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

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Message-ID: <aObK2MfxPyFcovwr@slm.duckdns.org>
Date: Wed, 8 Oct 2025 10:34:32 -1000
From: Tejun Heo <tj@...nel.org>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: Peter Zijlstra <peterz@...radead.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

Hello,

On Wed, Oct 08, 2025 at 05:22:42PM +0200, Vincent Guittot wrote:
> 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

If we pass in @prev into pick(), can't pick() decide whether to newidle
balance or not based on that?

Thanks.

-- 
tejun