Re: sched: Am I missing something?

From: Peter Williams
Date: Wed Sep 23 2009 - 10:36:51 EST


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.

The recent sched_reset_on_fork changes to sched_fork() invalidate my statement (above) about p->normal_prio as they do not set it to the correct value (in all cases). But it now means that the only useful work done by the above line in wake_up_new_task() is the side effect of setting p->normal_prio. I think it would be better if it was set properly in sched_fork(). I'll send a patch tomorrow.


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.

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.

Peter
--
Peter Williams pwil3058@xxxxxxxxxxxxxx

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