On Thu, 10 Oct 2013 18:53:37 +0200 Peter Zijlstra <pet...@infradead.org> wrote:
> On Thu, Oct 10, 2013 at 09:43:55AM -0700, Andrew Morton wrote: > > > But we would like to remove stomp machine from > > > CPU hotplug. > > > > We do? That's news. It wasn't mentioned in the changelog and should > > have been. Why? > > It would be an unrelated change to this and unrelated to the reasons as > to why I want a faster get_online_cpus(). Yeah, sorry for the confusion. My comment wasn't really about this change set but about stop machine and hotplug in general. Needing stop machine for hotplug has been a complaint by many, but off topic for this particular change set. > > > > Why prevent all CPUs from running when we want to remove > > > one? > > > > So get_online_cpus() goes away. Nothing is more scalable than nothing! > > Very much agreed; now stop_machine() wouldn't actually work for hotplug > because it will instantly preempt everybody, including someone who might > be in the middle of using per-cpu state of the cpu we're about to > remove. Well, stop machine doesn't instantly preempt everybody. Only those that don't have preemption disabled. Using per_cpu without preemption disabled can be dangerous. Except for the migrate disable we want to add for -rt. Then we can't rely on migrate disable and stop machine making sure per_cpu data isn't being used. Oh, and for threads that use per_cpu that are bound to a CPU. But then they need to be taken care of too for their CPU going off line. But this would also require the "get_online_cpu()s" to disable preemption as well. Not quite a "nothing" we are looking for. -- Steve -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
