Can't eek out that last bit of jitter

The process with the PREEMPT_RT kernel is rtapi. The one with -98 priority is the base thread and the one with -99 is the servo thread.
Some settings you could try:
Maximum Payload = Auto > to max
AMD 880 HD Audo = Enabled > Disabled
PME Wake Up From S5 = Enabled > Disabled
PS2 Keyboard PowerOn = Any Key > Disabled
Stroke Key = Wake Key > Disabled
PS2 Mouse PowerOn = Enabled > Disabled
USB Device Wake Up From S3/S4 = Enabled > Disabled
Unpopulated Links = Keep Link ON > Off

And the following kernel parameters instead:
skew_tick=1 nosmt=force kthread_cpus=0-1 irqaffinity=0-1 rcu_nocbs=2-7 nohz_full=2-7 isolcpus=2-7
intel_pstate=disable cpufreq.off=1 cpuidle.off=1 processor.max_cstate=0 idle=mwait clocksource=tsc
tsc=reliable nosoftlockup nowatchdog default_hugepagesz=1G hugepages=4 transparent_hugepage=never
threadirqs forcedeth.use_msi=0 mce=ignore_ce nmi_watchdog=0 audit=0 mitigations=off

So far, that works!  I don't think I ever would have gotten there without your help.  Thank you.

I didn't end up deleting everything in my grub setup, but I put in everything you added.  The 1090T only has 6 cores, so I reduced the 7s to 5s.  I ended up with:

 smt=1 ro quiet skew_tick=1 nosmt=force mds=nosmt mitigations=auto,nosmt irqaffinity=0-1 kthread_cpus=0 rcu_nocb_poll rcu_nocbs=2-5 nohz=on nohz_full=2-5 isolcpus=2-5 intel_pstate=disable amd_pstate=disable idle=mwait clocksource=tsc tsc=reliable cpufreq.off=1 cpuidle.off=1 intel_idle.max_cstate=0 amd_idle.max_cstate=0 processor.max_cstate=0 nowatchdog nosoftlockup acpi_irq_nobalance noirqbalance default_hugepagesz=1G hugepages=4 transparent_hugepage=never threadirqs forcedeth.use_msi=0 mce=ignore_ce nmi_watchdog=0 audit=0 initrd=/install/gtk/initrd.gz

Not only does it work, it is also running much cooler.  Now the CPU is around 31C, and before it was near 50C.

This result is using the A880GZ with integrated HD4250 graphics.  (I didn't need the R5 340X, but I may try it out just to give others info.)
 

Thank you so much!
 

For anyone wondering when I tried with the R5 340X graphics card, the max base jitter went up to +/-30us.  So I guess that isn't a good graphics card for linuxcnc, or maybe it just doesn't work with the A880GZ and linuxcnc.

Also, in case anyone is interested in, here is a screen shot including the watch of /proc/interrupts when using the integrated HD4250 graphics card with the biostar A880gz .

 

Oh, and on that screenshot, I did NOT run

swapoff --all
systemctl --user stop pulseaudio.socket
systemctl --user stop pulseaudio.service
sudo sysctl kernel.sched_rt_runtime_us=-1

I guess they were unnecessary. (Although maybe I had added that last one to startup in some way before, but I don't remember.)

TBH I used Deepseek on this one (and didn't filter out the 2-7 error), but I'm glad it helped you. The idle=mwait instead of poll seems to lower the temperature without much impact on the performance. Now the latency is good you could also try to only isolate the last core (so replace each 2-5 with 5), if the latency will be the same you'll have a much better responsive system.

That's cool. I had tried both Gemini and Copilot, and I didn't get anything close to as useful as that. Thank you again.