linux-kernel - Re: [PATCH RFC tip/core/rcu 11/11] rcu: move TREE

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Date:	Mon, 28 Feb 2011 10:47:17 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	paulmck@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	mingo@...e.hu, dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, rostedt@...dmis.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH RFC tip/core/rcu 11/11] rcu: move TREE_RCU from softirq
 to kthread

On Mon, 2011-02-28 at 11:29 +0800, Lai Jiangshan wrote:
> >>> +static int rcu_cpu_kthread_should_stop(int cpu)
> >>> +{
> >>> +   while (cpu_is_offline(cpu) || smp_processor_id() != cpu) {
> >>> +           if (kthread_should_stop())
> >>> +                   return 1;
> >>> +           local_bh_enable();
> >>> +           schedule_timeout_uninterruptible(1);
> >>> +           if (smp_processor_id() != cpu)
> >>> +                   set_cpus_allowed_ptr(current, cpumask_of(cpu));
> >>
> >> The current task is PF_THREAD_BOUND,
> >> Why do "set_cpus_allowed_ptr(current, cpumask_of(cpu));" ?
> > 
> > Because I have seen CPU hotplug operations unbind PF_THREAD_BOUND threads.

Correct, but that's on unplug, the rest of the story seems about plug,
so just detatch the thread on down/offline and let it die when its done.

> > In addition, I end up having to spawn the kthread at CPU_UP_PREPARE time,

Sure, that's a common time to create such treads :-), you can
kthread_ceate()+kthread_bind() in UP_PREPARE, just don't wake them yet.

> > at which point the thread must run unbound because its CPU isn't online
> > yet.  I cannot invoke kthread_create() within the stop-machine handler
> > (right?).

No you can not ;-)

>   I cannot wait until CPU_ONLINE time because that results in
> > hangs when other CPU notifiers wait for grace periods.
> > 
> > Yes, I did find out about the hangs the hard way.  Why do you ask?  ;-)

Right, so I assume that whoever needs the thread will:

 1) wake the thread,
 2) only do so after the cpu is actually online, how else could it be
executing code? :-)


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