Re: [PATCH RESEND] sched/fair: Fix overflow in vruntime_eligible()

[Peter Zijlstra]

On Mon, May 04, 2026 at 01:22:39PM +0200, Peter Zijlstra wrote:

> +#ifdef CONFIG_64BIT
> +#ifdef CONFIG_ARCH_SUPPORTS_INT128
> +	/* An __int128 mult should be cheaper than a division. */
> +	return avg >= (__int128)key * load;
> +#else
> +	/*
> +	 * Since the divisor is @load, which is guaranteed positive, the
> +	 * inequality: avg >= key * load, can be rewritten into a division
> +	 * like: avg/load > key || (avg/load == key && avg%load >= 0).
> +	 */
> +	s64 div = avg / load;
> +	if (div > key)
> +		return true;
> +	return div == key && avg % load >= 0;

Bah, I could of course have put the __builtin_mul_overflow() thing in
here. That probably generates better code than this for those
architectures not doing SUPPORT_INT128.

Just got focussed on keeping the thing exact, but that isn't needed.
That overflow thing is good enough.

---
Subject: sched/fair: Fix overflow in vruntime_eligible()
From: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>
Date: Fri, 1 May 2026 12:40:06 +0200

Zhan Xusheng reported running into sporadic a s64 mult overflow in
vruntime_eligible().

When constructing a worst case scenario:

If you have cgroups, then you can have an entity of weight 2 (per
calc_group_shares()), and its vlag should then be bounded by: (slice+TICK_NSEC)
* NICE_0_LOAD, which is around 44 bits as per the comment on entity_key().

The other extreme is 100*NICE_0_LOAD, thus you get:

{key, weight}[] := {
  puny: { (slice + TICK_NSEC) * NICE_0_LOAD, 2               },
  max:  { 0,                                 100*NICE_0_LOAD },
}

The avg_vruntime() would end up being very close to 0 (which is
zero_vruntime), so no real help making that more accurate.

vruntime_eligible(puny) ends up with:

 avg  = 2 * puny.key (+ 0)
 load = 2 + 100 * NICE_0_LOAD

 avg >= puny.key * load

And that is: (slice + TICK_NSEC) * NICE_0_LOAD * NICE_0_LOAD * 100, which will
overflow s64.

Zhan suggested using __builtin_mul_overflow(), however after staring at
compiler output for various architectures using godbolt, it seems that using an
__int128 multiplication often results in better code.

Specifically, a number of architectures already compute the __int128 product to
determine the overflow. Eg. arm64 already has the 'smulh' instruction used. By
explicitly doing an __int128 multiply, it will emit the 'mul; smulh' pattern,
which modern cores can fuse (armv8-a clang-22.1.0). x86_64 has less branches
(no OF handling).

Since Linux has ARCH_SUPPORTS_INT128 to gate __int128 usage, also provide the
__builtin_mul_overflow() variant as a fallback.

[peterz: Changelog and __int128 bits]
Fixes: 556146ce5e94 ("sched/fair: Avoid overflow in enqueue_entity()")
Reported-by: Zhan Xusheng <zhanxusheng1024@xxxxxxxxx>
Closes: https://patch.msgid.link/20260415145742.10359-1-zhanxusheng%40xiaomi.com
Signed-off-by: Zhan Xusheng <zhanxusheng@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/sched/fair.c |   39 ++++++++++++++++++++++++++++++++++-----
 1 file changed, 34 insertions(+), 5 deletions(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -882,11 +882,11 @@ bool update_entity_lag(struct cfs_rq *cf
  *
  * lag_i >= 0 -> V >= v_i
  *
- *     \Sum (v_i - v)*w_i
- * V = ------------------ + v
+ *     \Sum (v_i - v0)*w_i
+ * V = ------------------- + v0
  *          \Sum w_i
  *
- * lag_i >= 0 -> \Sum (v_i - v)*w_i >= (v_i - v)*(\Sum w_i)
+ * lag_i >= 0 -> \Sum (v_i - v0)*w_i >= (v_i - v0)*(\Sum w_i)
  *
  * Note: using 'avg_vruntime() > se->vruntime' is inaccurate due
  *       to the loss in precision caused by the division.
@@ -894,7 +894,7 @@ bool update_entity_lag(struct cfs_rq *cf
 static int vruntime_eligible(struct cfs_rq *cfs_rq, u64 vruntime)
 {
 	struct sched_entity *curr = cfs_rq->curr;
-	s64 avg = cfs_rq->sum_w_vruntime;
+	s64 key, avg = cfs_rq->sum_w_vruntime;
 	long load = cfs_rq->sum_weight;
 
 	if (curr && curr->on_rq) {
@@ -904,7 +904,36 @@ static int vruntime_eligible(struct cfs_
 		load += weight;
 	}
 
-	return avg >= vruntime_op(vruntime, "-", cfs_rq->zero_vruntime) * load;
+	key = vruntime_op(vruntime, "-", cfs_rq->zero_vruntime);
+
+	/*
+	 * The worst case term for @key includes 'NSEC_TICK * NICE_0_LOAD'
+	 * and @load obviously includes NICE_0_LOAD. NSEC_TICK is around 24
+	 * bits, while NICE_0_LOAD is 20 on 64bit and 10 otherwise.
+	 *
+	 * This gives that on 64bit the product will be at least 64bit which
+	 * overflows s64, while on 32bit it will only be 44bits and should fit
+	 * comfortably.
+	 */
+#ifdef CONFIG_64BIT
+#ifdef CONFIG_ARCH_SUPPORTS_INT128
+	/* This often results in simpler code than __builtin_mul_overflow(). */
+	return avg >= (__int128)key * load;
+#else
+	s64 rhs;
+	/*
+	 * On overflow, the sign of key tells us the correct answer: a large
+	 * positive key means vruntime >> V, so not eligible; a large negative
+	 * key means vruntime << V, so eligible.
+	 */
+	if (check_mul_overflow(key, load, &rhs))
+		return key <= 0;
+
+	return avg >= rhs;
+#endif
+#else /* 32bit */
+	return avg >= key * load;
+#endif
 }
 
 int entity_eligible(struct cfs_rq *cfs_rq, struct sched_entity *se)