Q: 2.8 from buildbot don't recognize PREEMPT_RT kernel

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20 Sep 2019 14:56 #145678 by LinuksGuru
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.

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20 Sep 2019 19:16 #145700 by tommylight

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20 Sep 2019 19:26 - 20 Sep 2019 19:30 #145701 by LinuksGuru
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.
Last edit: 20 Sep 2019 19:30 by LinuksGuru.

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20 Sep 2019 19:48 #145708 by tommylight
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.

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20 Sep 2019 19:56 #145709 by LinuksGuru
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.

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20 Sep 2019 20:53 #145712 by Todd Zuercher
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/

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21 Sep 2019 07:36 #145738 by tommylight
You should take Todd's advice and download that image, boot it from a USB and check the latency without installing it, that way you eliminate the HDD issues if there are any.

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24 Sep 2019 09:50 #146048 by LinuksGuru
HDD is WD SSD, and it is fine.

At first, I supposed it was problem with nVidia GT630 and nouveau driver.
I replaced GT630 with Radeon HD6450, as suggested here (post #418):
www.cnc-club.ru/forum/viewtopic.php?f=15&t=1078&start=400

Latency is still horrible.
I disabled onboard audio, firewire, tweaked grub kernel params - all same,

Finally, I found this thread, which describe latency spikes on Gigabyte P35-DS3 and DS4.
forums.tomshardware.com/threads/audio-la...ith-p35-ds3r.411939/

Its a buggy m/b BIOS after all.

I'll simply trash this m/b, its very old anyway, and buy something else.

Hope this info will be useful for someone.

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24 Sep 2019 10:57 #146055 by tommylight
Had the same processor on an Asus P5B Deluxe, it is still working perfectly well overclocked at 3.2 i think, latency was never over 40000.
I remember i did change the BIOS as it would not work with that processor.
Did you try flashing the BIOS ?

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24 Sep 2019 15:00 #146078 by LinuksGuru
Yes, I re-flashed latest available F14, no changes.

Here is another thread with similar latency issues, looks like the whole P35 family of Gigabyte m/b have been problematic.

forums.anandtech.com/threads/gigabyte-bo...-dpc-latency.171880/

Replacement m/b cost close to nothing, the most cumbersome part is a re-assembly.

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