Re: 2.6.13-rc6-rt9

["Paul E. McKenney" <paulmck@us.ibm.com>]

On Fri, Sep 30, 2005 at 03:07:29AM +0200, Thomas Gleixner wrote:
> On Thu, 2005-09-29 at 18:00 -0700, Paul E. McKenney wrote:
> > > Even with a current -rt (2.6.14-rc2-rt5) UML does not run. The issue is
> > > indeed (as jeff pointed out) that VTALRM is never send. The small test
> > > programm below illustrates this.
> > > 
> > > On a non-rt kernel it completed in 1 second.
> > > On a -rt kernel it waits at infinitum.
> > 
> > Will play with it and see what I broke...
> 
> Paul, 
> 
> you are not the culprit :)

Woo-hoo!!!  Exonerated!!!  This time, anyway...  ;-)

> The run_posix_cpu_timers(p) call is #ifdef'd out with PREEMPT_RT. 
> 
> Thats a hard to fix issue. 
> 
> It can not be run from hardirq context, as it takes a lot of locks
> (especially our favorites: tasklist_lock and sighand->siglock). :(
> 
> Maybe another playground for rcu, but it might also be solved by some
> other mechanism for accounting and delayed execution in the PREEMPT_RT
> case.

Certainly check_thread_timers() and check_process_timers() are playing
with a number of task_struct fields, so it is not immediately clear
to me how to safely replace tasklist_lock with RCU, at least not with
a simple and small patch.

What did you have in mind for delayed execution?

							Thanx, Paul