linux-kernel - Re: [PATCH 07/16] sched_ext: Add a DL server for sched

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Message-ID: <aLi0OOISvMNn7j59@gpd4>
Date: Wed, 3 Sep 2025 23:33:44 +0200
From: Andrea Righi <arighi@...dia.com>
To: Tejun Heo <tj@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
	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 07/16] sched_ext: Add a DL server for sched_ext tasks

On Wed, Sep 03, 2025 at 10:41:21AM -1000, Tejun Heo wrote:
> Hello,
> 
> On Wed, Sep 03, 2025 at 10:08:22PM +0200, Peter Zijlstra wrote:
> > > I'm a bit confused. This series doesn't have prep patches to add @rf to
> > > dl_server_pick_f. Is this the right patch?
> > 
> > Patch 14 seems to be the proposed alternative, and I'm not liking that
> > at all.
> > 
> > That rf passing was very much also needed for that other issue; I'm not
> > sure why that's gone away.
> 
> Using balance() was my suggestion to stay within the current framework. If
> we want to add @rf to pick_task(), that's more fundamental change. We
> dropped the discussion in the other thread but I found it odd to add @rf to
> pick_task() while disallowing the use of @rf in non-dl-server pick path and
> if we want to allow that, we gotta solve the race between pick_task()
> dropping rq lock and the ttwu inserting high pri task.

Yeah, patch 14 is fixing this, but this needs to be changed, because we
dropped the patch that adds @rf to pick_task(). I'll fix this in the next
version if we decide to stick with this way.

About balance() vs @rf, IIUC after pick_task() drops the rq lock a
concurrent ttwu() can already enqueue a higher-priority task, so the race
isn't really specific to @rf and it's more about making sure we don't start
using @rf in ways that rely on the pick being stable until the actual
switch, right?

If that’s correct, extending @rf to pick_task() wouldn't make things worse
than what we have, though sticking with balance() may still be the safer
incremental step...

Thanks,
-Andrea