT_ONE /
+					     (cgc->hweight ?: 1));
 		bpf_spin_unlock(&cgv_tree_lock);
 	} else {
 		stat_inc(FCG_STAT_CNS_GONE);