Query on Linux process sheduling

Query on Linux process sheduling

[B.Vinai Kumar]

Hi Guys,

I have embedded box running 2.6.28.9 linux kernel and running 6 Real
time task threads (With SCHED_RR as scheduling policy).
Together all the 6 threads running is consuming around 85% of CPU,
which is expected.

All the threads have a 10ms of cycle.

The issue is for every second I have observed that for 50ms these Real
time task threads are not scheduling at all. During this 50ms
the process swapper/idle task is being scheduled(pid 0).

Is this the behaviour is due to these values?

# cat /proc/sys/kernel/sched_rt_period_us
1000000
# cat /proc/sys/kernel/sched_rt_runtime_us
950000

What changes can be done to the kernel so that the Real time task
threads get scheduled in that 50ms interval too?

Thanks,
Vinai.

Re: Query on Linux process sheduling

[Jonathan Corbet]

On Sat, 24 Mar 2012 17:25:41 +0530
"B.Vinai Kumar" <vinaibangala2001@gmail.com> wrote:

> I have embedded box running 2.6.28.9 linux kernel and running 6 Real

Wow...ancient history :)

> time task threads (With SCHED_RR as scheduling policy).
> Together all the 6 threads running is consuming around 85% of CPU,
> which is expected.
> 
> All the threads have a 10ms of cycle.
> 
> The issue is for every second I have observed that for 50ms these Real
> time task threads are not scheduling at all. During this 50ms
> the process swapper/idle task is being scheduled(pid 0).
> 
> Is this the behaviour is due to these values?
> 
> # cat /proc/sys/kernel/sched_rt_period_us
> 1000000
> # cat /proc/sys/kernel/sched_rt_runtime_us
> 950000

Indeed, you're on the right track.  The scheduler preserves a little bit
of time for non-RT tasks to give you a chance to recover if something goes
totally nuts.

> What changes can be done to the kernel so that the Real time task
> threads get scheduled in that 50ms interval too?

Setting those two knobs to the same value should do the trick, but it does
increase the risk of losing control of the system.  See
Documentation/scheduler/sched-rt-group.txt for more information on how
these knobs work.

jon

Re: Query on Linux process sheduling

[B.Vinai Kumar]

Hi Jon,

Thanks for reply.

As you said all the real time process are scheduled in the 50ms, when
I set the period time value and run time value to "950000" us.

What is the risk when we change the default values?

When I completely disabled this feature. (by writing "-1" to run time
value), then also I see all the real time process are always
scheduled.

Thanks,
Vinai.