Re: [PATCH v3 sched_ext/for-7.3 35/36] tools/sched_ext: scx_qmap - Expand hierarchical sub-scheduling

On Tue, 07 Jul 2026 00:41:01 +0000, [email protected] wrote:
> - [Medium] Newly acquired exclusive CPUs are not added to `idle_cids.mask` if they were already idle when ownership changed, leading to lost wakeups.
> - [Medium] Tasks pinned exclusively to delegated CPUs will be permanently stranded in `SHARED_DSQ`.
> - [Medium] `qmap_sub_attach` and `qmap_sub_detach` modify the `sub_sched_ctxs` array without synchronization, causing data races with timer-driven partition updates.

1. Real - a cid already idle when it's reclaimed into the owned set was
   missed. Fixed by making idle_cids track physical idleness for every
   cid and masking it against the owned set at dispatch, so no reseed is
   needed.

2. Known limitation - a task affined only to delegated cids strands in
   SHARED_DSQ. This is the missing per-affinity fallback, now noted in
   the cover letter; a later series adds a root-reserved fallback slice.

3. Real but self-healing - the fields are re-derived by the following
   redistribute(). The clean fix (a bpf_spin_lock over the accesses)
   isn't usable because the cmask ops it would guard call kfuncs, which
   the verifier forbids inside a bpf_spin_lock section. Left as-is for
   the demo.

Thanks.

-- 
tejun