Re: [Xenomai-help] CONFIG_XENO_OPT_SELECT

[Philippe Gerum <rpm@xenomai.org>]

On Mon, 2011-01-31 at 15:47 +0100, Gilles Chanteperdrix wrote:
> roderik.wildenburg@domain.hid wrote:
> > In the Xenomai 2.4.x series there was a kernel option
> > CONFIG_XENO_OPT_SELECT. Unfortunately there is no help available for
> > this option and Google  wasn´t very fertile.
> 
> CONFIG_XENO_OPT_SELECT is an internal option of Xenomai. The options you
> should be looking at are
> 
> CONFIG_XENO_OPT_POSIX_SELECT
> CONFIG_XENO_OPT_RTDM_SELECT
> 
> These ones are documented, and selecting them, simply select
> CONFIG_XENO_OPT_SELECT. So, the fact that CONFIG_XENO_OPT_SELECT is off
> in your kernel configuration is off simply means that the other two
> options are off.
> 
>  Could somebody tell me,
> > what this option is for? I am asking, as we face some problems with
> > "select" in a Xenomai task (task is stuck in select, was permanently
> > marked as running (R in /proc/xenomai/sched) and lower priority tasks
> > weren´t  scheduled).
> 
> This looks strange. Each call to select should result in a call to
> Xenomai "select" syscall, if you are using the POSIX skin, which should
> return -ENOSYS, then select should call the glibc select. Since
> obviously what you want is to call glibc select directly, you can try
> calling __real_select instead.
> 
> Note that the issue with priorities you observe show that you have a
> task with a priority higher than 0 using a glibc syscall, thereby
> causing it to switch to secondary mode. The fact that lower priority
> threads do not run is the effect of the "priority coupling", but the
> real issue here is that a task with real-time priority is using a linux
> syscall, which is something you should normally avoid.
> 

The caller should lose its RPI boost as soon as the regular select()
syscall blocks it though - and I assume it does, so we do have a bug in
2.4.9 here (although I'm pretty sure it has been fixed in later
releases). Having a shadow stuck in 'R'unnable mode
in /proc/xenomai/sched is a good sign that something is broken deep
inside.

-- 
Philippe.