Re: [tip/core/rcu, 05/21] rcu: Make rcu_gp_cleanup() more accurately predict need for new GP

[Joel Fernandes <joel@joelfernandes.org>]

Hi Paul,

On Sun, Apr 22, 2018 at 08:03:28PM -0700, Paul E. McKenney wrote:
> Currently, rcu_gp_cleanup() scans the rcu_node tree in order to reset
> state to reflect the end of the grace period.  It also checks to see
> whether a new grace period is needed, but in a number of cases, rather
> than directly cause the new grace period to be immediately started, it
> instead leaves the grace-period-needed state where various fail-safes
> can find it.  This works fine, but results in higher contention on the
> root rcu_node structure's ->lock, which is undesirable, and contention
> on that lock has recently become noticeable.
> 
> This commit therefore makes rcu_gp_cleanup() immediately start a new
> grace period if there is any need for one.
> 
> It is quite possible that it will later be necessary to throttle the
> grace-period rate, but that can be dealt with when and if.
> 
> Reported-by: Nicholas Piggin <npiggin@gmail.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
>  kernel/rcu/tree.c        | 16 ++++++++++------
>  kernel/rcu/tree.h        |  1 -
>  kernel/rcu/tree_plugin.h | 17 -----------------
>  3 files changed, 10 insertions(+), 24 deletions(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 497f139056c7..afc5e32f0da4 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1763,14 +1763,14 @@ rcu_start_future_gp(struct rcu_node *rnp, struct rcu_data *rdp,
>   * Clean up any old requests for the just-ended grace period.  Also return
>   * whether any additional grace periods have been requested.
>   */
> -static int rcu_future_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
> +static bool rcu_future_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
>  {
>  	int c = rnp->completed;
> -	int needmore;
> +	bool needmore;
>  	struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
>  
>  	need_future_gp_element(rnp, c) = 0;
> -	needmore = need_future_gp_element(rnp, c + 1);
> +	needmore = need_any_future_gp(rnp);
>  	trace_rcu_future_gp(rnp, rdp, c,
>  			    needmore ? TPS("CleanupMore") : TPS("Cleanup"));
>  	return needmore;
> @@ -2113,7 +2113,6 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
>  {
>  	unsigned long gp_duration;
>  	bool needgp = false;
> -	int nocb = 0;
>  	struct rcu_data *rdp;
>  	struct rcu_node *rnp = rcu_get_root(rsp);
>  	struct swait_queue_head *sq;
> @@ -2152,7 +2151,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
>  		if (rnp == rdp->mynode)
>  			needgp = __note_gp_changes(rsp, rnp, rdp) || needgp;
>  		/* smp_mb() provided by prior unlock-lock pair. */
> -		nocb += rcu_future_gp_cleanup(rsp, rnp);
> +		needgp = rcu_future_gp_cleanup(rsp, rnp) || needgp;
>  		sq = rcu_nocb_gp_get(rnp);
>  		raw_spin_unlock_irq_rcu_node(rnp);
>  		rcu_nocb_gp_cleanup(sq);
> @@ -2162,13 +2161,18 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
>  	}
>  	rnp = rcu_get_root(rsp);
>  	raw_spin_lock_irq_rcu_node(rnp); /* Order GP before ->completed update. */
> -	rcu_nocb_gp_set(rnp, nocb);
>  
>  	/* Declare grace period done. */
>  	WRITE_ONCE(rsp->completed, rsp->gpnum);
>  	trace_rcu_grace_period(rsp->name, rsp->completed, TPS("end"));
>  	rsp->gp_state = RCU_GP_IDLE;
> +	/* Check for GP requests since above loop. */
>  	rdp = this_cpu_ptr(rsp->rda);
> +	if (need_any_future_gp(rnp)) {
> +		trace_rcu_future_gp(rnp, rdp, rsp->completed - 1,
> +				    TPS("CleanupMore"));
> +		needgp = true;

Patch makes sense to me.

I didn't get the "rsp->completed - 1" bit in the call to trace_rcu_future_gp.
The grace period that just completed is in rsp->completed. The future one
should be completed + 1. What is meaning of the third argument 'c' to the
trace event?

Also in rcu_future_gp_cleanup, we call:
	trace_rcu_future_gp(rnp, rdp, c,
			    needmore ? TPS("CleanupMore") : TPS("Cleanup"));
For this case, in the final trace event record, rnp->completed and c will be
the same, since c is set to rnp->completed before calling
trace_rcu_future_gp. I was thinking they should be different, do you expect
them to be the same?

thanks!

- Joel