[LITMUS^RT] preparing a new release

[Jonathan Herman]

It does still crash, yes. It looks like this is being caused by the
real-time balancing timer in linux
(see Documentation/scheduler/sched-rt-group.txt) which forces Linux
real-time (ie SCHED_FIFO) tasks to relinquish 5% of the CPU to SCHED_OTHER
tasks. It works like so:
1. A timer fires periodically.
2. The timer scans ALL cpus.
3. If any CPU has real-time work exceeding some threshold over the last
timer period, the currently running task on that CPU is rescheduled.

Step 3 is what caused our BUG. The BUG is hit if a remote processor, in
this case the processor on which the rebalancing timer fired, triggers a
reschedule of a SCHED_LITMUS task.

This can be disabled with:
echo -1 > /proc/sys/kernel/sched_rt_runtime_us

I will spend some time figuring out why this timer is considering
SCHED_LITMUS work when rebalancing. Somehow our work is counting towards
the 95% work which SCHED_FIFO tasks can use. That is why the BUG is only
hit when the system is fully utilized as well.


On Thu, Nov 29, 2012 at 5:39 PM, Björn Brandenburg <bbb at mpi-sws.org> wrote:

>
> On Nov 29, 2012, at 9:20 PM, Jonathan Herman <hermanjl at cs.unc.edu> wrote:
>
> > I have also confirmed that this only happens when shed-trace is running
> (not overhead tracing, just scheduling). Every commit I checked back to the
> 3.0 merge where my sched-trace still ran had this same issue.
>
> Yikes. Sounds like it could be a bug in the Feather-Trace triggers.
>
> Could you please try the following: edit arch/x86/Kconfig to remove
> ARCH_HAS_FEATHER_TRACE. This should disable the asm Feather-Trace hacks and
> replace it with a tame default implementation. Does it still crash?
>
> Thanks,
> Björn
>
>
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>



-- 
Jonathan Herman
Department of Computer Science at UNC Chapel Hill
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