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.f...@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?
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.f...@gmail.com>
> [ paulmck: Disable preemption to close offline race window. ]
> Signed-off-by: Paul E. McKenney <paul...@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;
> }
>
>
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