Re: [PATCH]sched: Isochronous class v2 for unprivileged soft rt scheduling

[Con Kolivas]

utz lehmann wrote:
> Hi
>
> I dislike the behavior of the SCHED_ISO patch that iso tasks are
> degraded to SCHED_NORMAL if they exceed the limit.
> IMHO it's better to throttle them at the iso_cpu limit.
>
> I have modified Con's iso2 patch to do this. If iso_cpu > 50 iso tasks
> only get stalled for 1 tick (1ms on x86).

Some tasks are so cache intensive they would make almost no forward 
progress running for only 1ms.

> Fortunately there is a currently unused task prio (MAX_RT_PRIO-1) [1]. I

Your implementation is not correct. The "prio" field of real time tasks 
is determined by MAX_RT_PRIO-1-rt_priority. Therefore you're limiting 
the best real time priority, not the other way around.

Throttling them for only 1ms will make it very easy to starve the system 
 with 1 or more short running (<1ms) SCHED_NORMAL tasks running. Lower 
priority tasks will never run.

Cheers,
Con

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