Re: Detecting PREEMPT_RT

On Sun, Jan 20, 2013 at 9:18 PM, Carsten Emde [off-list ref] wrote:

John,

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Unfortunately, this wiki article is old and unmaintained. You're
probably right to be a bit suspicious when checking for the "-rt"
suffix
of the kernel release (uname -r). But looking for the occurrence of
"PREEMPT RT" in the kernel version (uname -v) should work. An
application may use the system call uname() and check the version
element of the utsname structure.

the utsname.version string match is fine, however:

quoted

In addition you may want to make sure the system has
high-resolution

timers. If so, the timers in /proc/timer_list have ".resolution: 1
nsecs". An application may use the function check_timer() from
cyclictest for this purpose:

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static int check_timer(void)
{
   struct timespec ts;

   if (clock_getres(CLOCK_MONOTONIC, &ts))
     return 1;

   return (ts.tv_sec != 0 || ts.tv_nsec != 1);
}

Just tried this on a generic kernel (2.6.32-45-generic #102-Ubuntu
SMP Wed Jan 2 21:53:06 UTC 2013 i686 GNU/Linux) and I get ts.tv_sec == 0
and ts.tv_nsec == 1, so it looks this cant be used to tell an RT_PREEMPT
from a vanilla kernel

I meant boolean AND when I said "In addition".

Oh, I interpreted the timer check not as a PREEMPT_RT check, but a
separate check for timers, which we must confirm are available before
driving heavy machinery.
So, if utsname.version contains "PREEMPT RT", then load the preempt-rt
module.  Once the module is loaded, from an init() function, run sanity
checks, including the clock_getres check above.  Is that about right?

Yes. Sounds good to me.

Note we added a patch, called sysfs-realtime-entry.patch, but I'm not
sure how far back it goes, at least it's in 3.x-rt kernels.

This is the text from the patch

"Add a /sys/kernel entry to indicate that the kernel is a
realtime kernel.

Clark says that he needs this for udev rules, udev needs to evaluate
if its a PREEMPT_RT kernel a few thousand times and parsing uname
output is too slow or so.

Are there better solutions? Should it exist and return 0 on !-rt?"

To use it, you do something like
test -e /sys/kernel/realtime

If you need it in an older kernel that doesn't have it, it should port
trivially.

Thanks