Q: 2.8 from buildbot don't recognize PREEMPT_RT kernel

Hi !

I have installed stock (from buildbot repository stretch/2.8-rtpreempt) 2.8.0~pre1.5197.g08c06a0f9, and its seems it doesn't recognize that this kernel from Debian repository (4.9.0-11-rt-amd64) have PREEMPT_RT feature. Latency test shows huge 1588779 ns jitter for 25us thread. Runnung same PC with LinuxCNC 2.7 Live shows 21516 ns max jitter.

Installed 5.2-rt from testing, same problem.

How to fix this problem ?

Thanks in advance.

Computer specs ?

Intel Quad Core 2.4GHz Q6600, 8GB RAM, Gigabyte P35-DS4 m/b, Geforce GT630, SSD 120GB.
Someday it was my workstation which still has parallel port header, retired several years ago.

Grub boot options
quiet splash lapic idle=poll acpi_irq_nobalance noirqbalance noht isolcpus=1,2,3
drm_kms_helper.poll=0

I suspect linuxcnc from buildbot requires *exact* same kernel when it was compiled, not sure if I'm correct here.

BTW, changing grub boot options had little to no effect, latency is *huge* as with plain non-rt kernel.

Disable everything pertaining to power savings in BIOS, like speed stepping, hyperthreading, modem, serial ports etc.
Linuxcnc does not care much about the version of kernel, it just has to be fully preemptable.
Also the isolcpu will most probably worsen the latency in your case, so give it a try without it.
What graphic card are you using ? If by any chance an NVIDIA, do not install the proprietary drivers, they do mess up the latency.

GeFoce GT630, driver is nouveau.

Fresh install of Debian 9 with MATE desktop (ISO from Debian site). Disabled whatever possible, incl. animation in MATE desktop.

This is really strange - running LinuxCNC Live on the same PC doesn't show this latency problem.

Maybe the harddisk is dying and messing up latency?

If you want Debian 9 with preempt-rt why not use the Linuxcnc ISO?
www.linuxcnc.org/testing-stretch-rtpreempt/