Re: [PATCH] rcutorture: add random preemption

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

On Mon, Jun 21, 2010 at 06:50:11PM +0200, Peter Zijlstra wrote:
> On Mon, 2010-06-21 at 09:43 -0700, Paul E. McKenney wrote:
> > > +#ifdef CONFIG_PREEMPT
> > > +     if (!preempt_count() && !(rcu_random(rrsp) % (nrealreaders * 20000)))
> > > +             preempt_schedule();
> > > +#endif
> > 
> > This one scared me for a bit -- then I realized that preempt_schedule()
> > won't actually schedule if preemption is in any way disabled.  So the
> > above really is OK, because Classic RCU and RCU-bh disable preemption.
> > 
> > So, should we have a comment to this effect, or is my hypersensitivity to
> > RCU semantics unique to me? 
> 
> Well it seems to do a !preempt_count() test too, so I wouldn't worry too
> much about it, still using preempt_schedule() doesn't seem right, why
> not use cond_resched()?

My guess is that Lai wants to force a call to rcu_sched_qs() even if
!need_resched().  One reason for doing this would be to put more stress
on preemptible RCU's handling of context switches in RCU read-side
critical sections.

Another approach would be to have some high-priority threads that wake up
periodically, as I plan to make rcutorture do for RCU priority boosting.
Of course, this is not necessarily mutually exclusive with Lai's approach.

							Thanx, Paul