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

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Message-ID: <20110301001300.GD2331@linux.vnet.ibm.com>
Date:	Mon, 28 Feb 2011 16:13:00 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Lai Jiangshan <laijs@...fujitsu.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, Feb 28, 2011 at 10:47:17AM +0100, Peter Zijlstra wrote:
> 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.

I am OK doing the sched_setscheduler_nocheck() in UP_PREPARE, correct?

But yes, I can have the CPU_STARTING notifier wake up any kthreads that
the current CPU might have caused to be created.

> > > 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 ;-)

Glad I am maintaining at least a shred of sanity.  ;-)

> >   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? :-)

Ah, there is the rub -- I am using wait_event(), so I need to wake up the
kthread once before anyone uses it (or at least concurrently with anyone
using it).  Which I can presumably do from the CPU_STARTING notifier.

Make sense, or am I still missing something?

							Thanx, Paul
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