Messages in this thread | | | Date | Mon, 24 Jan 2022 08:44:52 -0800 | From | "Paul E. McKenney" <> | Subject | Re: synchronize_rcu_expedited gets stuck in hotplug path |
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On Mon, Jan 24, 2022 at 07:32:01PM +0530, Mukesh Ojha wrote: > > On 1/19/2022 3:11 AM, Paul E. McKenney wrote: > > On Tue, Jan 18, 2022 at 10:11:34AM -1000, Tejun Heo wrote: > > > Hello, > > > > > > On Tue, Jan 18, 2022 at 12:06:46PM -0800, Paul E. McKenney wrote: > > > > Interesting. Adding Tejun and Lai on CC for their perspective. > > > > > > > > As you say, the incoming CPU invoked synchronize_rcu_expedited() which > > > > in turn invoked queue_work(). By default, workqueues will of course > > > > queue that work on the current CPU. But in this case, the CPU's bit > > > > is not yet set in the cpu_active_mask. Thus, a workqueue scheduled on > > > > the incoming CPU won't be invoked until CPUHP_AP_ACTIVE, which won't > > > > be reached until after the grace period ends, which cannot happen until > > > > the workqueue handler is invoked. > > > > > > > > I could imagine doing something as shown in the (untested) patch below, > > > > but first does this help? > > > > > > > > If it does help, would this sort of check be appropriate here or > > > > should it instead go into workqueues? > > > Maybe it can be solved by rearranging the hotplug sequence but it's fragile > > > to schedule per-cpu work items from hotplug paths. Maybe the whole issue can > > > be side-stepped by making synchronize_rcu_expedited() use unbound workqueue > > > instead? Does it require to be per-cpu? > > Good point! > > > > And now that you mention it, RCU expedited grace periods already avoid > > using workqueues during early boot. The (again untested) patch below > > extends that approach to incoming CPUs. > > > > Thoughts? > > Hi Paul, > > We are not seeing the issue after this patch. > Can we merge this patch ?
It is currently in -rcu and should also be in -next shortly. Left to myself, and assuming further testing and reviews all go well, I would submit it during the upcoming v5.18 merge window.
Does that work for you? Or do you need it in mainline sooner?
Thanx, Paul
> -Mukesh > > > > > Thanx, Paul > > > > ------------------------------------------------------------------------ > > > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > > index 60197ea24ceb9..1a45667402260 100644 > > --- a/kernel/rcu/tree_exp.h > > +++ b/kernel/rcu/tree_exp.h > > @@ -816,7 +816,7 @@ static int rcu_print_task_exp_stall(struct rcu_node *rnp) > > */ > > void synchronize_rcu_expedited(void) > > { > > - bool boottime = (rcu_scheduler_active == RCU_SCHEDULER_INIT); > > + bool no_wq; > > struct rcu_exp_work rew; > > struct rcu_node *rnp; > > unsigned long s; > > @@ -841,9 +841,15 @@ void synchronize_rcu_expedited(void) > > if (exp_funnel_lock(s)) > > return; /* Someone else did our work for us. */ > > + /* Don't use workqueue during boot or from an incoming CPU. */ > > + preempt_disable(); > > + no_wq = rcu_scheduler_active == RCU_SCHEDULER_INIT || > > + !cpumask_test_cpu(smp_processor_id(), cpu_active_mask); > > + preempt_enable(); > > + > > /* Ensure that load happens before action based on it. */ > > - if (unlikely(boottime)) { > > - /* Direct call during scheduler init and early_initcalls(). */ > > + if (unlikely(no_wq)) { > > + /* Direct call for scheduler init, early_initcall()s, and incoming CPUs. */ > > rcu_exp_sel_wait_wake(s); > > } else { > > /* Marshall arguments & schedule the expedited grace period. */ > > @@ -861,7 +867,7 @@ void synchronize_rcu_expedited(void) > > /* Let the next expedited grace period start. */ > > mutex_unlock(&rcu_state.exp_mutex); > > - if (likely(!boottime)) > > + if (likely(!no_wq)) > > destroy_work_on_stack(&rew.rew_work); > > } > > EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
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