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SubjectRe: [8/8, v6] NUMA Hotplug Emulator: implement debugfs interface for memory probe
On Thu, 2 Dec 2010, Shaohui Zheng wrote:

> > > From: Shaohui Zheng <shaohui.zheng@intel.com>
> > >
> > > Implement a debugfs inteface /sys/kernel/debug/mem_hotplug/probe for meomory hotplug
> > > emulation. it accepts the same parameters like
> > > /sys/devices/system/memory/probe.
> > >
> >
> > NACK, we don't need two interfaces to do the same thing.
>
> You may not know the background, the sysfs memory/probe interface is a general
> interface. Even through we have a debugfs interface, we should still keep it.
>
> For test purpose, the sysfs is enough, according to the comments from Greg & Dave,
> we create the debugfs interface.
>

I doubt either Greg or Dave suggested adding duplicate interfaces for the
same functionality.

The difference is that we needed to add the add_node interface in a new
mem_hotplug debugfs directory because it's only useful for debugging
kernel code and, thus, doesn't really have an appropriate place in sysfs.
Nobody is going to use add_node unless they lack hotpluggable memory
sections in their SRAT and want to debug the memory hotplug callers. For
example, I already wrote all of this node hotplug emulation stuff when I
wrote the node hotplug support for SLAB.

Memory hotplug, however, does serve a non-debugging function and is
appropriate in sysfs since this is how people hotplug memory. It's an ABI
that we can't simply remove without deprecation over a substantial period
of time and in this case it doesn't seem to have a clear advantage. We
need not add special emulation support for something that is already
possible for real systems, so adding a duplicate interface in debugfs is
inappropriate.


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