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

[Kuba Piecuch <jpiecuch@google.com>]

Hi Cheng-Yang,

On Sun Apr 26, 2026 at 1:47 AM UTC, Cheng-Yang Chou wrote:
> Hi Kuba,
>
> On Thu, Apr 23, 2026 at 01:32:20PM +0000, Kuba Piecuch wrote:
>> > On Mon, Mar 23, 2026 at 01:13:20PM -1000, Tejun Heo 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.
>> >
>> > Since this has been stale for a while, I prepared a patch to implement
>> > scx_bpf_dsq_insert_begin() as suggested.
>> 
>> Thanks for creating the patch. A couple of thoughts:
>> 
>> 1. Do we have a use case that requires dsq_insert_begin() that isn't
>>    satisfied using the "insert and then cancel if needed" approach?
>
> IIUC, yes. scx_bpf_dispatch_cancel() is only registered in 
> scx_kfunc_ids_dispatch, so it is only callable from ops.dispatch().
> dsq_insert_begin(), on the other hand, is available from both
> ops.enqueue() and ops.dispatch() (SCX_KF_ENQUEUE | SCX_KF_DISPATCH).
> Since there is nothing to cancel in ops.enqueue(), the insert-and-cancel
> approach simply doesn't work there.

Wouldn't the natural thing then be to extend scx_bpf_dispatch_cancel() to
work for direct dispatch? Instead of introducing a whole new mechanism, let's
extend the one we have by functionality that it (arguably) should have had
from the beginning.

>
>> 
>> 2. Do we want to restrict ourselves through the one qseq slot provided by
>>    dsq_insert_begin()? The most flexible approach IMO would be to simply
>>    allow BPF to read the qseq directly via a kfunc and then supply it to
>>    dsq_insert() later. With this, we can have multiple qseqs saved at the
>>    same time, and we can even pass them between CPUs, e.g. if one CPU
>>    dequeues a task for a sibling CPU, but we want the checks to be made inside
>>    the sibling's ops.dispatch() (I just made this use case it up, it may not
>>    be practical.)
>>    That said, exposing an internal thing like qseq to BPF may be a step too far.
>
> In Tejun's reply back in [1], he suggested dsq_insert_begin() precisely
> to avoid promoting qseq into the BPF ABI — which matches your own concern.
> The single per-CPU slot is sufficient for the one-task-per-iteration
> dispatch loops used by existing schedulers (e.g., scx_central).
> If a concrete cross-CPU use case materializes later, we can always extend
> dsq_insert() to accept an explicit qseq without breaking the current,
> simpler path.
>
> [1]: https://lore.kernel.org/all/acHJED4iAeytdC2l@slm.duckdns.org/
>

Well, Tejun doesn't explicitly say there that he's against exposing qseq, but
I won't be surprised if he is.

FWIW, ghOSt (our Google-internal BPF scheduling solution) uses exactly this
approach to guard the dispatch path against racing dequeues/enqueues.
Every task has a seqnum that gets incremented on each "event" pertaining to
the task. In the dispatch path, the BPF scheduler reads the task seqnum,
does whatever checks it needs to do, and passes the seqnum to ghOSt at the end.

Admittedly, what works downstream doesn't have to work upstream, but I still
wanted to provide this data point :-)

Thanks,
Kuba