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

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Message-ID: <CAKfTPtApJuw=Ad8aX=p3VLvMojyoxVBVRbMG80ADXR-NVL0PTw@mail.gmail.com>
Date: Thu, 9 Oct 2025 09:17:44 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Tejun Heo <tj@...nel.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

On Wed, 8 Oct 2025 at 22:34, Tejun Heo <tj@...nel.org> wrote:
>
> 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?

The problem is that with dl_server, you can has a prev of a lower prio
but still want to run a newidle balance :
-cfs task preempted by dl server
-cfs task migrates to another cpu
-we want to run newidle balance when cpu become idle

>
> Thanks.
>
> --
> tejun