Messages in this thread | | | Date | Thu, 5 Apr 2012 10:39:18 -0700 | From | "Paul E. McKenney" <> | Subject | Re: CPU Hotplug rework |
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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
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