Experimental Physics and
Industrial Control System

Michael Davidsaver via Core-talk <core-talk at aps.anl.gov> · Tue, 2 Feb 2021 10:01:21 -0800

I've not paid serious attention to RT Linux in the past as the cases where
I needed RT were already using RTEMS.  Now I have a project where I may
want RT latency bounds when receiving high rate UDP traffic (where a short
delay can easily mean a socket buffer overflow).

While running a test to see that RT priorities were actually being set, I happened
to mis-type a 'ps' and list priorities for all threads.  I noticed that some of
my (IOC) threads had a higher priority than the OS threaded interrupt handlers.

It looks like the ISR threads default to SCHED_FIFO/50, which is also what
epicsThreadPriorityMedium currently maps to.  So eg. the 1 second scan task
has a higher priority (63) than the NIC ISR thread (50).  This seems like
a recipe for some unexpected priority inversions.

Has anyone else looked into this?

I'm looking at the 'rtprio' and 'cls' columns.  The "[...]" entire are kernel threads.

> ps -eLo rtprio,pri,nice,cls,pid,cmd|sort -n
...
>     10  50   -  FF 31826 ../../bin/linux-x86_64/pscdemo st.cmd
>     10  50   -  FF 31826 ../../bin/linux-x86_64/pscdemo st.cmd
>     10  50   -  RR 17801 /usr/lib/firefox-esr/firefox-esr ...
>     10  50   -  RR 24728 /usr/lib/firefox-esr/firefox-esr ...
>     12  52   -  FF 31826 ../../bin/linux-x86_64/pscdemo st.cmd
...
>     35  75   -  FF 31826 ../../bin/linux-x86_64/pscdemo st.cmd
>     50  90   -  FF   100 [irq/123-pciehp]
>     50  90   -  FF   101 [irq/125-pciehp]
>     50  90   -  FF   224 [card0-crtc0]
>     50  90   -  FF   225 [card0-crtc1]
>     50  90   -  FF   226 [card0-crtc2]
>     50  90   -  FF   230 [irq/171-DLL096D]
>     50  90   -  FF 26049 [irq/172-mei_me]
>     50  90   -  FF 31826 ../../bin/linux-x86_64/pscdemo st.cmd
...
>     50  90   -  FF 31826 ../../bin/linux-x86_64/pscdemo st.cmd
>     50  90   -  FF   461 [watchdogd]
>     50  90   -  FF   753 [irq/176-iwlwifi]
...
>     50  90   -  FF   768 [irq/185-iwlwifi]
>     50  90   -  FF    99 [irq/122-pciehp]
>     58  98   -  FF 31826 ../../bin/linux-x86_64/pscdemo st.cmd
...
>     89 129   -  FF 31826 ../../bin/linux-x86_64/pscdemo st.cmd
>     99 139   -  FF    12 [migration/0]
...
>     99 139   -  FF    45 [migration/7]
>     99 139   -  RR  1427 /usr/lib/rtkit/rtkit-daemon

Now I'm also wondering how firefox is able to set SCHED_RR/10

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System