linux-kernel - Re: [8/8, v6] NUMA Hotplug Emulator: implement debugfs interface for memory probe

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Message-ID: <alpine.DEB.2.00.1012011716550.22420@chino.kir.corp.google.com>
Date:	Wed, 1 Dec 2010 17:21:48 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Shaohui Zheng <shaohui.zheng@...el.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, lethal@...ux-sh.org,
	Andi Kleen <ak@...ux.intel.com>, dave@...ux.vnet.ibm.com,
	Greg KH <gregkh@...e.de>, Haicheng Li <haicheng.li@...el.com>
Subject: Re: [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@...el.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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