Re: RCU_BOOST not working for me

[Joel Fernandes <joel@joelfernandes.org>]

On Fri, Jan 17, 2020 at 08:54:36PM -0800, Paul E. McKenney wrote:
> On Fri, Jan 17, 2020 at 08:34:58PM -0800, Paul E. McKenney wrote:
> > On Fri, Jan 17, 2020 at 09:34:34PM -0500, Joel Fernandes wrote:
> > > On Fri, Jan 17, 2020 at 03:17:56PM -0800, Paul E. McKenney wrote:
> > > [...] 
> > > > But rcutorture already has tests for RCU priority boosting.  Or are those
> > > > failing in some way?
> > > 
> > > Yes there are tests, but I thought of just a simple experiment to study this.
> > > Purely since it is existing RCU kernel code that I'd like to understand. And
> > > me/Daniel are also looking into possibly using run-time / trace-based
> > > verification some of these behaviors.
> > 
> > The functionality of rcu_state.cbovld should make that more entertaining.
> > 
> > But I would guess that the initial model would ignore memory footprint
> > and just model RCU priority boosting as kicking in a fixed time after
> > the beginning of the grace period.
> > 
> > Or do you guys have something else in mind?
> > 
> > 							Thanx, Paul
> > 
> > PS.  Steve, yes, I do well remember our earlier discussions about readers
> >      inheriting priority from the highest-priority synchronize_rcu().  ;-)
> 
> To see the reason why RCU priority boosting does not work like that,
> consider a (stupid but legal) situation with way more tasks like this
> than the system can handle:
> 
> 	for (;;) {
> 		cond_resched();
> 		p = kmalloc(sizeof(*p), GFP_ATOMIC);
> 		if (!p)
> 			continue;
> 		rcu_read_lock();
> 		kfree(&p->rh, my_func);
> 		rcu_read_unlock(;
> 	}
> 
> Nothing is ever waiting on the RCU grace period, so there is no natural
> place for priority to be inherited from.
> 
> But the current RCU priority boosting works just fine in this situation,
> at least assuming rcutree.kthread_prio is set suitably.  And if it is
> not working fine in some other situation, it would be good for someone
> to let me in on the secret.  ;-)

But in this example, you don't have anyone starting a grace period. So how
would the boosting be done? Somebody has to kick boosting into action?

thanks,

 - Joel