Re: [RFC][PATCH 2/3] sched: Add support to pick functions to take rf

[Vincent Guittot]

On Wed, 8 Oct 2025 at 15:58, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Wed, Oct 08, 2025 at 03:16:58PM +0200, Vincent Guittot wrote:
>
> > > +static struct task_struct *
> > > +fair_server_pick_task(struct sched_dl_entity *dl_se, struct rq_flags *rf)
> > >  {
> > > -       return pick_next_task_fair(rq, prev, NULL);
> >
> > The special case of a NULL rf pointer is used to skip
> > sched_balance_newidle() at the end of pick_next_task_fair() in the
> > pick_next_task() slo path when prev_balance has already it. This means
> > that it will be called twice if prev is not a fair task.
>
> Oh right. I suppose we can simply remove balance_fair.

That was the option that I also had in mind but this will change from
current behavior and I'm afraid that sched_ext people will complain.
Currently, if prev is sched_ext, we don't call higher class.balance()
which includes the fair class balance_fair->sched_balance_newidle.  If
we now always call sched_balance_newidle() at the end
pick_next_task_fair(), we will try to pull a fair task at each
schedule between sched_ext tasks

>
> > While reviewing this series, I also noticed an older issue that we
> > have with check pelt lost idle time [1]
> > [1] https://lore.kernel.org/all/20251008131214.3759798-1-vincent.guittot@xxxxxxxxxx/
>
> Let me go have a look.