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SubjectRe: CPU Hotplug rework
On Sat, Apr 07, 2012 at 01:22:23AM +0530, Srivatsa S. Bhat wrote:
> On 04/05/2012 11:09 PM, Paul E. McKenney wrote:
> > On Mon, Mar 19, 2012 at 08:18:42PM +0530, Srivatsa S. Bhat wrote:
> >> On 03/19/2012 08:14 PM, Srivatsa S. Bhat wrote:
> >>
> >>> Hi,
> >>>
> >>> There had been some discussion on CPU Hotplug redesign/rework
> >>> some time ago, but it was buried under a thread with a different
> >>> subject.
> >>> (
> >>>
> >>> So I am opening a new thread with an appropriate subject to discuss
> >>> what needs to be done and how to go about it, as part of the rework.
> >>>
> >>> Peter Zijlstra and Paul McKenney had come up with TODO lists for the
> >>> rework, and here are their extracts from the previous discussion:
> >
> > Finally getting around to looking at this in more detail...
> >
> >> Additional things that I would like to add to the list:
> >>
> >> 1. Fix issues with CPU Hotplug callback registration. Currently there
> >> is no totally-race-free way to register callbacks and do setup
> >> for already online cpus.
> >>
> >> I had posted an incomplete patchset some time ago regarding this,
> >> which gives an idea of the direction I had in mind.
> >>
> >
> > Another approach is to have the registration function return the
> > CPU mask corresponding to the instant at which registration occurred,
> > perhaps via an additional function argument that points to a
> > cpumask_var_t that can be NULL if you don't care. Then you can
> > do setup for the CPUs indicated in the mask.
> That would look something like this right?
> register_cpu_notifier(nb, mask);
> do_setup(mask);
> If yes, then here is our problem:
> 1. A CPU hotplug operation can happen, in-between these 2 function calls.
> 2. No matter how we try to wrap these up with get/put_online_cpus(), it
> will still lead to either a race, or a deadlock, depending on how we
> do it.

Hmmm... The call to register_cpu_notifier() excludes CPU-hotplug
operations, so any CPU-hotplug operation that happens after the
register_cpu_notifier() will invoke the notifier. This does mean that
we can then see concurrent invocation of the notifier from do_setup()
and from a CPU-hotplug operation, but it should not be hard to create
a reasonably locking scheme to prevent this.

BTW, I am assuming that it is OK for the notifiers to run out of order
in this case. Perhaps you are trying to preserve ordering?

The reason that I believe that it should be OK to allow misordering of
notifiers is that the subsystem in question is not fully initialized yet.
It is therefore probably not yet actually servicing requests, for example,
something like RCU would might be refusing to start grace periods.
If it is not yet servicing requests, it should not matter what order
it sees the CPUs come online. Once do_setup() returns, it will see
subsequent CPU-hotplug operations in order, so at that point it might
be able to start servicing requests.

Or am I missing something else?

> Wrapping only do_setup() within get/put_online_cpus() wouldn't serve our
> purpose, since the race with CPU Hotplug would still exist, just like
> before. So, let's consider what happens when we wrap both the functions
> within get/put_online_cpus():
> get_online_cpus();
> register_cpu_notifier(nb, mask);
> do_setup(mask);
> put_online_cpus();
> Unfortunately this leads to an ABBA deadlock (see below).
> > Or am I missing the race you had in mind? Or is the point to make
> > sure that the notifiers execute in order?
> No, I wasn't referring to the order of execution of the notifiers here.

OK, then I am still missing something...

> Well, the race I was concerned about was the one between a CPU hotplug
> operation and a callback registration operation, which might lead to an
> ABBA deadlock, with the 2 locks in question being cpu hotplug lock and
> cpu_add_remove_lock.
> The heart of the problem is that, something as simple as the following
> is prone to an ABBA deadlock (note that I am not even talking about
> do_setup() here):
> get_online_cpus()
> register_cpu_notifier() or any other variant
> put_online_cpus()
> In the above, the lock ordering is:
> grab cpu_hotplug lock -> grab cpu_add_remove_lock.

But why do we need the get_online_cpus() in this case? What would it be
preventing from happening and why do we care?

> But in a usual CPU hotplug operation, the lock ordering is the exact reverse:
> grab cpu_add_remove_lock -> grab cpu_hotplug lock.
> Hence, we have an ABBA deadlock possibility.
> I had posted a pictorial representation of the deadlock condition here:

And Peter replied to this saying that he can remove the get_online_cpus()
call, which gets rid of the ABBA deadlock condition, correct?

> So, to get rid of this deadlock, the approach I proposed came out of the
> following observation:
> A code such as:
> register_cpu_notifier();
> //Go on doing stuff that are irrelevant to CPU hotplug.
> is totally safe from any kind of race or deadlock, the reason being that
> CPU Hotplug operations begin by taking the cpu_add_remove_lock, and
> callback registration functions like the above also take only this
> particular lock. IOW, the mutex that protects a race between CPU hotplug
> operation vs callback registration is the cpu_add_remove_lock. The
> cpu_hotplug lock is actually irrelevant here!

Yep, agreed, despite my confusion some weeks ago.

> CPU0 CPU 1
> register_cpu_notifier() cpu_down()
> ----------------------------------------------------------------------
> acquire cpu_add_remove_lock
> Blocked on cpu_add_remove_lock
> do callback registration
> release cpu_add_remove_lock
> Only now we can proceed and acquire
> the cpu_add_remove_lock.
> acquire cpu_hotplug lock
> do cpu_down related work
> release cpu_hotplug lock
> release cpu_add_remove_lock
> So my approach is: consider whatever we want to do while registering our
> callback, including doing setup for already online cpus; - and do *all of it*
> within the section marked as "do callback registration". IOW, we piggyback
> on something _known_ to be perfectly fine and hence avoid all races and
> deadlocks.

OK, interesting approach. But does this actually save anything in
any real situation, given that CPU hotplug notifiers are run during
initialization, before the subsystem being initialized is really doing

> >> 2. There is a mismatch between the code and the documentation around
> >> the difference between [un/register]_hotcpu_notifier and
> >> [un/register]_cpu_notifier. And I remember seeing several places in
> >> the code that uses them inconsistently. Not terribly important, but
> >> good to fix it up while we are at it.
> >
> > The following lead me to believe that they were the same:
> >
> > #define register_hotcpu_notifier(nb) register_cpu_notifier(nb)
> > #define unregister_hotcpu_notifier(nb) unregister_cpu_notifier(nb)
> >
> > What am I missing here?
> At first, even I thought that they were exactly same. But then looking at the
> #ifdef magic in include/linux/cpu.h, I realized that there is a subtle
> difference between the two:
> The register_cpu_notifier() family of functions are defined whenever:
> b. or, CONFIG_HOTPLUG_CPU is not set, but we are in core code
> ie., !defined(MODULE)
> The hotcpu family of functions are different in the sense that they are
> defined only when CONFIG_HOTPLUG_CPU is set. However, if CONFIG_HOTPLUG_CPU
> is not set, they are defined as pretty much empty functions, irrespective of
> whether we are calling them from core code or from module code.
> And the reasoning behind this difference is: strictly speaking,
> CONFIG_HOTPLUG_CPU needs to be set only if we want to *offline* CPUs at
> some point in time, like suspend/resume for example. Otherwise, we don't need
> But there are subsystems/arch specific stuff which need to take action even
> during CPU online operations (like booting), and hence their callback
> registrations must work even when CONFIG_HOTPLUG_CPU is not set. So they must
> use the native register_cpu_notifier() family of functions.
> If some code doesn't really care much when offlining of CPUs is not required,
> then they can use the hotcpu_* family and optimize out for the !HOTPLUG_CPU
> case.
> None of this seems to be documented in Documentation/cpu-hotplug.txt. In fact
> what that document explains (something about early init/late init) is actually
> misleading because it is out-dated beyond recognition! :-(

Indeed, it appears that the last substantive change to this file was more
than two years ago, and most of the work on the file happened more than five
years ago. It certainly needs attention! I have added documentation
updates to the list.

> Now coming to the inconsistent uses of these functions, if some core code which
> needs to register its callback irrespective of CONFIG_HOTPLUG_CPU, uses
> hotcpu* variants instead, it'll break stuff in the !HOTPLUG_CPU case.
> The other case (using register_cpu* variants in every place) is less worrisome,
> in fact harmless, but could have been optimized out in the !HOTPLUG_CPU case,
> if the hotcpu* variants had been used.

OK, got it, thank you for the tutorial!

> >> 3. There was another thread where stuff related to CPU hotplug had been
> >> discussed. It had exposed some new challenges to CPU hotplug, if we
> >> were to support asynchronous smp booting.
> >>
> >>
> >>
> >>
> >>
> >
> > Good points! ;-)
> Thank you :-)
> >> 4. Because the current CPU offline code depends on stop_machine(), every
> >> online CPU must cooperate with the offline event. This means, whenever
> >> we do a preempt_disable(), it ensures not only that that particular
> >> CPU won't go offline, but also that *any* CPU cannot go offline. This
> >> is more like a side-effect of using stop_machine().
> >>
> >> So when trying to move over to stop_one_cpu(), we have to carefully audit
> >> places where preempt_disable() has been used in that manner (ie.,
> >> preempt_disable used as a light-weight and non-blocking form of
> >> get_online_cpus()). Because when we move to stop_one_cpu() to do CPU offline,
> >> a preempt disabled section will prevent only that particular CPU from
> >> going offline.
> >>
> >> I haven't audited preempt_disable() calls yet, but one such use was there
> >> in brlocks (include/linux/lglock.h) until quite recently.
> >
> > I was thinking in terms of the offline code path doing a synchronize_sched()
> > to allow preempt_disable() to retain a reasonable approximation of its
> > current semantics. This would require a pair of CPU masks, one for code
> > using CPU-based primitives (e.g., sending IPIs) and another for code
> > implementing those primitives.
> >
> > Or is there some situation that makes this approach fail?
> Oh, right, using synchronize_sched() in the offline path would pretty much
> solve our problem.. I guess I overlooked that. Thanks for pointing it out!

Here is hoping. ;-)

Thanx, Paul

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