Re: [PATCH v3 3/3] sched_ext: Allow scx_bpf_reenqueue_local() to be called from anywhere

[Tejun Heo <tj@kernel.org>]

Hello, Kuba.

On Thu, Dec 11, 2025 at 02:24:04PM +0000, Kuba Piecuch wrote:
> Since we're doing a direct dispatch, the user scheduler task will be
> inserted into the dispatch queue in enable_task_scx(), without dropping the rq
> lock between example_enqueue() and the insertion, which means reenq_local()
> will run afterwards (since it's deferred using irq_work), removing all tasks
> from the DSQ, including the userspace scheduler.
> 
> A similar problem arises even if we don't do direct dispatch and drop the rq
> lock after example_enqueue(): since dispatching and reenq_local() are deferred
> using different irq_work entries, and irq_work_run() processes entries from
> newest to oldest, dispatching will be handled before reenq_local(), yielding
> the same result.

Oh yeah, the asynchronity can become pretty confusing.

> The user may be unaware of this behavior (it's not mentioned anywhere) and
> expect the reenqueue to happen before dispatching the new task.
> 
> I think at the very least we should make users aware of this in the comment
> for scx_bpf_reenqueue_local___v2().

Documentation is always helpful but I wonder whether this can be improved by
the reenqueue function capturing the dsq seq number and re-enqueueing only
the ones that were enqueued before the reenqueue was called.

Thanks.

-- 
tejun