Re: [PATCH] sched/fair: clamp rescaled vlag in reweight_entity() to bound entity_key()

[Peter Zijlstra]

On Wed, Jun 10, 2026 at 10:55:29AM -0400, Rik van Riel wrote:

> reweight_entity() preserves an entity's lag across a weight change by
> scaling vlag in rescale_entity():
> 
> 	se->vlag = vl' = vl * old_weight / new_weight;
> 
> and then, for an on_rq entity, recomputes:
> 
> 	se->vruntime = avruntime - se->vlag;
> 
> On a large weight decrease (w' << w) this inflates vlag without bound;
> nothing re-clamps it to the per-entity lag limit that entity_lag() and
> update_entity_lag() enforce everywhere else.  The deadline is rescaled
> and re-based separately, so only vruntime drifts.

So the vlag limit is:

	limit = calc_delta_fair(max_slice, se)

Right? But that is in fact:

          max_slice * NICE_0_LOAD
  limit = -----------------------
	       se->weight

Now, rescale does:

          vlag * old_weight
  vlag =  -----------------
             new_weight

So then transform the limit using that same:

          max_slice * NICE_0_LOAD   old_weight   max_slice * NICE_0_LOAD
  limit = ----------------------- * ---------- = -----------------------
                old_weight          new_weight          new_weight

IOW, the transform totally preserves the limit. If the vlag was bound
before, it must be bound after.

What am I missing?