Re: CPU Hotplug rework

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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.
> > (http://thread.gmane.org/gmane.linux.kernel/1246208/focus=1246404)
> > 
> > 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.
>    http://thread.gmane.org/gmane.linux.kernel/1258880/focus=15826

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.

Or am I missing the race you had in mind?  Or is the point to make
sure that the notifiers execute in order?

> 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?

> 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.
> 
>    http://thread.gmane.org/gmane.linux.kernel/1246209/focus=48535
>    http://thread.gmane.org/gmane.linux.kernel/1246209/focus=48542
>    http://thread.gmane.org/gmane.linux.kernel/1246209/focus=1253241
>    http://thread.gmane.org/gmane.linux.kernel/1246209/focus=1253267

Good points!  ;-)

> 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?

							Thanx, Paul