Re: [Xenomai] question: XENO_OPT_TIMING_SCHEDLAT

[Christophe Blaess <mailist@logilin.fr>]

On 30/06/2012 19:04, Gilles Chanteperdrix wrote:
> On 06/30/2012 06:52 PM, Christophe Blaess wrote:
>> On 30/06/2012 12:44, Gilles Chanteperdrix wrote:
>>>
>>>> /proc/xenomai/latency is an estimation of the minimum scheduling latency
>>>> on your system. In order to know what to put there you should do:
>>>>
>>>> echo 0 > /proc/xenomai/latency
>>>>
>>>> Run latency under load for several hour
>> Is it necessary to run the test under heavy load?
>> We're looking for the minimal latency, not the worst.
>>
>> I thought that in this case a normal, even light load would be sufficient.
> It is not obvious which path is the shortest. For instance, having to
> wake up from a "wait for interrupt" state to handle the timer interrupt
> may induce a certain latency, and so may not be the shortest path. So,
> running the test for a long time, and with a lot of different activities
> is an empiric way to try many paths so that extreme paths have more
> chances to be closest to the extrema paths.
>
> And anyway, you usually want to know the worst case latency for your
> system as well.
>

I ran two 6-hours latency tests on a Pandaboard. After echoing 0 in 
/proc/xenomai/latency

For the first one the system load was very weak. I ran the test on the 
second core because most of the interrupts are processed on the first 
core. So the latency process was almost alone.

#*/usr/xenomai/bin/latency -p 100 -c 1 -T 21600*
[...]
RTS|      2.388|      3.134|     18.706|       0|     0|    06:00:00/06:00:00


For the second test, the system was under a very high load. I used a 
shell script very close to dohell and sent a lot of external interrupts 
(ping flood, GPIO interrupt...). This time latency ran on the first 
core, and was subject to a lot of preemptions from interrupt requests.

#*/usr/xenomai/bin/latency -p 100 -c 0 -T 21600*
[...]
RTS|      2.908|      7.797|     51.579|       0|     0|    06:00:00/06:00:00

I would recommend, as you said, for long time tests to alternate high 
and low pressure stages to get the min and max values.


Apart from that, on my board (Panda with Xenomai 2.6.0), I note that 
writing in /proc/xenomai/latency such as

# echo 2388 > /proc/xenomai/latency


gives a segmentation fault, even if the value is indeed written. Here is 
the dmesg log :

[65108.190551] Unable to handle kernel paging request at virtual address 00004a5a
[65108.190551] pgd = ef138000
[65108.190551] [00004a5a] *pgd=af101831, *pte=00000000, *ppte=00000000
[65108.190582] Internal error: Oops: 80000007 [#1] PREEMPT SMP
[65108.190582] last sysfs file: /sys/devices/virtual/vc/vcs2/dev
[65108.190612] CPU: 0    Not tainted  (2.6.38.8-xenomai-cpb #2)
[65108.190612] PC is at 0x4a5a
[65108.190643] LR is at simple_strtoul+0x8/0xc
[65108.190643] pc : [<00004a5a>]    lr : [<c024ebf4>]    psr: 60000033
[65108.190643] sp : ef06df08  ip : 00000002  fp : 00000100
[65108.190673] r10: 401ae600  r9 : efbffe48  r8 : 00000fff
[65108.190673] r7 : 00000fff  r6 : 1b207478  r5 : 742e4a5b  r4 : 1b262074
[65108.190673] r3 : 00000002  r2 : c0456918  r1 : 00000000  r0 : ffffffea
[65108.190704] Flags: nZCv  IRQs on  FIQs on  Mode SVC_32  ISA Thumb  Segment user
[65108.190704] Control: 10c53c7d  Table: af13804a  DAC: 00000015
[65108.190704] Process sh (pid: 86, stack limit = 0xef06c2f8)
[65108.190734] Stack: (0xef06df08 to 0xef06e000)
[65108.190734] df00:                   00000000 00000000 00000000 00000000 00000000 00000000
[65108.190734] df20: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
[65108.190765] df40: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
[65108.190765] df60: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
[65108.190795] df80: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
[65108.190795] dfa0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
[65108.190795] dfc0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
[65108.190826] dfe0: 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
[65108.190826] [<c024ebf4>] (simple_strtoul+0x8/0xc) from [<00000000>] (  (null))
[65108.190856] Code: bad PC value
[65108.190979] ---[ end trace 29e0859033dc848c ]---