linux-kernel - Re: [PATCH v5 tip/core/rcu 11/16] rcu: Defer rcu_tasks_kthread() creation till first call_rcu

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Message-ID: <CAJhHMCAvzXUgCuiO5nDB0oZYkFsyJ1bdJmz5JML4jrLrC5F0qw@mail.gmail.com>
Date:	Thu, 14 Aug 2014 18:28:53 -0400
From:	Pranith Kumar <bobby.prani@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Josh Triplett <josh@...htriplett.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Howells <dhowells@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>, dvhart@...ux.intel.com,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v5 tip/core/rcu 11/16] rcu: Defer rcu_tasks_kthread()
 creation till first call_rcu_tasks()

On Mon, Aug 11, 2014 at 6:49 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
>
> It is expected that many sites will have CONFIG_TASKS_RCU=y, but
> will never actually invoke call_rcu_tasks().  For such sites, creating
> rcu_tasks_kthread() at boot is wasteful.  This commit therefore defers
> creation of this kthread until the time of the first call_rcu_tasks().
>
> This of course means that the first call_rcu_tasks() must be invoked
> from process context after the scheduler is fully operational.
>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---
>  kernel/rcu/update.c | 33 ++++++++++++++++++++++++++-------
>  1 file changed, 26 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index 1256a900cd01..d997163c7e92 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -378,7 +378,12 @@ DEFINE_SRCU(tasks_rcu_exit_srcu);
>  static int rcu_task_stall_timeout __read_mostly = HZ * 60 * 10;
>  module_param(rcu_task_stall_timeout, int, 0644);
>
> -/* Post an RCU-tasks callback. */
> +static void rcu_spawn_tasks_kthread(void);
> +
> +/*
> + * Post an RCU-tasks callback.  First call must be from process context
> + * after the scheduler if fully operational.
> + */
>  void call_rcu_tasks(struct rcu_head *rhp, void (*func)(struct rcu_head *rhp))
>  {
>         unsigned long flags;
> @@ -391,8 +396,10 @@ void call_rcu_tasks(struct rcu_head *rhp, void (*func)(struct rcu_head *rhp))
>         *rcu_tasks_cbs_tail = rhp;
>         rcu_tasks_cbs_tail = &rhp->next;
>         raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
> -       if (needwake)
> +       if (needwake) {
> +               rcu_spawn_tasks_kthread();
>                 wake_up(&rcu_tasks_cbs_wq);
> +       }
>  }
>  EXPORT_SYMBOL_GPL(call_rcu_tasks);
>
> @@ -618,15 +625,27 @@ static int __noreturn rcu_tasks_kthread(void *arg)
>         }
>  }
>
> -/* Spawn rcu_tasks_kthread() at boot time. */
> -static int __init rcu_spawn_tasks_kthread(void)
> +/* Spawn rcu_tasks_kthread() at first call to call_rcu_tasks(). */
> +static void rcu_spawn_tasks_kthread(void)
>  {
> -       struct task_struct __maybe_unused *t;
> +       static DEFINE_MUTEX(rcu_tasks_kthread_mutex);
> +       static struct task_struct *rcu_tasks_kthread_ptr;
> +       struct task_struct *t;
>
> +       if (ACCESS_ONCE(rcu_tasks_kthread_ptr)) {
> +               smp_mb(); /* Ensure caller sees full kthread. */
> +               return;
> +       }

I don't see the need for this smp_mb(). The caller has already seen
that rcu_tasks_kthread_ptr is assigned. What are we ensuring with this
barrier again?

an smp_rmb() before this ACCESS_ONCE() and an smp_wmb() after
assigning to rcu_tasks_kthread_ptr should be enough, right?

> +       mutex_lock(&rcu_tasks_kthread_mutex);
> +       if (rcu_tasks_kthread_ptr) {
> +               mutex_unlock(&rcu_tasks_kthread_mutex);
> +               return;
> +       }
>         t = kthread_run(rcu_tasks_kthread, NULL, "rcu_tasks_kthread");
>         BUG_ON(IS_ERR(t));
> -       return 0;
> +       smp_mb(); /* Ensure others see full kthread. */
> +       ACCESS_ONCE(rcu_tasks_kthread_ptr) = t;

Isn't it better to reverse these two statements and change as follows?

ACCESS_ONCE(rcu_tasks_kthread_ptr) = t;
smp_wmb();

or

smp_store_release(rcu_tasks_kthread_ptr, t);

will ensure that this write to rcu_task_kthread_ptr is ordered with
the previous read. I recently read memory-barriers.txt, so please
excuse me if I am totally wrong. But I am confused! :(

> +       mutex_unlock(&rcu_tasks_kthread_mutex);
>  }
> -early_initcall(rcu_spawn_tasks_kthread);
>
>  #endif /* #ifdef CONFIG_TASKS_RCU */
> --
> 1.8.1.5
>



-- 
Pranith
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