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

[Paul E. McKenney]

On Wed, Jun 27, 2018 at 09:15:31AM -0700, Paul E. McKenney wrote:
> 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@xxxxxxxxx>
> > > 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.

Hearing none, I have made 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@xxxxxxxxx>
> > >     [ paulmck: Disable preemption to close offline race window. ]
> > >     Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > >     [ 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;
> > >  	}
> > >  
> > > 
> 
>