Re: [PATCH tip/core/rcu 2/2] rcu: Make expedited GPs handle CPU 0 being offline

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

On Wed, Jun 27, 2018 at 10:42:01AM +0800, Boqun Feng wrote:
> On Tue, Jun 26, 2018 at 12:27:47PM -0700, Paul E. McKenney wrote:
> > On Tue, Jun 26, 2018 at 07:46:52PM +0800, Boqun Feng wrote:
> > > On Tue, Jun 26, 2018 at 06:44:47PM +0800, Boqun Feng wrote:
> > > > On Tue, Jun 26, 2018 at 11:38:20AM +0200, Peter Zijlstra wrote:
> > > > > On Mon, Jun 25, 2018 at 03:43:32PM -0700, Paul E. McKenney wrote:
> > > > > > +		preempt_disable();
> > > > > > +		for_each_leaf_node_possible_cpu(rnp, cpu) {
> > > > > > +			if (cpu_is_offline(cpu)) /* Preemption disabled. */
> > > > > > +				continue;
> > > > > 
> > > > > Create for_each_node_online_cpu() instead? Seems a bit pointless to
> > > > > iterate possible mask only to then check it against the online mask.
> > > > > Just iterate the online mask directly.
> > > > > 
> > > > > Or better yet, write this as:
> > > > > 
> > > > > 	preempt_disable();
> > > > > 	cpu = cpumask_next(rnp->grplo - 1, cpu_online_mask);
> > > > > 	if (cpu > rnp->grphi)
> > > > > 		cpu = WORK_CPU_UNBOUND;
> > > > > 	queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work);
> > > > > 	preempt_enable();
> > > > > 
> > > > > Which is what it appears to be doing.
> > > > > 
> > > > 
> > > > Make sense! Thanks ;-)
> > > > 
> > > > Applied this and running a TREE03 rcutorture. If all go well, I will
> > > > send the updated patch.
> > > > 
> > > 
> > > So the patch has passed one 30 min run for TREE03 rcutorture. Paul,
> > > if it looks good, could you take it for your next spin or pull request
> > > in the future? Thanks.
> > 
> > I ended up with the following, mostly just rewording the comment and
> > adding a one-liner on the change.  Does this work for you?
> 
> Looks good to me. Only one thing I think we need to modify a little,
> please see below:
> 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > commit ef31fa78032536d594630d7bd315d3faf60d98ca
> > Author: Boqun Feng <boqun.feng@gmail.com>
> > Date:   Fri Jun 15 12:06:31 2018 -0700
> > 
> >     rcu: Make expedited GPs handle CPU 0 being offline
> >     
> >     Currently, the parallelized initialization of expedited grace periods uses
> >     the workqueue associated with each rcu_node structure's ->grplo field.
> >     This works fine unless that CPU is offline.  This commit therefore
> >     uses the CPU corresponding to the lowest-numbered online CPU, or just
> >     reports the quiescent states if there are no online CPUs on this rcu_node
> >     structure.
> 
> better write "or just queue the work on WORK_CPU_UNBOUND if there are
> no online CPUs on this rcu_node structure"? Because we currently don't
> report the QS directly if all CPU are offline.
> 
> Thoughts?

Any objections?  If I don't hear any by tomorrow morning (Pacific Time),
I will make this change.

							Thanx, Paul

> Regards,
> Boqun
> 
> >     
> >     Note that this patch uses cpu_is_offline() instead of the usual
> >     approach of checking bits in the rcu_node structure's ->qsmaskinitnext
> >     field.  This is safe because preemption is disabled across both the
> >     cpu_is_offline() check and the call to queue_work_on().
> >     
> >     Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> >     [ paulmck: Disable preemption to close offline race window. ]
> >     Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> >     [ paulmck: Apply Peter Zijlstra feedback on CPU selection. ]
> > 
> > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> > index c6385ee1af65..b3df3b770afb 100644
> > --- a/kernel/rcu/tree_exp.h
> > +++ b/kernel/rcu/tree_exp.h
> > @@ -472,6 +472,7 @@ static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
> >  static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
> >  				     smp_call_func_t func)
> >  {
> > +	int cpu;
> >  	struct rcu_node *rnp;
> >  
> >  	trace_rcu_exp_grace_period(rsp->name, rcu_exp_gp_seq_endval(rsp), TPS("reset"));
> > @@ -493,7 +494,13 @@ static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
> >  			continue;
> >  		}
> >  		INIT_WORK(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
> > -		queue_work_on(rnp->grplo, rcu_par_gp_wq, &rnp->rew.rew_work);
> > +		preempt_disable();
> > +		cpu = cpumask_next(rnp->grplo - 1, cpu_online_mask);
> > +		/* If all offline, queue the work on an unbound CPU. */
> > +		if (unlikely(cpu > rnp->grphi))
> > +			cpu = WORK_CPU_UNBOUND;
> > +		queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work);
> > +		preempt_enable();
> >  		rnp->exp_need_flush = true;
> >  	}
> >  
> >