linux-kernel - Re: [PATCH v5 08/14] sched/ext: Add a DL server for sched

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Message-ID: <aGKvW08eOgK-RyQ3@slm.duckdns.org>
Date: Mon, 30 Jun 2025 05:38:03 -1000
From: Tejun Heo <tj@...nel.org>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: linux-kernel@...r.kernel.org, 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>,
	David Vernet <void@...ifault.com>, Andrea Righi <arighi@...dia.com>,
	Changwoo Min <changwoo@...lia.com>,
	Luigi De Matteis <ldematteis123@...il.com>,
	sched-ext@...ts.linux.dev
Subject: Re: [PATCH v5 08/14] sched/ext: Add a DL server for sched_ext tasks

Hello, Joel.

On Mon, Jun 30, 2025 at 11:12:52AM -0400, Joel Fernandes wrote:
...
> > One thing that confuses me is that all that the patchset needs to do seems
> > to be adding @rf to dl_server_pick_f and that seems fine to me. Why is it
> > necessary to add @rf to sched_class->pick_task()?
> 
> Because ext_server_pick_task is called via DL's pick_task?
> 
> In deadline.c, pick_task_dl -> _pick_task_dl -> ext_server_pick_task

Ah, right, sorry about being so dense.

> This changes the signature of the pick_task_dl function, which in turn
> changes the signature of class->pick_task.
> 
> How about I pass NULL to pick_task_scx() from ext_server_pick_task(), and
> also annotate all functions where rf is unused, by naming the argument as
> rf_unused (except for DL), would that make it more clear that the rq lock
> should not be arbitrarily dropped just because rf was passed? And perhaps
> sprinkling some more code comments.

I think what bothers me is that this erases the distinction between
->balance() and ->pick_task(). However, I'm not sure the distinction means
anything anymore especially given the removal of !CONFIG_SMP paths. Looking
at the balance callsites, I think we can just fold each ->balance() into the
head of the corresponding ->pick_task(). Peter, what do you think?

Thanks.

-- 
tejun