Re: [PATCH v2 5/7] sched/fair: Increase weight bits for avg_vruntime

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Feb 23, 2026 at 11:56:33AM +0100, Vincent Guittot wrote:
> On Thu, 19 Feb 2026 at 09:10, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > Due to the zero_vruntime patch, the deltas are now a lot smaller and
> > measurement with kernel-build and hackbench runs show about 45 bits
> > used.
> >
> > This ensures avg_vruntime() tracks the full weight range, reducing
> > numerical artifacts in reweight and the like.
> 
> Instead of paranoid, would it be better to add WARN_ONCE ?
> 
> I'm afraid that we will not notice any potential overflow without a
> long study of the regression with SCHED_FEAT(PARANOID_AVG, false)
> 
> Couldn't we add a cheaper WARN_ONCE (key > 2^50)  in __sum_w_vruntime_add ?
> 
> We should always have
> key < 110ms (max slice+max tick) * nice_0 (2^20) / weight (2)
> key < 2^46
> 
> We can use 50 bits to get margin
> 
> Weight is always less than 27bits and key*weight gives us 110ms (max
> slice+max tick) * nice_0 (2^20) so we should never add more than 2^47
> to ->sum_weight
> 
> so a WARN_ONCE (cfs_rq->sum_weight > 2^63) should be enough

Ha, I was >< close to pushing out these patches when I saw this.

The thing is signed, so bit 63 is the sign bit, but I suppose we can
test bit 62 like so:

Let me go build and boot that.

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -679,9 +679,13 @@ static inline void
 __sum_w_vruntime_add(struct cfs_rq *cfs_rq, struct sched_entity *se)
 {
 	unsigned long weight = avg_vruntime_weight(cfs_rq, se->load.weight);
-	s64 key = entity_key(cfs_rq, se);
+	s64 w_vruntime, key = entity_key(cfs_rq, se);
 
-	cfs_rq->sum_w_vruntime += key * weight;
+	w_vruntime = key * weight;
+
+	WARN_ON_ONCE((w_vruntime >> 63) != (w_vruntime >> 62));
+
+	cfs_rq->sum_w_vruntime += w_vruntime;
 	cfs_rq->sum_weight += weight;
 }