Messages in this thread | | | Date | Thu, 5 Apr 2012 10:55:49 -0700 | From | "Paul E. McKenney" <> | Subject | Re: CPU Hotplug rework |
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On Thu, Apr 05, 2012 at 10:39:18AM -0700, 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. > > > (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?
Hmmm... I suppose that -rt's use of migrate_disable() needs some other approach in any case, unless -rt's offlining waits for all pre-existing migrate_disable() sections to finish.
Thanx, Paul
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