Re: [PATCHSET sched_ext/for-7.3-fixes] sched_ext: Fix missing ops.dequeue() on remote local DSQ moves

[Tejun Heo]

Hello,

On Tue, Sep 29, 2026 at 04:17:23PM +0000, Kuba Piecuch wrote:
> Since ebf1ccff79c4 ("sched_ext: Fix ops.dequeue() semantics"), every task
> entering the BPF scheduler's custody gets exactly one ops.dequeue() when
> it leaves it. A dispatch to a terminal DSQ ends custody with a flag-less
> ops.dequeue() at insertion time.

The fix looks good to me. It effectively reverts the enqueue_task_scx()
half of b75aaea24c9f ("sched_ext: Properly mark SCX-internal migrations
via sticky_cpu"), which as far as I can see only ever suppressed this
ops.dequeue(). Andrea, can you confirm?

- dequeue_remote.c isn't built until 3/3, so 1/3 can't be built or run.
  Can you put the fix first, followed by the test with its Makefile entry?

- 2/3: 7.1.y also needs 18d62044cda7 ("sched_ext: Preserve rq tracking
  across local DSQ dispatch"). Without it, the nested ops.dequeue() trips
  lockdep when ops.dispatch() uses scx_bpf_dsq_move() to another CPU's
  local DSQ. It's tagged for stable too, but maybe note it as a
  prerequisite?

- 2/3: With sub-scheds, scx_resolve_local_dsq() can divert the task to the
  reject or rescue DSQ, so "inserted into the local DSQ" in the comments
  isn't always accurate. Maybe "destination DSQ"? The new comment in
  enqueue_task_scx() could be two lines, and the description could lead
  with the late SCX_DEQ_CORE_SCHED_EXEC and be a lot shorter.

- 1/3: A task can only be picked straight out of custody through
  sched_core_find(), which only returns tasks with a core cookie. Checking
  p->core_cookie on SCX_DEQ_CORE_SCHED_EXEC would be exact and would
  remove core_sched_in_use() and the skip.

- 1/3: _SC_NPROCESSORS_ONLN ignores affinity. With the runner confined to
  one CPU, the test fails instead of skipping. sched_getaffinity() and
  CPU_COUNT()?

- 1/3: Nits. If the /proc scan stays, PR_SCHED_CORE_GET writes a u64, so
  the cookie should be u64. ops.dispatch() pops one entry per call, so a
  stale one idles the CPU until the next kick. Maybe loop a few times?
  missed_dequeue_cnt and core_sched_exec_dequeue_cnt aren't printed
  per-scenario like the other counters.

- 1/3: The variants, error conditions and core-sched caveat are repeated
  across the cover, description, file header and comments. Can you say
  each once? Also, single-line comments are usually lowercase in
  sched_ext.

Thanks.

--
tejun