Re: [PATCH] sched/fair: Avoid overflow in place_entity()

[Peter Zijlstra]

On Mon, Sep 28, 2026 at 09:47:54PM +0900, Hui Su wrote:
> place_entity() inflates an entity's virtual lag with
> 
>     lag * (load + weight) / load
> 
> Commit 4823725d9d1d ("sched/fair: Increase weight bits for avg_vruntime")
> replaced the scaled-down weights used in this calculation with
> avg_vruntime_weight(), removing the arithmetic headroom provided by
> scale_load_down().
> 
> The multiplication can overflow s64 before the division even when the
> final quotient is representable. A task's hierarchical effective weight
> can be as small as 2 after avg_vruntime_weight() scaling under group
> scheduling, while entity_lag() bounds |vlag| by
> 
>     calc_delta_fair(cfs_rq_max_slice(cfs_rq) + TICK_NSEC, se)
> 
> For example, with HZ=1000, NICE_0_LOAD=1048576, and the minimum scaled
> hierarchical weight scale_load(2)=2048, this bound is 891289600. With 114
> maximum-weight entities, the runqueue load is 10361604096. The
> intermediate product is:
> 
>     891289600 * (10361604096 + 2)
>         = 9235189971864780800
>         > S64_MAX
> 
> The signed overflow corrupts the entity's placement.
> 
> Rewrite
> 
>     lag * (load + weight) / load
> 
> as
> 
>     lag + lag * weight / load
> 
> The two expressions are equivalent, but the latter avoids multiplying the
> virtual lag by the total runqueue weight. entity_lag() bounds vlag against
> the entity's effective weight, so the remaining lag * weight product stays
> bounded. rescale_entity() preserves this scale across h_load changes and
> already relies on the same product when rescaling vlag.
> 
> Keep the original zero-load warning semantics by multiplying lag by weight
> after replacing the divisor with one.
> 
> Fixes: 4823725d9d1d ("sched/fair: Increase weight bits for avg_vruntime")
> Signed-off-by: Hui Su <sh_def@xxxxxxx>
> ---

Indeed so; however I've rewritten things like below. Hope that works for
you.

---
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7455a83a6a99..6ce7233cd339 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6266,12 +6266,20 @@ place_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
 		load = cfs_rq->sum_weight;
 		if (curr && curr->on_rq)
 			load += avg_vruntime_weight(cfs_rq, curr->h_load.weight);
-
-		weight = avg_vruntime_weight(cfs_rq, se->h_load.weight);
-		lag *= load + weight;
 		if (WARN_ON_ONCE(!load))
 			load = 1;
-		lag = div64_long(lag, load);
+
+		weight = avg_vruntime_weight(cfs_rq, se->h_load.weight);
+
+		/*
+		 * Rewrite, because the product (W + w_i)*vl'_i might
+		 * overflow s64:
+		 *
+		 *   vl_i = (W + w_i)*vl'_i / W
+		 *        = (W*vl'_i + w_i*vl'_i) / W
+		 *        = vl'_i + w_i*vl'_i/W
+		 */
+		lag += div64_long(lag * weight, load);
 
 		/*
 		 * A heavy entity (relative to the tree) will pull the