Re: [Xenomai-help] CONFIG_PREEMPT & irqbench

[Jan Kiszka <jan.kiszka@domain.hid>]

Markus Franke wrote:
> Jan Kiszka wrote:
>> Markus Franke wrote:
>>> I am running some latency tests with irqbench/irqloop. I am wondering
>>> whether it would be possible to achieve better results when activating
>>> CONFIG_PREEMPT and CONFIG_PREEMPT_VOLUNTARILY during the kernel
>>> configuration and running irqloop in User Mode over this kernel?
>>
>> Nope.
>>
>>
>>> Does it make any sense?
>>
>> Nope. :)
>> The Linux kernel is already fully preemptible by Xenomai once you
>> applied the I-pipe patch. Therefore, you are free to pick the Linux
>> preemption strategy according to your *Linux* load, independent of what
>> the real-time part needs.
> 
> Well, "Native-API-Tour.pdf" states that everytime a mode switch from
> Primary Domain to Secondary Domain is made (e.g. Linux system call like
> ioctl()), Xenomai can ease from the
> "continuous trend of improvements of Linux 2.6 regarding
> preemptability". So CONFIG_PREEMPT should have an impact on measurements
> with Xenomai, maybe not with irqbench/irqloop. The problem here is that
> we don't have a mode switch, right? When running irqloop as User-Task it
> simply runs in Secondary Mode, right?

Of course, irqloop runs in *primary* mode to be able to handle the
events deterministically. So it is not directly affected by CONFIG_PREEMPT.

Yes, if you have an RT thread that issues syscalls and wishes to have
them handled as fast as possible, CONFIG_PREEMPT should be enabled (and
CONFIG_XENO_OPT_RPIDISABLE should remain off, maybe you even want to
consider CONFIG_XENO_OPT_ISHIELD then). Such RT application designs are
tricky to get correct and deterministic, so it's often better to not
rely on these properties and rather seek a clear separation of pure RT
threads on the one side and Linux syscall issuing non-RT or RT
borderline threads (low-prio RT threads that are being switched back and
forth between primary and secondary mode) on the other.

> 
>> Do you have CONFIG_DEBUG_PREEMPT set as well then? This option still as
>> a small but measurable impact on Xenomai due to micro-dependencies that
>> as scheduled to be removed in the near future.
> 
> I cannot see anything like
> 
> CONFIG_DEBUG_PREEMPT=y
> or
> #CONFIG_DEBUG_PREEMPT is not set
> 
> in my config-file. The parameter simply doesn't exist.

Shadowed by CONFIG_DEBUG=n likely.

Jan