linux-kernel - Re: sched: Am I missing something?

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Date:	Tue, 22 Sep 2009 09:53:14 +1000
From:	Peter Williams <pwil3058@...pond.net.au>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Ingo Molnar <mingo@...e.hu>, Mike Galbraith <efault@....de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: sched: Am I missing something?

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@...pond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
  -- Ambrose Bierce
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