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

[Andrea Righi <arighi@nvidia.com>]

On Mon, Feb 02, 2026 at 05:19:51AM -1000, Tejun Heo wrote:
> Hello,
> 
> On Sat, Jan 31, 2026 at 09:26:56PM +0100, Andrea Righi wrote:
> > I agree that SCX_DSQ_GLOBAL behaves differently from user-created DSQs at
> > the implementation level, but I think that difference shouldn't leak into
> > the logical model.
> > 
> > From a semantic point of view, dispatching a task to SCX_DSQ_GLOBAL does
> > not mean that the task leaves the "enqueued by BPF" state. The task is
> > still under the BPF scheduler's custody, not directly dispatched to a
> > specific CPU, and remains sched_ext-managed. The scheduler has queued the
> > task and it hasn't relinquished control over it.
> > 
> > That said, I don't have a strong opinion here. If we prefer to treat
> > SCX_DSQ_GLOBAL as a "direct dispatch" DSQ for the purposes of ops.dequeue()
> > semantics, then I'm fine with adjusting the logic accordingly (with proper
> > documentation).
> > 
> > Tejun, thoughts?
> 
> I think putting a task into GLOBAL means that the BPF scheduler is done with
> it. Another data point in this direction is that when insertion into a local
> DSQ can't be done, the task falls back to the global DSQ although all the
> current ones also trigger error.

Alright, it seems that the general consensus, based on your feedback and
Kuba's, is to treat SCX_DSQ_GLOBAL as a "terminal" DSQ for the purpose of
triggering ops.dequeue().

I'll update the logic to do the following:
 - When a task is dispatched to SCX_DSQ_GLOBAL, the BPF scheduler is
   considered done with it (similar to local DSQ dispatches).
 - ops.dequeue() will not be called for SCX_DSQ_GLOBAL dispatches.
 - This aligns with the fallback behavior where tasks that fail local DSQ
   insertion end up in the global DSQ as a terminal destination.

Thanks,
-Andrea