Re: [PATCH] rcu: refine structure rcu_node field for rcu boost

[Paul E. McKenney]

On Thu, Jan 18, 2018 at 06:33:43PM +0800, Liu, Changcheng wrote:
> Do not allocate space for rcu boost field when
> RCU BOOST is not configured.
> 
> Signed-off-by: Liu Changcheng <changcheng.liu@xxxxxxxxx>

The added #ifdef in rcu_read_unlock_special() is a deal-breaker.
Just out of curiosity, is this decrease in storage measurable at
the system level?

							Thanx, Paul

> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index 46a5d19..88f087e 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -129,6 +129,7 @@ struct rcu_node {
>  				/*  if there is no such task.  If there */
>  				/*  is no current expedited grace period, */
>  				/*  then there can cannot be any such task. */
> +#ifdef CONFIG_RCU_BOOST
>  	struct list_head *boost_tasks;
>  				/* Pointer to first task that needs to be */
>  				/*  priority boosted, or NULL if no priority */
> @@ -153,6 +154,8 @@ struct rcu_node {
>  				/* Number of tasks boosted for expedited GP. */
>  	unsigned long n_normal_boosts;
>  				/* Number of tasks boosted for normal GP. */
> +#endif/* #ifdef CONFIG_RCU_BOOST*/
> +
>  #ifdef CONFIG_RCU_NOCB_CPU
>  	struct swait_queue_head nocb_gp_wq[2];
>  				/* Place for rcu_nocb_kthread() to wait GP. */
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index db85ca3..fee0b1e 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -506,8 +506,10 @@ void rcu_read_unlock_special(struct task_struct *t)
>  		if (IS_ENABLED(CONFIG_RCU_BOOST)) {
>  			/* Snapshot ->boost_mtx ownership w/rnp->lock held. */
>  			drop_boost_mutex = rt_mutex_owner(&rnp->boost_mtx) == t;
> +#ifdef CONFIG_RCU_BOOST
>  			if (&t->rcu_node_entry == rnp->boost_tasks)
>  				rnp->boost_tasks = np;
> +#endif
>  		}
> 
>  		/*
> -- 
> 2.7.4
>