Re: [patch] Real-Time Preemption, -RT-2.6.12-rc1-V0.7.41-07

Esben Nielsen
2005-03-24T23:06:00Z

On Thu, 24 Mar 2005, Ingo Molnar wrote:

>
> * Steven Rostedt <rostedt@goodmis.org> wrote:
>
> > Here we have more unnecessary schedules. So the condition to grab a
> > lock should be:
> >
> > 1. not owned.
> > 2. partially owned, and the owner is not RT.
> > 3. partially owned but the owner is RT and so is the grabber, and the
> > grabber's priority is >= the owner's priority.
>
> there's another approach that could solve this problem: let the
> scheduler sort it all out. Esben Nielsen had this suggestion a couple of
> months ago - i didnt follow it because i thought that technique would
> create too many runnable tasks, but maybe that was a mistake. If we do
> the owning of the lock once the wakee hits the CPU we avoid the 'partial
> owner' problem, and we have the scheduler sort out priorities and
> policies.
>
> but i think i like the 'partial owner' (or rather 'owner pending')
> technique a bit better, because it controls concurrency explicitly, and
> it would thus e.g. allow another trick: when a new owner 'steals' a lock
> from another in-flight task, then we could 'unwakeup' that in-flight
> thread which could thus avoid two more context-switches on e.g. SMP
> systems: hitting the CPU and immediately blocking on the lock. (But this
> is a second-phase optimization which needs some core scheduler magic as
> well, i guess i'll be the one to code it up.)
>

I checked the implementation of a mutex I send in last fall. The unlock
operation does give ownership explicitly to the highest priority waiter,
as Ingo's implementation does.

Originally I planned for just having unlock to wake up the highest
priority owner and set lock->owner = NULL. The lock operation would be
something like
while(lock->owner!=NULL)
{
schedule();
}
grap the lock.

Then the first task, i.e. the one with highest priority on UP, will get it
first. On SMP a low priority task on another CPU might get in and take it.

I like the idea of having the scheduler take care of it - it is a very
optimal coded queue-system after all. That will work on UP but not on SMP.
Having the unlock operation to set the mutex in a "partially owned" state
will work better. The only problem I see, relative to Ingo's
implementation, is that then the awoken task have to go in and
change the state of the mutex, i.e. it has to lock the wait_lock again.
Will the extra schedulings being the problem happen offen enough in
practise to have the extra overhead?

> Ingo

Esben