Xrun reporting broken in `--sync` mode

[fps]

1. Start jackd in --sync mode:

jackd --sync -R -P 70 -d alsa -d hw:iXR -p 64 -n 2

2. In a second terminal run:

jack_cpu -t 5 -c 99

Or to make it even more obvious instead run jackd as:

jackd --sync -R -P 70 -d alsa -d hw:iXR -p 12 -n 2

Notice the period size of 12

I would expect jackd to go bonkers and spam the terminal with xruns once the jack_cpu program runs. You can go even more bananas and use jack_cpu -t 5 -c 110.

But interestingly when --sync is given no xruns are reported. With the sync option disabled I do get the behaviour that I expect (excerpt):

[nix-shell:~]$ jackd  -R -P 70  -d alsa -d hw:iXR -p 12 -n 2
jackdmp 1.9.19
Copyright 2001-2005 Paul Davis and others.
Copyright 2004-2016 Grame.
Copyright 2016-2021 Filipe Coelho.
jackdmp comes with ABSOLUTELY NO WARRANTY
This is free software, and you are welcome to redistribute it
under certain conditions; see the file COPYING for details
no message buffer overruns
no message buffer overruns
no message buffer overruns
JACK server starting in realtime mode with priority 70
self-connect-mode is "Don't restrict self connect requests"
audio_reservation_init
Acquire audio card Audio1
creating alsa driver ... hw:iXR|hw:iXR|12|2|48000|0|0|nomon|swmeter|-|32bit
configuring for 48000Hz, period = 12 frames (0.2 ms), buffer = 2 periods
ALSA: final selected sample format for capture: 32bit integer little-endian
ALSA: use 2 periods for capture
ALSA: final selected sample format for playback: 32bit integer little-endian
ALSA: use 2 periods for playback
JackEngine::XRun: client = jack-cpu was not finished, state = Running
JackAudioDriver::ProcessGraphAsyncMaster: Process error
JackEngine::XRun: client = jack-cpu was not finished, state = Running
JackAudioDriver::ProcessGraphAsyncMaster: Process error
[...]

Version info:

jackdmp version 1.9.19 tmpdir /dev/shm protocol 9

OS: Nixos

Installed the nixos package of jack2

Kernel: Linux ogfx86 6.0.5-rt14 #1-NixOS SMP PREEMPT_RT Wed Oct 26 10:53:32 UTC 2022 x86_64 GNU/Linux

IRQ tuning:

$ top -b | grep xhci
    180 root     -70   0       0      0      0 S   0.0   0.0 679:32.07 irq/126-xhci_hcd