Re: [PATCH v2] rcu: Use rcu_gp_kthread_wake() to wake up grace period kthreads

[Paul E. McKenney]

On Tue, Aug 12, 2014 at 01:07:47PM -0400, Pranith Kumar wrote:
> Updated changelog regarding the removed comment about the implied barrier
> provided by wake_up() logic.
> 
> --
> Pranith

Queued for 3.18, thank you!

							Thanx, Paul

> 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().
> It was found that there are quite a few avoidable wake ups both during
> idle time and under stress induced by rcutorture.
> 
> Idle:
> 
> Total:66000, unnecessary:66000, case1:61827, case2:66000, case3:0
> Total:68000, unnecessary:68000, case1:63696, case2:68000, case3:0
> 
> rcutorture:
> 
> Total:254000, unnecessary:254000, case1:199913, case2:254000, case3:0
> Total:256000, unnecessary:256000, case1:201784, case2:256000, case3:0
> 
> Here case{1-3} are the cases listed above. We can avoid these wake ups by using
> rcu_gp_kthread_wake() to conditionally wake up the grace period kthreads.
> 
> There is a comment about an implied barrier supplied by the wake_up() logic.
> This barrier is necessary for the awakened thread to see the updated ->gp_flags.
> This flag is always being updated with the root node lock held. Also, the
> awakened thread tries to acquire the root node lock before reading ->gp_flags
> because of which there is proper ordering.
> 
> Hence this commit tries to avoid calling wake_up() whenever we can by using
> rcu_gp_kthread_wake() function.
> 
> Signed-off-by: Pranith Kumar <bobby.prani@xxxxxxxxx>
> CC: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> ---
>  kernel/rcu/tree.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index b63517c..6b68d2d 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1938,7 +1938,7 @@ 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. */
> +	rcu_gp_kthread_wake(rsp);
>  }
> 
>  /*
> @@ -2516,7 +2516,7 @@ 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. */
> +	rcu_gp_kthread_wake(rsp);
>  }
> 
>  /*
> -- 
> 1.9.1
> 

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