Re: [PATCH v5 tip/core/rcu 09/16] rcu: Improve RCU-tasks energy efficiency

[Pranith Kumar]

On Thu, Aug 14, 2014 at 5:55 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Aug 14, 2014 at 05:42:06PM -0400, Pranith Kumar wrote:
>> On Mon, Aug 11, 2014 at 6:48 PM, Paul E. McKenney
>> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
>> > From: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
>> >
>> > The current RCU-tasks implementation uses strict polling to detect
>> > callback arrivals.  This works quite well, but is not so good for
>> > energy efficiency.  This commit therefore replaces the strict polling
>> > with a wait queue.
>> >
>> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
>> > ---
>> >  kernel/rcu/update.c | 14 ++++++++++++--
>> >  1 file changed, 12 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
>> > index f1535404a79e..1256a900cd01 100644
>> > --- a/kernel/rcu/update.c
>> > +++ b/kernel/rcu/update.c
>> > @@ -368,6 +368,7 @@ early_initcall(check_cpu_stall_init);
>> >  /* Global list of callbacks and associated lock. */
>> >  static struct rcu_head *rcu_tasks_cbs_head;
>> >  static struct rcu_head **rcu_tasks_cbs_tail = &rcu_tasks_cbs_head;
>> > +static DECLARE_WAIT_QUEUE_HEAD(rcu_tasks_cbs_wq);
>> >  static DEFINE_RAW_SPINLOCK(rcu_tasks_cbs_lock);
>> >
>> >  /* Track exiting tasks in order to allow them to be waited for. */
>> > @@ -381,13 +382,17 @@ module_param(rcu_task_stall_timeout, int, 0644);
>> >  void call_rcu_tasks(struct rcu_head *rhp, void (*func)(struct rcu_head *rhp))
>> >  {
>> >         unsigned long flags;
>> > +       bool needwake;
>> >
>> >         rhp->next = NULL;
>> >         rhp->func = func;
>> >         raw_spin_lock_irqsave(&rcu_tasks_cbs_lock, flags);
>> > +       needwake = !rcu_tasks_cbs_head;
>> >         *rcu_tasks_cbs_tail = rhp;
>> >         rcu_tasks_cbs_tail = &rhp->next;
>> >         raw_spin_unlock_irqrestore(&rcu_tasks_cbs_lock, flags);
>> > +       if (needwake)
>> > +               wake_up(&rcu_tasks_cbs_wq);
>> >  }
>> >  EXPORT_SYMBOL_GPL(call_rcu_tasks);
>>
>> I think you want
>>
>> needwake = !!rcu_tasks_cbs_head;
>>
>> otherwise it will wake up when rcu_tasks_cbs_head is null, no?
>
> Well, that is exactly what we want.  Note that we do the test -before-
> the enqueue.  This means that we do the wakeup if the list -was-
> empty before the enqueue, which is exactly the case where the task
> might be asleep without having already been sent a wakeup.
>
> Assuming that wakeups are reliably delivered, of course.  But if they
> are not reliably delivered, that is a bug that needs to be fixed.
>

Ohk, I did not notice the modification through rcu_tasks_cbs_tail! All is well.

-- 
Pranith
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