Messages in this thread | | | Date | Tue, 23 Jul 2013 15:05:21 -0400 | From | Konrad Rzeszutek Wilk <> | Subject | Re: [PATCH v3 1/2] xen/balloon: set a mapping for ballooned out pages |
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On Tue, Jul 23, 2013 at 07:00:09PM +0100, Ian Campbell wrote: > On Tue, 2013-07-23 at 18:27 +0100, Stefano Stabellini wrote: > > +static int __cpuinit balloon_cpu_notify(struct notifier_block *self, > > + unsigned long action, void *hcpu) > > +{ > > + int cpu = (long)hcpu; > > + switch (action) { > > + case CPU_UP_PREPARE: > > + if (per_cpu(balloon_scratch_page, cpu) != NULL) > > + break; > > Thinking about this a bit more -- do we know what happens to the per-cpu > area for a CPU which is unplugged and then reintroduced? Is it preserved > or is it reset? > > If it is reset then this gets more complicated :-( We might be able to > use the core mm page reference count, so that when the last reference is > removed the page is automatically reclaimed. We can obviously take a > reference whenever we add a mapping of the trade page, but I'm not sure > we are always on the path which removes such mappings... Even then you > could waste pages for some potentially large amount of time each time > you replug a VCPU. > > Urg, I really hope the per-cpu area is preserved!
It is. During bootup time you see this:
[ 0.000000] smpboot: Allowing 128 CPUs, 96 hotplug CPU [ 0.000000] setup_percpu: NR_CPUS:512 nr_cpumask_bits:512 nr_cpu_ids:128 nr_node_ids:1
which means that all of the per_CPU are shrunk down to 128 (from CONFIG_NR_CPUS=512 was built with) and stays for the lifetime of the kernel.
You might have to clear it when the vCPU comes back up though - otherwise you will have garbage.
Or you can use the zalloc_cpumask_var_node which will allocate a dynamic version of this. (based on the possible_cpus - so in this case 128). > > Ian. >
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