[PATCH 0/3] irq_work: CPU-hotplug improvements on PREEMPT_RT

[Sebastian Andrzej Siewior]

There was an unrelated thread which made me look into how
!IRQ_WORK_HARD_IRQ is processed on PREEMPT_RT. Turns out the queue of
callbacks gets never flushed on CPU shutdown. !PREEMPT_RT has a flush
but on PREEMPT_RT the thread context is required.
This almost never happens but if it happens it is a pain.

A small fix is for the smpboot thread to invoke the thread-function
before parking. This is an improvement. There is still a small window if
callbacks are added after the irq_work thread has been shutdown.
As a fix I added an explicit flush on the control CPU once the CPU is
dead.
>From what I've seen there are two users that don't behave as expected if
invoked from the "wrong" CPU:
- cgrp_dead_tasks_iwork()
  This shouldn't be a problem because no task terminates after the
  smpboot thread parked. Everythng that was queued up before will be
  flushed during parking.
  Regardless posted
     https://lore.kernel.org/all/20260911101900.984420-1-bigeasy@xxxxxxxxxxxxx/

- wake_up_klogd_work()
  This will not cause data curruption but console printing will be
  delayed until the following printk. Not pretty, posted
     https://lore.kernel.org/all/20260911103832.w6C8cT4L@xxxxxxxxxxxxx/

Sebastian Andrzej Siewior (3):
  irq_work: Update a comment regarding CPU hotplug invocation
  irq_work: Flush lazy work CPU down on PREEMPT_RT
  smpboot: Don't park the thread if work is pending

 include/linux/irq_work.h |  2 ++
 kernel/irq_work.c        | 16 ++++++++++++----
 kernel/smp.c             |  1 +
 kernel/smpboot.c         |  6 ++++--
 4 files changed, 19 insertions(+), 6 deletions(-)

-- 
2.55.0