On Sat, Jan 18, 2020 at 03:21:31PM -0500, Joel Fernandes wrote:
> 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?
Gah!
Right you are. Please s/kfree/kfree_rcu/.
It was late and I was tired. ;-)
Thanx, Paul
