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

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Message-ID: <20250903205646.GR4067720@noisy.programming.kicks-ass.net>
Date: Wed, 3 Sep 2025 22:56:46 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Tejun Heo <tj@...nel.org>
Cc: Andrea Righi <arighi@...dia.com>, 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.

I thought the idea was to add rf unconditionally, dl-server or not, it
is needed in both cases.

Yes, that race needs dealing with. We have this existing pattern that
checks if a higher class has runnable tasks and restarting the pick.
This is currently only done for pick_next_task_fair() but that can
easily be extended.

You suggested maybe moving this to the ttwu side -- but up to this point
I thought we were in agreement. I'm not sure moving it to the ttwu side
makes things better; it would need ttwu to know a pick is in progress
and for which class. The existing restart pick is simpler, I think.

Yes, the restart is somewhat more complicated if we want to deal with
the dl-server, but not terribly so. It could just store a snapshot of
rq->dl.dl_nr_running from before the pick and only restart if that went
up.