Re: [PATCH v2 sched_ext/for-7.1] sched_ext: Invalidate dispatch decisions on CPU affinity changes

[Tejun Heo]

Hello,

On Fri, Mar 20, 2026 at 09:18:20AM +0000, Kuba Piecuch wrote:
...
> The simple way to do this is to do scx_bpf_dsq_insert() at the very beginning,
> once we know which task we would like to dispatch, and cancel the pending
> dispatch via scx_bpf_dispatch_cancel() if any of the pre-dispatch checks fail
> on the BPF side. This way, the "critical section" includes BPF-side checks, and
> SCX will ignore the dispatch if there was a dequeue/enqueue racing with the
> critical section.
> 
> With this solution, we can throw an error if task_can_run_on_remote_rq() is
> false, because we know that there was no racing cpumask change (if there was,
> it would have been caught earlier, in finish_dispatch()).

Yeah, I think this makes more sense. qseq is already there to provide
protection against these events. It's just that the capturing of qseq is too
late. If insert/cancel is too ugly, we can introduce another kfunc to
capture the qseq - scx_bpf_dsq_insert_begin() or something like that - and
stash it in a per-cpu variable. That way, qseq would be cover the "current"
queued instance and the existing qseq mechanism would be able to reliably
ignore the ones that lost race to dequeue.

Thanks.

-- 
tejun