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

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Date:	Thu, 2 Dec 2010 15:34:01 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	"Zheng, Shaohui" <shaohui.zheng@...el.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"lethal@...ux-sh.org" <lethal@...ux-sh.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Greg KH <gregkh@...e.de>,
	"Li, Haicheng" <haicheng.li@...el.com>
Subject: RE: [8/8, v6] NUMA Hotplug Emulator: implement debugfs interface
 for memory probe

On Thu, 2 Dec 2010, Zheng, Shaohui wrote:

> Why should we add so many interfaces for memory hotplug emulation?

Because they are functionally different from real memory hotplug and we 
want to support different configurations such as mapping memory to a 
different node id or onlining physical nodes that don't exist.

They are in debugfs because the emulation, unlike real memory hotplug, is 
used only for testing and debugging.

> If so, we should create both sysfs and debugfs 
> entries for an online node, we are trying to add redundant code logic.
> 

We do not need sysfs triggers for onlining a node, that already happens 
automatically if the memory that is being onlined has a hotpluggable node 
entry in the SRAT that has an offline node id.

> We need not make a simple thing such complicated, Simple is beautiful, I'd prefer to rename the mem_hotplug/probe 
> interface as mem_hotplug/add_memory.
> 
> 	/sys/kernel/debug/mem_hotplug/add_node (already exists)
> 	/sys/kernel/debug/mem_hotplug/add_memory (rename probe as add_memory)
> 

No, add_memory would then require these bizarre lines that you've been 
parsing like

	echo 'physical_addr=0x80000000 node_id=3' > /sys/kernel/debug/mem_hotplug/add_memory

which is unnecessary if you introduce my proposal for per-node debugfs 
directories similar to that under /sys/devices/system/node that is 
extendable later if we add additional per-node triggers under 
CONFIG_DEBUG_FS.

Adding /sys/kernel/debug/mem_hotplug/node2/add_memory that you write a 
physical address to is a much more robust, simple, and extendable 
interface.
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