Re: Real Time scheduler infos

[Peter Zijlstra <peterz@infradead.org>]

On Thu, 2009-12-17 at 15:22 +0100, Ivan Gualandri wrote:
> Hi, 
> 
> i have a question about priorities in RT patchset. 
> 
> In linux priorites are from 1 to 139 (1 highest - 139 lowest)
> And RT prorities are between 1 to 99. An i think 1 is highest and 139
> lowest.
> 
> But now if i try to launch three tasks with 3 different priorities: 
> for example 
> A has priority 10
> B has priority 20
> C has priority 30 
> I notice that the tasks B and C are always the first two to run. And A
> always wait the end of B or C (also if A is the first task launched).
> 
> Now my question is: 
> when i set priority of a RT task (changing the sched_priority field of my
> task) the highest priority is 99 or 1? 

The user exposed priorities are from low to high, that is 1 is the
lowest and 99 (POSIX compliant apps use
sched_get_priority_max(SCHED_FIFO)) is the highest priority, as
specified by POSIX.

The scheduler implementation has this reversed but that should be of no
consequence.