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 <[email protected]>
> > 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 <[email protected]>
> > [ paulmck: Disable preemption to close offline race window. ]
> > Signed-off-by: Paul E. McKenney <[email protected]>
> > [ 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;
> > }
> >
> >
