Re: [PATCH v2] sched: fair: Prevent negative lag increase during delayed dequeue

[Peter Zijlstra]

On Wed, Apr 22, 2026 at 04:06:42PM +0200, Vincent Guittot wrote:
> On Wed, 22 Apr 2026 at 15:39, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> > That said, on IRQ you mentioned that this wasn't quite good enough and
> > that your original patch is best.
> 
> Yes, it fixes one case but breaks the other one :-(
> 
> >
> > The trouble is, your original patch can update vlag (!se->sched_delayed)
> > and report it hasn't changed; because then vlag == se->clag, obviously.
> 
> In the  (!se->sched_delayed),  we don't care because the entity is not
> enqueued so we don't need to place it with new vlag

I am confused more :-) this could be dequeue_entity() doing a normal
dequeue, in which case it very much is enqueued. We only delay for
!eligible, !special etc..

> > This invalidates the comment on the return value of the function. In
> > fact, it makes the function have a very non-obvious return meaning.
> >
> > So I'm a little confused -- what do we actually want this function to
> > do?
> 
> I want update_entity_lag() to return true if we have modified the vlag
> of an enqueued entity. In this case we need to dequeue, place entity
> with new vlag and enqueue it.
> 
> we don't need to test se->on_rq because update_entity_lag() is called
> for enqueued task only with delayed dequeue entity so
> se->sched_delayed implies se->on_rq
> 
> In fact we should test :
> (vlag != se->vlag) && se->on_rq
> but  && se->on_rq is useless
> 
> That being said, this probably deserves a comment

But but, dequeue_entity()'s second update_entity_lag() call can have:

  se->sched_delayed == 0 && se->on_rq == 1

Think dequeue of eligible or special or...