Re: [PATCH v2] rcu: Only boost rcu reader tasks with lower priority than boost kthreads

[Uladzislau Rezki <urezki@gmail.com>]

> When RCU_BOOST is enabled, the boost kthreads will boosting readers
> who are blocking a given grace period, if the current reader tasks
> have a higher priority than boost kthreads(the boost kthreads priority
> not always 1, if the kthread_prio is set), boosting is useless, skip
> current task and select next task to boosting, reduce the time for a
> given grace period.
> 
> Signed-off-by: Zqiang <qiang1.zhang@intel.com>
> ---
>  v1->v2:
>  rename label 'end' to 'skip_boost'.
>  add 'boost_exp_tasks' pointer to point 'rnp->exp_tasks'
>  do the similar thing as normal grace period.
> 
>  kernel/rcu/tree.h        |  2 ++
>  kernel/rcu/tree_plugin.h | 31 +++++++++++++++++++++++--------
>  2 files changed, 25 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index b8d07bf92d29..862ca09b56c7 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -103,6 +103,8 @@ struct rcu_node {
>  				/*  queued on this rcu_node structure that */
>  				/*  are blocking the current grace period, */
>  				/*  there can be no such task. */
> +	struct list_head *boost_exp_tasks;
> +
>  	struct rt_mutex boost_mtx;
>  				/* Used only for the priority-boosting */
>  				/*  side effect, not as a lock. */
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index c3d212bc5338..22bf5a8040f5 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -12,6 +12,7 @@
>   */
>  
>  #include "../locking/rtmutex_common.h"
> +#include <linux/sched/deadline.h>
>  
>  static bool rcu_rdp_is_offloaded(struct rcu_data *rdp)
>  {
> @@ -535,6 +536,8 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
>  			drop_boost_mutex = rt_mutex_owner(&rnp->boost_mtx.rtmutex) == t;
>  			if (&t->rcu_node_entry == rnp->boost_tasks)
>  				WRITE_ONCE(rnp->boost_tasks, np);
> +			if (&t->rcu_node_entry == rnp->boost_exp_tasks)
> +				WRITE_ONCE(rnp->boost_exp_tasks, np);
>  		}
>  
>  		/*
> @@ -1022,7 +1025,7 @@ static int rcu_boost(struct rcu_node *rnp)
>  	struct task_struct *t;
>  	struct list_head *tb;
>  
> -	if (READ_ONCE(rnp->exp_tasks) == NULL &&
> +	if (READ_ONCE(rnp->boost_exp_tasks) == NULL &&
>  	    READ_ONCE(rnp->boost_tasks) == NULL)
>  		return 0;  /* Nothing left to boost. */
>  
> @@ -1032,7 +1035,7 @@ static int rcu_boost(struct rcu_node *rnp)
>  	 * Recheck under the lock: all tasks in need of boosting
>  	 * might exit their RCU read-side critical sections on their own.
>  	 */
> -	if (rnp->exp_tasks == NULL && rnp->boost_tasks == NULL) {
> +	if (rnp->boost_exp_tasks == NULL && rnp->boost_tasks == NULL) {
>  		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>  		return 0;
>  	}
> @@ -1043,8 +1046,8 @@ static int rcu_boost(struct rcu_node *rnp)
>  	 * expedited grace period must boost all blocked tasks, including
>  	 * those blocking the pre-existing normal grace period.
>  	 */
> -	if (rnp->exp_tasks != NULL)
> -		tb = rnp->exp_tasks;
> +	if (rnp->boost_exp_tasks != NULL)
> +		tb = rnp->boost_exp_tasks;
>  	else
>  		tb = rnp->boost_tasks;
>  
> @@ -1065,14 +1068,24 @@ static int rcu_boost(struct rcu_node *rnp)
>  	 * section.
>  	 */
>  	t = container_of(tb, struct task_struct, rcu_node_entry);
> +	if (dl_task(t) || t->prio <= current->prio) {
This is a bit confusing to me. We know that "current" has SCHED_FIFO
class. In this scenario if "t->prio" has lower value(higher priority)
the task falls into all real-time categories anyway and is either:
  - SCHED_FIFO
  - SCHED_RR
  - SCHED_DEADLINE

Checking whether the task is dl_task() sounds like unnecessary there.
Am i missing something? Could you please comment on it?

Thanks!

--
Vlad Rezki