Re: [PATCH v5 08/14] sched/ext: Add a DL server for sched_ext tasks

[Tejun Heo]

On Fri, Jun 20, 2025 at 04:32:23PM -0400, Joel Fernandes wrote:
> +static struct task_struct *ext_server_pick_task(struct sched_dl_entity *dl_se,
> +						void *flags)
> +{
> +	struct rq_flags *rf = flags;
> +
> +	balance_scx(dl_se->rq, dl_se->rq->curr, rf);
> +	return pick_task_scx(dl_se->rq, rf);
> +}
...
> +void ext_server_init(struct rq *rq)
> +{
> +	struct sched_dl_entity *dl_se = &rq->ext_server;
> +
> +	init_dl_entity(dl_se);
> +
> +	dl_server_init(dl_se, rq, ext_server_has_tasks, ext_server_pick_task);
> +}

Needing to pass in @rf to ext_server_pick_task() makes sense as SCX always
needs to balance first. However, I still don't understand why that
necessitates adding @rf to sched_class->pick_task(). The existing assumption
is that ->pick_task() cannot release and regrab the rq lock and that's why
->balance() exists in the first place. Breaking this can lead to other
misbehaviors - e.g. if a higher priority class ->pick_task() releases the rq
lock, a lower one may end up losing a task that its ->balance() saw. This
can lead to lower priority class's ->pick_task() being called without its
->balance() being called which can lead to stalls.

One thing that confuses me is that all that the patchset needs to do seems
to be adding @rf to dl_server_pick_f and that seems fine to me. Why is it
necessary to add @rf to sched_class->pick_task()?

Thanks.

-- 
tejun