Re: [RFC PATCH 1/1] rcu: Use rcu_gp_kthread_wake() to wake up kthreads

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, Jul 25, 2014 at 01:06:58AM -0400, Pranith Kumar wrote:
> The rcu_gp_kthread_wake() function checks for three conditions before waking up
> grace period kthreads:
> 
> *  Is the thread we are trying to wake up the current thread?
> *  Are the gp_flags zero? (all threads wait on non-zero gp_flags condition)
> *  Is there no thread created for this flavour, hence nothing to wake up?
> 
> If any one of these condition is true, we do not call wake_up(). 
> 
> In rcu_report_qs_rsp(), I added a pr_info() call testing if any of the above
> conditions is true, in which case we can avoid calling wake_up(). It turns out
> that quite a few actually are. Most of the cases where we can avoid is condition 2
> above and condition 1 also occurs quite often. Condition 3 never happens.
> 
> I could not test the wake_up() in force_quiescent_state() as that is not
> triggered trivially, but I am assuming we can replace wake_up() there too.
> 
> Hence this commit tries to avoid calling wake_up() whenever we can by using
> rcu_gp_kthread_wake() function.

This one does sound much more plausible than the earlier one.  I have
a few more questions that I will ask in your follow-up message.

> One concern is the comment which states that we need a memory barrier at that
> location which is being implied by the wake_up(). Should we put an smp_mb() and
> just not rely on the barrier provided by wake_up()? Thoughts?

Let's see...  The memory barriers are unnecessary for your case 1
and case 3.  That leaves your case 2, which is all about ->gp_flags.
It is quite possible that this case is now fully covered by locking,
so that the comment is obsolete.  But why don't you check?

							Thanx, Paul

> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
> ---
>  kernel/rcu/tree.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 72e0b1f..d0e0d6e 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1938,7 +1938,8 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
>  {
>  	WARN_ON_ONCE(!rcu_gp_in_progress(rsp));
>  	raw_spin_unlock_irqrestore(&rcu_get_root(rsp)->lock, flags);
> -	wake_up(&rsp->gp_wq);  /* Memory barrier implied by wake_up() path. */
> +	/* Memory barrier implied by wake_up() path. */
> +	rcu_gp_kthread_wake(rsp);
>  }
> 
>  /*
> @@ -2516,7 +2517,8 @@ static void force_quiescent_state(struct rcu_state *rsp)
>  	ACCESS_ONCE(rsp->gp_flags) =
>  		ACCESS_ONCE(rsp->gp_flags) | RCU_GP_FLAG_FQS;
>  	raw_spin_unlock_irqrestore(&rnp_old->lock, flags);
> -	wake_up(&rsp->gp_wq);  /* Memory barrier implied by wake_up() path. */
> +	/* Memory barrier implied by wake_up() path. */
> +	rcu_gp_kthread_wake(rsp);
>  }
> 
>  /*
> -- 
> 2.0.1
>