Re: [PATCH v2] sched_ext: Rebuild fair weight on ext to fair switches

[Zicheng Qu <quzicheng315@gmail.com>]

On Wed, May 27, 2026 at 07:26PM +0800, Peter Zijlstra wrote:

> This is truly horrible. We have 4 class methods involved with switching
> classes and you stick in a random call in a place that is called when no
> class is changed.
>
> Would not something like this work?
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 62a2dcb0d03e..a2eb43bd73b9 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -14957,6 +14957,11 @@ static void switched_from_fair(struct rq *rq, struct task_struct *p)
>   	detach_task_cfs_rq(p);
>   }
>   
> +static void switching_to_fair(struct rq *rq, struct task_struct *p)
> +{
> +	set_load_weight(p, false);
> +}
> +
>   static void switched_to_fair(struct rq *rq, struct task_struct *p)
>   {
>   	WARN_ON_ONCE(p->se.sched_delayed);
> @@ -15351,6 +15356,7 @@ DEFINE_SCHED_CLASS(fair) = {
>   	.prio_changed		= prio_changed_fair,
>   	.switching_from		= switching_from_fair,
>   	.switched_from		= switched_from_fair,
> +	.switching_to		= switching_to_fair,
>   	.switched_to		= switched_to_fair,
>   
>   	.get_rr_interval	= get_rr_interval_fair,
Yes, from the class switch point of view, `switching_to_fair()` is a better
fit.

Before v2, I was weighing three possible places for the fix:

1. Updating `p->se.load` from `reweight_task_scx()`. This would keep the 
fair
weight in sync while the task is on sched_ext, so switching back to fair 
would
not need any extra fixup. However, it would also make sched_ext maintain 
fair
class state even when fair is not using it, which does not seem like the 
right
ownership model.

2. Rebuilding `p->se.load` from fair's `switching_to` hook. This is the most
natural place semantically, since the task is entering fair and fair 
prepares
its own state before enqueue. My only concern was that, for non-ext -> fair
paths, `__setscheduler_params()` may have already updated `p->se.load` 
through
`set_load_weight(p, true)`, so calling `set_load_weight(p, false)`
unconditionally here can be redundant logically. Functionally, though, it is
harmless.

3. Rebuilding in `sched_change_end()` based on the old/new classes. That was
the v2 choice because both classes are available there, the task has not 
been
enqueued yet, and it covers both `scx_root_disable()` and the partial-mode
`sched_setscheduler()` path. In hindsight, though, this makes the generic
sched_change path handle a scx & fair-specific fixup. That is more awkward
than letting fair prepare its own state in `switching_to_fair()`.

I'll respin v3 as you suggested.


Thanks,

Zicheng