Re: [PATCH tip/core/rcu 3/4] rcu: add expedited grace-period support for preemptible RCU

[Lai Jiangshan <laijs@cn.fujitsu.com>]

Paul E. McKenney wrote:
> From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> 
> Implement an synchronize_rcu_expedited() for preemptible RCU that actually
> is expedited.  This uses synchronize_sched_expedited() to force all
> threads currently running in a preemptible-RCU read-side critical section
> onto the appropriate ->blocked_tasks[] list, then takes a snapshot of
> all of these lists and waits for them to drain.
> 

> 3.	Add an implementation of synchronize_rcu_expedited() that
> 	actually expedites preemptible-RCU grace periods.

It's very nice.

But I don't understand all things.

1) Why it can be speeded up (in theory)?
synchronize_sched_expedited() does speed up, it is due to
migration_threads are the most highest priority threads.

But for synchronize_rcu_expedited(), some preempted tasks in ->blocked_tasks[]
may be waiting at runqueue for long long time because some other
higher priority threads comes.

simply comparison:
synchronize_sched_expedited()
==> wake_up_process(rq->migration_thread) to force schedule on cpus.
	which forces read-sides notify the end earlier,
	or we can say "it forces read-sides run to end faster"

synchronize_rcu_expedited()
==> Nothing to force preempted read-site run to end faster.

2) Why we introduce a API which no one use it.
I remember that Net guys request a expedited synchronize_rcu().
but currently there is still no one use it.


Beware my thinking may be wrong!
Thanks, Lai