Re: Issue with the priority of the sound cards using rtirq

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On 05/02/2012 12:19 AM, Jeremy Jongepier wrote:
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.

That's how we all would like things to work, regretfully that is not how things actually work.

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.

For soundcard drivers there is no convention for naming the irq process that services its interrupt. The name of the irq process does not match the name of the kernel module (which would be an obvious choice). The ascii that is printed by ps is defined in the kernel code for each driver, and driver authors pick different names that are not consistent. That means you can't pattern match for the right irq process if the interrupt number is shared by several devices.

It would be possible to pattern match if, for example, all soundcards used a "snd_" prefix for the name of the irq process, but that is not the case (at least until last time I looked at the kernel code for this very same reason).

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.

The rtirq code does the following (simplified):

- rtirq gets the irq number from /proc/asound/cards
- that number is used to search the output of ps looking for matches with that particular irq number - there is no way to tell which irq process corresponds to a soundcard, rtirq does not try AFAICT
- so, all irq processes that match the number get the elevated priority

For example, in my laptop, from /proc/interrupts:

16: 2551 516 113 95 IO-APIC-fasteoi nouveau, snd_hda_intel, mmc0

rtirq happily gives them 75, 74 and 73:

  222 FF      75   - 115  0.0 S    irq/16-nouveau
11077 FF      74   - 114  0.0 S    irq/16-snd_hda_
11082 FF      73   - 113  0.0 S    irq/16-mmc0

Please let me know if there is a way to, given an irq number, know what is the pid of the irq process that services that irq number for a particular _soundcard_, I have found no way of doing that so far[*].

Note that in all other cases other than soundcards rtirq looks for the name, not the number of the irq (for example for usb the names are well known and unique and hardwired into rtirq).

-- Fernando

[*] other than scanning the kernel source code for the names of all possible soundcard driver irq processes, making a list and matching against that - it would work but it would depend on those names not changing, and of having the list that matches the kernel you are running.
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