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

Ingo Molnar
2005-03-24T11:40:15Z

* 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.)

Ingo