On Sun, Jan 20, 2013 at 9:18 PM, Carsten Emde <C.Emde@xxxxxxxxx> wrote:
> John,
>
>
>>>>> 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:
>>>>>
>>>>> 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:
>>>>>
>>>>> 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
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