[LAU] Issue with the priority of the sound cards using rtirq

Jeremy Jongepier jeremy at autostatic.com
Wed May 2 07:19:01 UTC 2012

On 05/01/12 20:32, Fernando Lopez-Lezcano wrote:
> (I don't seem to find the start of the thread in my mailbox)
> ohci_hcd is there is you specify usb in the list of priorities rtirq
> needs to raise. And nvidia is there probably because it shares an
> interrupt with a real soundcard. Rtirq does not raise the priority of
> only the soundcard, but everything that uses the interrupt that the
> soundcard uses.

Afaik this is not true. The threadirqs kernel option (or a real-time 
kernel) creates bottom halves (or tasklets, or softirqs) of each device 
that has or shares an IRQ. So in Ralf's case four devices share IRQ 18, 
two USB hubs, a soundcard and a graphics card. Each of those devices 
gets its own specific softirq which then can get prioritized with rtirq 
accordingly. Default rtprio is 50 afaik, so nvidia should get rtprio 50 
and not 82.

> AFAIK it is not possible (or simple) to raise _just_ the soundcard as it
> is impossible to know which irq process corresponds to a soundcard (and
> which one does not). That is because the labeling of the irq process
> names in the ps listing is arbitrary.

It is not arbitrary, the softirqs get specific names derived from the 
loaded kernel module for that device iirc. So the graphics card gets a 
label 18-nvidia, the soundcard 18-snd_hdsp and both usb hubs 
18-ohci_usb. And it is those labels that rtirq uses to set the rtprio 
for the different softirqs.

  At least that is what I remember
> seeing last I looked at this issue. You can of course manually lower the
> priorities of non-soundcard irq processes with chrt.
> -- Fernando

More info: 

If that Wiki page contains errors, please let me know so I can correct 



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