Re: [PATCH 12/12] sched_ext: scx_qmap - Add rescue support

On Sat, Aug 01, 2026 at 09:06:02AM +0000, [email protected] wrote:
> [Severity: High]
> Does removing the !(enq_flags & SCX_ENQ_REENQ) check here cause an
> infinite bounce loop when rescue is disabled kernel-side?
>
> If scx_qmap is executed with -B 0, the kernel capability check rejects the
> insert and bounces the task back to ops.enqueue() with the SCX_ENQ_REENQ
> flag.
>
> Without the check for SCX_ENQ_REENQ, qmap_enqueue() will immediately hit
> this condition again and force-insert the task back to the target CPU.
>
> Could this lead to an endless, unthrottled loop that locks up the core?

The loop is bounded. Each bounce goes back through scx_do_enqueue_task()
which counts it in p->scx.reenq_cnt, and crossing SCX_REENQ_MAX_REPEAT
ejects the owning scheduler. With -B 0, a stranded task takes down qmap
instead of starving or burning the CPU, which is the intended failure mode
when rescue is turned off.

Thanks.

--
tejun