Re: RCU_BOOST not working for me

From: "Paul E. McKenney" <paulmck@kernel.org>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: rcu@vger.kernel.org, rostedt@goodmis.org, bristot@redhat.com
Subject: Re: RCU_BOOST not working for me
Date: Sat, 18 Jan 2020 14:29:59 -0800	[thread overview]
Message-ID: <20200118222959.GW2935@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <20200118202131.GE244899@google.com>

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