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Message-ID: <4B178440.5020701@cn.fujitsu.com>
Date: Thu, 03 Dec 2009 17:26:24 +0800
From: Lai Jiangshan <laijs@...fujitsu.com>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
CC: linux-kernel@...r.kernel.org, mingo@...e.hu, dipankar@...ibm.com,
akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
josh@...htriplett.org, dvhltc@...ibm.com, niv@...ibm.com,
tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
Valdis.Kletnieks@...edu, dhowells@...hat.com
Subject: Re: [PATCH tip/core/rcu 3/4] rcu: add expedited grace-period support
for preemptible RCU
Paul E. McKenney wrote:
> From: Paul E. McKenney <paulmck@...ux.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
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