linux-kernel - Re: [PATCH 13/14] sched: Add {DE,EN}QUEUE

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Message-ID: <20250911094240.GW3289052@noisy.programming.kicks-ass.net>
Date: Thu, 11 Sep 2025 11:42:40 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Tejun Heo <tj@...nel.org>
Cc: linux-kernel@...r.kernel.org, mingo@...hat.com, juri.lelli@...hat.com,
	vincent.guittot@...aro.org, 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
Subject: Re: [PATCH 13/14] sched: Add {DE,EN}QUEUE_LOCKED

On Wed, Sep 10, 2025 at 04:01:55PM -1000, Tejun Heo wrote:
> Hello, Peter.
> 
> On Wed, Sep 10, 2025 at 05:44:22PM +0200, Peter Zijlstra wrote:
> > Provide a LOCKED queue flag, indicating that the {en,de}queue()
> > operation is in task_rq_lock() context.
> > 
> > Note: the sched_change in scx_bypass() is the only one that does not
> > use task_rq_lock(). If that were fixed, we could have sched_change
> > imply LOCKED.
> 
> I don't see any harm in doing task_rq_lock() in the scx_bypass() loop.
> Please feel free to switch that for simplicity.

I didn't immediately see how to do that. Doesn't that
list_for_each_entry_safe_reverse() rely on rq->lock to retain integrity?

Moreover, since the goal is to allow:

 __schedule()
   lock(rq->lock);
   next = pick_task() := pick_task_scx()
     lock(dsq->lock);
     p = some_dsq_task(dsq);
     task_unlink_from_dsq(p, dsq);
     set_task_cpu(p, cpu_of(rq));
     move_task_to_local_dsq(p, ...);
     return p;

without dropping rq->lock, by relying on dsq->lock to serialize things,
I don't see how we can retain the runnable list at all.

And at this point, I'm not sure I understand ext well enough to know
what this bypass stuff does at all, let alone suggest means to
re architect this.