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

[Boqun Feng]

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?

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;
>  	}
>  
>

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