Messages in this thread | | | Date | Tue, 22 Sep 2009 09:53:14 +1000 | From | Peter Williams <> | Subject | Re: sched: Am I missing something? |
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On 21/09/09 23:53, Peter Zijlstra wrote: > On Mon, 2009-09-21 at 23:22 +1000, Peter Williams wrote: >> Or is the line: >> >> p->prio = effective_prio(p); >> >> in wake_up_new_task() an expensive no op. >> >> As far as I can tell from reading the code, it will always be the case >> that EITHER rt_prio(p->prio) is true OR p->prio == p->normal_prio when >> this call is made and, in either case, the value of p->prio will be >> unchanged. In addition, when this call is made p->normal_prio is >> already equal to to normal_prio(p), so the side effects of the function >> (setting p->normal_prio) are also unnecessary. >> >> Am I correct or have I missed something? > > Yuck @ all that prio code.. > > I think you're right, sched_fork() resets the prio, so poking at it in > wake_up_new_task() seems superfluous.
After more thought, I also think it would be dangerous if it did actually change the value from/to a real time priority to/from a non real time priority as it runs the risk of leaving p with an inappropriate sched_class if CONFIG_RT_MUTEXES is defined. It seems to me that if CONFIG_RT_MUTEXES is defined then any changes to a task's prio field needs to be accompanied by code to ensure the task has the correct sched_class value as well.
> > I've been meaning to re-write most of the PI code one of these days, but > so far I've not had time to. > > My initial goal is to replace plist with a rb-tree and fix some of the > boost paths to be inside the scheduler. That is, we currently have the > fun situation that we boost a lock owner, which becomes runnable, gets > pushed to another cpu, then current blocks and reschedules, leaving this > cpu to again sort out work. > > It would be much easier if we'd first dequeue current, then boost and > then select the owner. Saves a bit of bouncing around. > > The rb-tree is needed for things like PI on CFS (yes, you can do a form > of PI on proportional schedulers), and we're going to look at doing a > full sporadic task model deadline scheduler, which needs both deadline > inheritance and bandwidth inheritance.
I think that the __normal_prio(), normal_prio() and effective_prio() code are inefficient remnants of the old cpu scheduler that missed out on being cleaned up during the switch to CFS. I think that they can be cleaned up a bit independently of the changes to the PI code that you mention. I'll look at it further and see if I can come up with a patch.
Peter -- Peter Williams pwil3058@bigpond.net.au
"Learning, n. The kind of ignorance distinguishing the studious." -- Ambrose Bierce
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