isolcpus, what should be running in the isolated core?

I am chasing a problem with my Vista P4-SE USB pendant.  I was thinking that it was an issue with the USB disconnecting but I am not seeing any disconnect messages in the dmesg output unless I actually disconnect the pendant.   That caused me to start looking at what was running where.  I noticed in top that the vc-p4s process is running on core 1  (I have isolcpus=2 at the moment.  It was set to 3 previously) and it consistently shows  99.7-100.3 CPU% .

What process or processes should be running in the isolated core?  And maybe just as important, which ones should not be running in the isolated core?

Thanks
John

andypugh wrote:

What processes were shown as running in the isolated core?
100% utilisation might be normal for an isolated core running the realtime threads.
I have seen busy-wait hacks to improve latency, too, have you set that up?

I will check that again today and report back.  The latency on this machine per the histogram is really good.  I certainly don't need to chase that any further.  My question arises from curiosity and the issue I am having with my P4-SE pendant on this machine.  dmesg is not showing any USB disconnects but the pendant does become inoperable.  I am using Wireshark to diagnose that further.   My thought was could there be something that is preventing the pendant from receiving the heartbeat message that it needs to stay alive.  Vista (the mfg) gave a version of the pendant firmware that has the heartbeat requirement turned off and the pendant still goes INOP after a varying amount of time.  You just can't tell it is INOP by looking at the pendant display.

Thanks
John
 

While isolcpus=3 this is what is running on core 3 per top.

cpuhp/3
irq_work/3
migration/3
rcuc/3
ktimers/3
ksoftirqd/3
kworker/3:0-events
irq/8-rtc0
kworker/3:1-events
irq/23-ehci_hcd:usb2
irq/32-mei_me
irq/29-enp2s0