Re: [PATCH tip/core/rcu 05/15] rcu: Improve boost selection whenmoving tasks to root rcu_node

[Josh Triplett]

On Thu, Aug 30, 2012 at 11:56:18AM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paul.mckenney@xxxxxxxxxx>
> 
> The rcu_preempt_offline_tasks() moves all tasks queued on a given leaf
> rcu_node structure to the root rcu_node, which is done when the last CPU
> corresponding the the leaf rcu_node structure goes offline.  Now that
> RCU-preempt's synchronize_rcu_expedited() implementation blocks CPU-hotplug
> operations during the initialization of each rcu_node structure's
> ->boost_tasks pointer, rcu_preempt_offline_tasks() can do a better job
> of setting the root rcu_node's ->boost_tasks pointer.
> 
> The key point is that rcu_preempt_offline_tasks() runs as part of the
> CPU-hotplug process, so that a concurrent synchronize_rcu_expedited() is
> guaranteed to either have not started on the one hand (in which case there
> is no boosting on behalf of the expedited grace period) to be completely

Missing word: s/to be/or to be/

> initialized on the other (in which case, in absence of other priority

s/absence/the absence/

> boosting, all ->boost_tasks pointers will be initialized).  Therefore,
> if rcu_preempt_offline_tasks() finds that the ->boost_tasks pointer is
> equal to the ->exp_tasks pointer, it can be sure that it is correcty
> placed.
> 
> The case where there was boosting ongoing at the time that the

s/The/In the/

> synchronize_rcu_expedited() function started, different nodes might
> start boosting the tasks blocking the expedited grace period at different
> times.  In this mixed case, the root node will either be boosting tasks
> for the expedited grace period already, or it will start as soon as it
> gets done boosting for the normal grace period -- but in this latter
> case, the root node's tasks needed to be boosted in any case.
> 
> This commit therefore adds a check of the ->boost_tasks pointer against
> the ->exp_tasks pointer to the list that prevents updating ->boost_tasks.

Seems like some hint of this explanation really ought to end up in a
comment somewhere...

> Signed-off-by: Paul E. McKenney <paul.mckenney@xxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>

> ---
>  kernel/rcutree_plugin.h |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> index b1b4851..c930a47 100644
> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -591,7 +591,8 @@ static int rcu_preempt_offline_tasks(struct rcu_state *rsp,
>  	/* In case root is being boosted and leaf was not. */
>  	raw_spin_lock(&rnp_root->lock); /* irqs already disabled */
>  	if (rnp_root->boost_tasks != NULL &&
> -	    rnp_root->boost_tasks != rnp_root->gp_tasks)
> +	    rnp_root->boost_tasks != rnp_root->gp_tasks &&
> +	    rnp_root->boost_tasks != rnp_root->exp_tasks)
>  		rnp_root->boost_tasks = rnp_root->gp_tasks;
>  	raw_spin_unlock(&rnp_root->lock); /* irqs still disabled */
>  #endif /* #ifdef CONFIG_RCU_BOOST */
> -- 
> 1.7.8
> 
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