I have been attempting to get passed through sound cards working and
have been encountering issues. Apart from the extra complication of
windows, I've been testing with a real-time audio app like yoshimi, and
I get audio lag/glitches. I've been tracking down the causes of these
glitches and have found several things that contribute. So far, on two
setups I have, I've found:
* >1ms long SMIs on a GA-990FX motherboard, caused exclusively by
parallel disk writes (I am running a 5-disk array on it)
* The linux radeon driver (on the host) occasionally locking up a core
for 10-35ms at a time (I think this has been fixed since the 4.19
series)
* When console messages get sent to a serial port, it locks up a core
for the duration of the message. (This is a known problem that is
still being worked on)
* qemu's emulated (uhci) USB seems to hold up a vCPU for a few ms
somewhere in the guest's interrupt handler
I've been able to track these down with some combination of
- the SMI (hwlat) detector in ftrace
- the irqsoff tracer in ftrace
- cyclictest
- guesswork
I have found workarounds for the above, however I am still having
trouble with one box - I get glitches every minute or two. My
investigation is so far stuck at hits from the irqsoff tracer that
contain lines like this:
alsa-sin-31611 1d... 0us#: _raw_spin_lock_irq
alsa-sin-31611 1d... 4448us : snd_pcm_group_unlock_irq
So, questions:
- when I get output like the above from an irqsoff trace, how do I probe
further?
- are there any other tools that could help with debugging these sorts
of problems?
- I understand that for IRQs to get routed properly from host to guest,
the device needs to have a per-device (per-function?) IRQ status bit
that can be used to aid in routing. How do I confirm that the device in
question has this bit?
- how do I check that IRQs are being routed to the guest optimally?
What do I watch out for? How do I check if it's using irqfd?
- does MSI really help? Will it save any more more than, say, 100us in
IRQ response latency?
(I am able to swap in a sound card that works; I have a cheap USB dongle
that hasn't given me any problems yet. I still want to see what options
I have though.)
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