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

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Message-ID: <CAKfTPtDG9Fz8o1TVPe3w2eNA+Smhmq2utSA_c6X4GJJgt_dAJA@mail.gmail.com>
Date: Wed, 8 Oct 2025 17:22:42 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: tj@...nel.org, linux-kernel@...r.kernel.org, mingo@...nel.org, 
	juri.lelli@...hat.com, dietmar.eggemann@....com, rostedt@...dmis.org, 
	bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com, longman@...hat.com, 
	hannes@...xchg.org, mkoutny@...e.com, void@...ifault.com, arighi@...dia.com, 
	changwoo@...lia.com, cgroups@...r.kernel.org, sched-ext@...ts.linux.dev, 
	liuwenfang@...or.com, tglx@...utronix.de, 
	Joel Fernandes <joelagnelf@...dia.com>
Subject: Re: [RFC][PATCH 2/3] sched: Add support to pick functions to take rf

On Wed, 8 Oct 2025 at 15:58, Peter Zijlstra <peterz@...radead.org> 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@linaro.org/
>
> Let me go have a look.