Date:	Sun, 8 Mar 2009 21:28:24 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] rcu_barrier VS cpu_hotplug: Ensure callbacks in dead
	cpu are migrated to online cpu

On Mon, Mar 09, 2009 at 10:56:12AM +0800, Lai Jiangshan wrote:
> Paul E. McKenney wrote:
> > On Sun, Mar 08, 2009 at 10:58:43AM +0800, Lai Jiangshan wrote:
> >> Paul E. McKenney wrote:
> >>> On Sat, Mar 07, 2009 at 06:54:38PM +0800, Lai Jiangshan wrote:
> >>>> [RFC]
> >>>> I don't like this patch, but I thought for some days and I can't
> >>>> thought out a better one.
> >>>>
> >>>> I'm very hope rcu_barrier() can be called anywhere(any sleepable context).
> >>>> But get_online_cpus() is a very large lock, it limits rcu_barrier().
> >>>>
> >>>> We can avoid get_online_cpus() easily for rcupreempt by using a new rcu_barrier:
> >>>> void rcu_barrier(void)
> >>>> {
> >>>> 	for each rcu_data {
> >>>> 		lock rcu_data;
> >>>> 		if rcu_data is not empty, queue a callback for rcu_barrier;
> >>>> 		unlock rcu_data;
> >>>> 	}
> >>>> }
> >>>> But we cannot use this algorithm for rcuclassic and rcutree,
> >>>> rcu_data in rcuclassic and rcutree have not a spinlock for queuing callback.
> >>>>
> >>>> From: Lai Jiangshan <laijs@...fujitsu.com>
> >>>>
> >>>> cpu hotplug may be happened asynchronously, some rcu callbacks are maybe
> >>>> still in dead cpu, rcu_barrier() also needs to wait for these rcu callbacks
> >>>> to complete, so we must ensure callbacks in dead cpu are migrated to
> >>>> online cpu.
> >>> Hmmm...  I thought that on_each_cpu() took care of interlocking with
> >>> CPU hotplug via smp_call_function().  During a CPU-hotplug operation,
> >>> the RCU callbacks do get migrated from the CPU going offline.  Are you
> >>> seeing a sequence of events that finds a hole in this approach?
> >>>
> >>> Now, if a CPU were to go offline in the middle of smp_call_function()
> >>> there could be trouble, but I was under the impression that the
> >>> preempt_disable() in on_each_cpu() prevented this from happening.
> >>>
> >>> So, please tell me more!
> >>>
> >> preempt_disable() ensure online cpu is still online until preempt_enable(),
> >> but preempt_disable()/preempt_enable() can't ensure rcu callbacks migrated.
> >>
> >>
> >> rcu_barrier()			|	_cpu_down()
> >> 				|		__cpu_die() (cpu D is dead)
> >> 	........................|............................
> >> 	on_each_cpu()		|
> >> 	........................|...........................
> >> 	wait_for_completion()	|		rcu_offline_cpu() (move cpu D's
> >> 				|			rcu callbacks to A,B,or C)
> >>
> >>
> >> on_each_cpu() does not queue rcu_barrier_callback to cpu D(it's dead).
> >> So rcu_barrier() will not wait for callbacks which are original at cpu D.
> >>
> >> We need ensure callbacks in dead cpu are migrated to online cpu before
> >> we call on_each_cpu().
> > 
> > Good catch!!!  I did indeed miss that possibility.  :-/
> > 
> > Hmmmm...  rcu_barrier() already acquires a global mutex, and is an
> > infrequent operation, so I am not all that worried about the scalability.
> 
> I do not worry about the scalability either.
> When we use get_online_cpus(), rcu_barrier() can not be called anywhere
> (any sleepable context), this is what I worry about.
> 
> Most locks in kernel are locked after cpu_hotplug.lock,
> if a path has required one of these lock, it cannot call get_online_cpus().
> (to avoid ABBA deadlock)
> So, if we use get_online_cpus() in rcu_barrier(), we cannot use rcu_barrier()
> in most area in kernel.

Yes, that could be painful.

> > But I agree that there should be a better way to do this.  One approach
> > might be to the dying CPU enqueue the rcu_barrier() callback on its
> > own list when it goes offline, during the stop_machine() time period.
> > This enqueuing operation would require some care -- it would be necessary
> > to check to see if the callback was already on the list, for example,
> > as well as to properly adjust the rcu_barrier_completion() state.
> > 
> > Of course, it would also be necessary to handle the case where an
> > rcu_barrier() callback was enqueued when there was no rcu_barrier()
> > in flight, preferably by preventing this from happening.
> > 
> > An entirely different approach would be to steal a trick from CPU
> > designers, and use a count of the number of rcu_barrier() calls (this
> > counter could be a single bit).  Have a per-CPU counter of the number
> > of callbacks outstanding for each counter value.  Then rcu_barrier()
> > simply increments the rcu_barrier() counter, and waits until the
> > number of outstanding callbacks corresponding to the old value drops
> > to zero.  This would get rid of the need for rcu_barrier() to enqueue
> > callbacks, preventing the scenario above from arising in the first
> > place.
> 
> Will you implement it with one of better ways?

I would start after I get the idr crash resolved -- first patch already
done (http://lkml.org/lkml/2009/3/7/133), but problems remain.  So if
you want to get started, I would be happy to review.  Your tail-pointer
trick would be needed to allow the counts to be correctly maintained
when migrating callbacks, of course.

							Thanx, Paul
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