linux-kernel - Re: [PATCH 1/2] sched_ext: Fix ops.dequeue() semantics

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Message-ID: <aYowT16VM-JnU6dJ@gpd4>
Date: Mon, 9 Feb 2026 20:06:55 +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 1/2] sched_ext: Fix ops.dequeue() semantics

On Mon, Feb 09, 2026 at 07:28:50AM -1000, Tejun Heo wrote:
> On Sat, Feb 07, 2026 at 10:26:17AM +0100, Andrea Righi wrote:
> > Hi Emil,
> > 
> > On Fri, Feb 06, 2026 at 03:35:34PM -0500, Emil Tsalapatis wrote:
> > > On Fri Feb 6, 2026 at 8:54 AM EST, Andrea Righi wrote:
> > ...
> > > > diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
> > > > index bcb962d5ee7d8..c48f818eee9b8 100644
> > > > --- a/include/linux/sched/ext.h
> > > > +++ b/include/linux/sched/ext.h
> > > > @@ -84,6 +84,7 @@ struct scx_dispatch_q {
> > > >  /* scx_entity.flags */
> > > >  enum scx_ent_flags {
> > > >  	SCX_TASK_QUEUED		= 1 << 0, /* on ext runqueue */
> > > > +	SCX_TASK_NEED_DEQ	= 1 << 1, /* in BPF custody, needs ops.dequeue() when leaving */
> > > 
> > > Can we make this "SCX_TASK_IN_BPF"? Since we've now defined what it means to be
> > > in BPF custody vs the core scx scheduler (terminal DSQs) this is a more
> > > general property that can be useful to check in the future. An example:
> > > We can now assert that a task's BPF state is consistent with its actual 
> > > kernel state when using BPF-based data structures to manage tasks.
> > 
> > Ack. I like SCX_TASK_IN_BPF and I also like the idea of resuing the flag
> > for other purposes. It can be helpful for debugging as well.
> 
> One problem with the name is that when a task is in the BPF scheduler's
> custody, it can be still be on the kernel side in a DSQ or can be on the BPF
> side on a BPF data structure. This is currently distinguished by SCX_OPSS
> state (queued on the ops side or not). We do say things like "the task is in
> BPF" to note that the task is not on a DSQ but in BPF proper, so I think
> SCX_TASK_IN_BPF can become confusing.
> 
> I don't know what the right name is. When we write it out, we say "in BPF
> sched's custody" where "BPF sched" means the whole SCX scheduler. Maybe just
> SCX_TASK_IN_CUSTODY?

Yeah, I agree that the "task in BPF" concept is a bit too overloaded. I
think SCX_TASK_IN_CUSTODY is clear enough and it doesn't overlap with the
"in BPF" concept. I'll rename the flag to SCX_TASK_IN_CUSTODY.

Thanks,
-Andrea