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Message-ID: <20190810033814.GP28441@linux.ibm.com>
Date: Fri, 9 Aug 2019 20:38:14 -0700
From: "Paul E. McKenney" <paulmck@...ux.ibm.com>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: linux-kernel@...r.kernel.org, Rao Shoaib <rao.shoaib@...cle.com>,
max.byungchul.park@...il.com, byungchul.park@....com,
kernel-team@...roid.com, kernel-team@....com,
Davidlohr Bueso <dave@...olabs.net>,
Josh Triplett <josh@...htriplett.org>,
Lai Jiangshan <jiangshanlai@...il.com>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
rcu@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH RFC v1 1/2] rcu/tree: Add basic support for kfree_rcu
batching
On Fri, Aug 09, 2019 at 10:42:32PM -0400, Joel Fernandes wrote:
> On Wed, Aug 07, 2019 at 10:52:15AM -0700, Paul E. McKenney wrote:
> [snip]
> > > > > @@ -3459,6 +3645,8 @@ void __init rcu_init(void)
> > > > > {
> > > > > int cpu;
> > > > >
> > > > > + kfree_rcu_batch_init();
> > > >
> > > > What happens if someone does a kfree_rcu() before this point? It looks
> > > > like it should work, but have you tested it?
> > > >
> > > > > rcu_early_boot_tests();
> > > >
> > > > For example, by testing it in rcu_early_boot_tests() and moving the
> > > > call to kfree_rcu_batch_init() here.
> > >
> > > I have not tried to do the kfree_rcu() this early. I will try it out.
> >
> > Yeah, well, call_rcu() this early came as a surprise to me back in the
> > day, so... ;-)
>
> I actually did get surprised as well!
>
> It appears the timers are not fully initialized so the really early
> kfree_rcu() call from rcu_init() does cause a splat about an initialized
> timer spinlock (even though future kfree_rcu()s and the system are working
> fine all the way into the torture tests).
>
> I think to resolve this, we can just not do batching until early_initcall,
> during which I have an initialization function which switches batching on.
> >From that point it is safe.
Just go ahead and batch, but don't bother with the timer until
after single-threaded boot is done. For example, you could check
rcu_scheduler_active similar to how sync_rcu_exp_select_cpus() does.
(See kernel/rcu/tree_exp.h.)
If needed, use an early_initcall() to handle the case where early boot
kfree_rcu() calls needed to set the timer but could not.
> Below is the diff on top of this patch, I think this should be good but let
> me know if anything looks odd to you. I tested it and it works.
Keep in mind that a call_rcu() callback can't possibly be invoked until
quite some time after the scheduler is up and running. So it will be
a lot simpler to just skip setting the timer during early boot.
Thanx, Paul
> have a great weekend! thanks,
> -Joel
>
> ---8<-----------------------
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index a09ef81a1a4f..358f5c065fa4 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2634,6 +2634,7 @@ struct kfree_rcu_cpu {
> };
>
> static DEFINE_PER_CPU(struct kfree_rcu_cpu, krc);
> +int kfree_rcu_batching_ready;
>
> /*
> * This function is invoked in workqueue context after a grace period.
> @@ -2742,6 +2743,17 @@ static void kfree_rcu_monitor(struct work_struct *work)
> spin_unlock_irqrestore(&krcp->lock, flags);
> }
>
> +/*
> + * This version of kfree_call_rcu does not do batching of kfree_rcu() requests.
> + * Used only by rcuperf torture test for comparison with kfree_rcu_batch()
> + * or during really early init.
> + */
> +void kfree_call_rcu_nobatch(struct rcu_head *head, rcu_callback_t func)
> +{
> + __call_rcu(head, func, -1, 1);
> +}
> +EXPORT_SYMBOL_GPL(kfree_call_rcu_nobatch);
> +
> /*
> * Queue a request for lazy invocation of kfree() after a grace period.
> *
> @@ -2764,6 +2775,10 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> unsigned long flags;
> struct kfree_rcu_cpu *krcp;
> bool monitor_todo;
> + static int once;
> +
> + if (!READ_ONCE(kfree_rcu_batching_ready))
> + return kfree_call_rcu_nobatch(head, func);
>
> local_irq_save(flags);
> krcp = this_cpu_ptr(&krc);
> @@ -2794,16 +2809,6 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
> }
> EXPORT_SYMBOL_GPL(kfree_call_rcu);
>
> -/*
> - * This version of kfree_call_rcu does not do batching of kfree_rcu() requests.
> - * Used only by rcuperf torture test for comparison with kfree_rcu_batch().
> - */
> -void kfree_call_rcu_nobatch(struct rcu_head *head, rcu_callback_t func)
> -{
> - __call_rcu(head, func, -1, 1);
> -}
> -EXPORT_SYMBOL_GPL(kfree_call_rcu_nobatch);
> -
> /*
> * During early boot, any blocking grace-period wait automatically
> * implies a grace period. Later on, this is never the case for PREEMPT.
> @@ -3650,17 +3655,6 @@ static void __init rcu_dump_rcu_node_tree(void)
> pr_cont("\n");
> }
>
> -void kfree_rcu_batch_init(void)
> -{
> - int cpu;
> -
> - for_each_possible_cpu(cpu) {
> - struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
> - spin_lock_init(&krcp->lock);
> - INIT_DELAYED_WORK(&krcp->monitor_work, kfree_rcu_monitor);
> - }
> -}
> -
> struct workqueue_struct *rcu_gp_wq;
> struct workqueue_struct *rcu_par_gp_wq;
>
> @@ -3668,8 +3662,6 @@ void __init rcu_init(void)
> {
> int cpu;
>
> - kfree_rcu_batch_init();
> -
> rcu_early_boot_tests();
>
> rcu_bootup_announce();
> @@ -3700,6 +3692,21 @@ void __init rcu_init(void)
> srcu_init();
> }
>
> +static int __init kfree_rcu_batch_init(void)
> +{
> + int cpu;
> +
> + for_each_possible_cpu(cpu) {
> + struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
> + spin_lock_init(&krcp->lock);
> + INIT_DELAYED_WORK(&krcp->monitor_work, kfree_rcu_monitor);
> + }
> +
> + WRITE_ONCE(kfree_rcu_batching_ready, 1);
> + return 0;
> +}
> +early_initcall(kfree_rcu_batch_init);
> +
> #include "tree_stall.h"
> #include "tree_exp.h"
> #include "tree_plugin.h"
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