linux-kernel - Re: [PATCH 2/2] selftests/sched_ext: Add test to validate ops.dequeue() semantics

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Message-ID: <aYpeG8ZUHKizSLAE@gpd4>
Date: Mon, 9 Feb 2026 23:22:19 +0100
From: Andrea Righi <arighi@...dia.com>
To: Tejun Heo <tj@...nel.org>
Cc: Emil Tsalapatis <emil@...alapatis.com>,
	David Vernet <void@...ifault.com>,
	Changwoo Min <changwoo@...lia.com>,
	Kuba Piecuch <jpiecuch@...gle.com>,
	Christian Loehle <christian.loehle@....com>,
	Daniel Hodges <hodgesd@...a.com>, sched-ext@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] selftests/sched_ext: Add test to validate
 ops.dequeue() semantics

On Mon, Feb 09, 2026 at 10:10:30AM -1000, Tejun Heo wrote:
> Hello,
> 
> On Mon, Feb 09, 2026 at 08:17:24PM +0100, Andrea Righi wrote:
> > Agreed. And just to be clear, for the purpose of triggering ops.dequeue(),
> > **all** direct dispatches from ops.select_cpu() should be consistently
> > ignored, including dispatches to user DSQs. I'll update this behavior in
> > the next version, because this one treats direct dispatches to user DSQs
> > from ops.select_cpu() as if the task is in the scheduler's custody, which
> > shouldn't be the case for consistency.
> 
> I'm not sure about that. ops.select_cpu() doing direct dispatch is just a
> shortcut and should be treated like the same operation being done at the
> head of ops.enqueue(). That's what's happening semantically and I think we
> should stick with what's happening underneath - ie. make ops.select_cpu()'s
> shortcut the special case, not whether tasks in a user DSQ get ops.dequeue()
> or not.

Ok, what you're saying is that a direct dispatch from ops.select_cpu() is
just a shortcut for work that would otherwise happen at the head of
ops.enqueue().

So, while ops.select_cpu() itself is not "being in scheduler custody", the
semantic operation of dispatching a task is still the scheduler taking
control of the task. As a result, a dispatch to a user DSQ from
ops.select_cpu() should be treated the same as a dispatch to a user DSQ
from ops.enqueue() for the purpose of triggering ops.dequeue(). The fact
that this happens in ops.select_cpu() rather than ops.enqueue() is an
implementation detail, not a semantic boundary.

Under this interpretation, storing a task in BPF internal data structures
from ops.select_cpu() should not trigger ops.dequeue(), since the task has
not been put under scheduler control yet. However, dispatching a task to a
user DSQ, regardless of whether it happens from ops.select_cpu() or
ops.enqueue(), does explicitly place the task in the scheduler's custody.

If this matches what you mean, then I agree with this approach.

Thanks,
-Andrea