Messages in this thread | | | Date | Thu, 28 Jun 2018 09:52:41 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/rcu 2/2] rcu: Make expedited GPs handle CPU 0 being offline |
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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@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.
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@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; > > > } > > > > > > > >
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