Messages in this thread | | | Date | Sun, 10 Aug 2003 08:41:18 +0200 | From | Mike Galbraith <> | Subject | Re: [patch] SCHED_SOFTRR starve-free linux scheduling policy ... |
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At 01:41 AM 8/10/2003 +0100, Daniel Phillips wrote: >On Saturday 09 August 2003 18:47, Mike Galbraith wrote: > > > But the patch has a much bigger problem: there is no way a SOFTRR > task can > > > be realtime as long as higher priority non-realtime tasks can preempt it. > > > The new dynamic priority adjustment makes it certain that we will > > > regularly see normal tasks with priority elevated above so-called > > > realtime tasks. Even without dynamic priority adjustment, any higher > > > priority system task can unwttingly make a mockery of realtime schedules. > > > > Not so. > >Yes so. A SCHED_NORMAL task with priority n can execute even when a >SCHED_FIFO/RR/SOFTRR task of priority n-1 is ready. In the case of FIFO and >RR we don't care because they're already unusable by normal users but in the >case of SOFTRR it defeats the intended realtime gaurantee.
No, _not_ so. How is the SCHED_NORMAL (didn't that used to be called SCHED_OTHER?) task ever going to receive the cpu when a realtime task is runnable given that 1. task selection is done via sched_find_first_bit(), and 2. realtime queues reside at the top of the array?
> > Dynamic priority adjustment will not put a SCHED_OTHER task above > > SCHED_RR, SCHED_FIFO or SCHED_SOFTRR, so they won't preempt. > >Are you sure? I suppose that depends on the particular flavor of dynamic >priority adjustment. The last I saw, dynamic priority can adjust the task >priority by 5 up or down. If I'm wrong, please show me why and hopefully >point at specific code.
See the definition of rt_task() in sched.c, and the comments in sched.h beginning at line 266.
-Mike
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