linux-kernel - Re: [PATCH 06/14] sched_ext: Add a DL server for sched

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Message-ID: <aPKTsSd8uk6RB28s@gpd4>
Date: Fri, 17 Oct 2025 21:06:25 +0200
From: Andrea Righi <arighi@...dia.com>
To: Tejun Heo <tj@...nel.org>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	Joel Fernandes <joelagnelf@...dia.com>,
	David Vernet <void@...ifault.com>,
	Changwoo Min <changwoo@...lia.com>, Shuah Khan <shuah@...nel.org>,
	sched-ext@...ts.linux.dev, bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Luigi De Matteis <ldematteis123@...il.com>
Subject: Re: [PATCH 06/14] sched_ext: Add a DL server for sched_ext tasks

On Fri, Oct 17, 2025 at 09:04:23AM -1000, Tejun Heo wrote:
> On Fri, Oct 17, 2025 at 08:58:35PM +0200, Andrea Righi wrote:
> > On Fri, Oct 17, 2025 at 05:47:45AM -1000, Tejun Heo wrote:
> > > On Fri, Oct 17, 2025 at 11:25:53AM +0200, Andrea Righi wrote:
> > > > +static struct task_struct *
> > > > +ext_server_pick_task(struct sched_dl_entity *dl_se, struct rq_flags *rf)
> > > > +{
> > > > +	return pick_task_scx(dl_se->rq, rf);
> > > > +}
> > > 
> > > I wonder whether we should tell pick_task_scx() to suppress the
> > > rq_modified_above() test in this case as a fair or RT task being enqueued
> > > has no reason to restart the picking process. While it will behave fine on
> > > retry, it's probably useful to be explicit here.
> > 
> > Yeah, that's a valid point. Maybe we can add a new flag to rq->scx.flags?
> > Something like SCX_RQ_DL_SERVER_PICK?
> 
> We can factor out the internals of pick_task_scx() into a separate function
> and add a flag there?

Much better, I like that. Ok, I'll incorporate this change and send a new
version.

Thanks,
-Andrea