On 2025-02-18 11:50:55 [-0300], Wander Lairson Costa wrote:

These logs are for the test case of booting the kernel with nr_cpus=1:

     kworker/0:0-8       [000] d..2.  2120.708145: process_one_work <-worker_thread
     kworker/0:0-8       [000] ...1.  2120.708145: igbvf_reset_task <-process_one_work

This looks like someone broke the function tracer because the preemtion
level should be 0 here not 1. So we would have to substract one… This
does remind me of something else…

…

     kworker/0:0-8       [000] b..13  2120.718620: e1000_reset_hw_vf <-igbvf_reset

…

     kworker/0:0-8       [000] D.h.3  2120.718626: irq_handler_entry: irq=63 name=ens14f0

^ the interrupt.
…

     kworker/0:0-8       [000] b..13  2120.719133: e1000_check_for_ack_vf <-e1000_write_posted_mbx
  irq/63-ens14f0-1112    [000] b..12  2121.730652: igb_msix_other <-irq_thread_fn
  irq/63-ens14f0-1112    [000] b..12  2121.730652: igb_rd32 <-igb_msix_other
  irq/63-ens14f0-1112    [000] b..13  2121.730653: igb_check_for_rst <-igb_msix_other
  irq/63-ens14f0-1112    [000] b..13  2121.730653: igb_check_for_rst_pf <-igb_msix_other

The threaded interrupt is postponed due to the BH-off section. I am
working on lifting that restriction. Therefore it gets on CPU right
after kworker's bh-enable.

…

The threaded interrupt handler is called right after (during?)
spin_unlock_bh(). I wonder what the 'f' means in the preempt-count
field there.

The hardware interrupt handler gets there while worker is in the wait
loop. The threaded interrupt handler gets postponed until after the last
spin_unlock_bh(). The BH part is the important part.
With that log, I expect the same hold-off part with threaded interrupts
and the same BH-off synchronisation.

I am currently working on something else that has a higher priority, so
I don't have time right now to go deeper on that. But feel free to ask
me for any test or trace you may need.

I would need to check if it is safe to explicitly request the threaded
handler but this is what I would suggest. It works around the issue for
threaded interrupts and PREEMPT_RT as its user.
You confirmed that it works, right?

Sebastian