[yocto] Can I disable RT throttling?

[Bruce Ashfield]

On Sat, Mar 9, 2013 at 12:12 PM, Vin Shelton <acs at alumni.princeton.edu> wrote:
> On Fri, Mar 8, 2013 at 10:28 PM, Bruce Ashfield
> <bruce.ashfield at windriver.com> wrote:
>> On 13-03-08 5:36 PM, David Mulder wrote:
>>> I’m running a 10us control loop by (under vxWorks) setting one thread’s
>>> priority to max and not yielding ever (letting other tasks run on other
>>> cores), but Linux seems to thwart that capability: Ubuntu swaps out my
>>> thread occasionally for hundreds of microseconds; Yocto prints a
>>> “[sched_delayed] sched: RT throttling activated” message as soon as I
>>> start my thread, so it seems likely that it will swap out my thread
>>> periodically (I’m not quite able to confirm that yet), and even the
>>> fastest thread swap that I’ve heard of is too slow.
>>
>>
>> You are looking for kernel functionality that doesn't exist yet. In
>> any SMP system there are sources of cross cpu interference that can't
>> be removed, if you do, the global state machines of the kernel will
>> break and the system will eventually come to a halt.
>>
>> The system is trying to save you from yourself, by throttling the
>> RT task from taking the entire system down.
>>
>> There is work in the mainline kernel and -rt communities around cpu
>> and cpu isolation (some of which we'll try and make available via
>> the yocto meta-virtualization or meta-realtime layer, when they are
>> ready), it goes by names such as task_nohz or adaptive_nohz. It's
>> a complex problem to solve (interrupts, rcu, lapic), and there's not
>> likely to be anything available in the short term.
>
> At ELC, Steve Rostedt said that this feature is currently targeted at
> the June mainline kernel release, but there's a lot of work still to
> be done, so that's not a firm date.

Absolutely. There's no such thing as a firm date for anything, I (among
others) are following the development closely, testing, and helping wherever
possible.

Cheers,

Bruce

>
> Regards,
>   Vin Shelton
> _______________________________________________
> yocto mailing list
> yocto at yoctoproject.org
> https://lists.yoctoproject.org/listinfo/yocto



-- 
"Thou shalt not follow the NULL pointer, for chaos and madness await
thee at its end"