Re: [PATCH 1/2] [REPOST] mm: show node to memory section relationship with symlinks in sysfs

On Tue, Oct 14, 2008 at 08:54:21PM +0900, Yasunori Goto wrote:
On Fri, Oct 10, 2008 at 04:32:30PM -0700, Andrew Morton wrote:
On Fri, 10 Oct 2008 16:18:44 -0700
Gary Hade <garyhade@xxxxxxxxxx> wrote:

On Fri, Oct 10, 2008 at 02:59:50PM -0700, Andrew Morton wrote:
On Fri, 10 Oct 2008 14:33:57 -0700
Gary Hade <garyhade@xxxxxxxxxx> wrote:

On Fri, Oct 10, 2008 at 12:42:39PM -0700, Andrew Morton wrote:
On Thu, 9 Oct 2008 12:21:15 -0700
Gary Hade <garyhade@xxxxxxxxxx> wrote:

Show node to memory section relationship with symlinks in sysfs

Add /sys/devices/system/node/nodeX/memoryY symlinks for all
the memory sections located on nodeX. For example:
/sys/devices/system/node/node1/memory135 -> ../../memory/memory135
indicates that memory section 135 resides on node1.

I'm not seeing here a description of why the kernel needs this feature.
Why is it useful? How will it be used? What value does it have to
our users?

Sorry, I should have included that. In our case, it is another
small step towards eventual total node removal. We will need to
know which memory sections to offline for whatever node is targeted
for removal. However, I suspect that exposing the node to section
information to user-level could be useful for other purposes.
For example, I have been thinking that using memory hotremove
functionality to modify the amount of available memory on specific
nodes without having to physically add/remove DIMMs might be useful
to those that test application or benchmark performance on a
multi-node system in various memory configurations.


hm, OK, thanks. It does sound a bit thin, and if we merge this then
not only do we get a porkier kernel,

Would you feel the same about the size increase if patch 2/2 (include
memory section subtree in sysfs with only sparsemem enabled) was
withdrawn?

Without patch 2/2 the size increase for non-Sparsemem or Sparsemem
wo/memory hotplug kernels is extremely small. Even for memory hotplug
enabled kernels there is only a little extra code in ./drivers/base/node.o
which only gets linked into NUMA enabled kernels. I can gather some numbers
if necessary.

Size is probably a minor issue on memory-hotpluggable machines.

we also get a new userspace interface which we're then locked into.

True.

That's a bigger issue. The later we leave this sort of thing, the more
information we have.

I understand your concerns about adding possibly frivolous interfaces
but in this case we are simply eliminating a very obvious hole in the
existing set of memory hot-add/remove interfaces. In general, it
makes absolutely no sense to provide a resource add/remove mechanism
without telling the user where the resource is physically located.
i.e. providing the _maximum_ possible amount of location information
available for the add/remove controllable resource. This is especially
critical for large multi-node systems and for resources that can impact
application or overall system performance.

The kernel already exports node location information for CPUs
(e.g. /sys/devices/system/node/node0/cpu0 -> ../../cpu/cpu0) and
PCI devices (e.g. ./devices/pci0000:00/0000:00:00.0/numa_node).
Why should memory be treated any differently?

The memory hot-add/remove interfaces include physical device files
(e.g. /sys/devices/system/memory/memory0/phys_device) which are not
yet fully implemented. When systems that support removable memory
modules force this interface to mature, node location information
will become even more critical. This feature will not be very useful
on multi-node systems if the user does not know what node a specific
memory module is installed in. It may be possible to encode the
node ID into the string provided by the phys_device file but a
more general node to memory section association as provided by this
patch is better since it can be used for other purposes.

Sorry for late responce.

Our fujitsu box can hot-add a node. This means a user/script has to
find which memory sections and cpus belong to added node when node hot-add
is executed.

Current my hotplug script is very poor. It onlines all offlined cpus and memories.
However if user offlined one memory section intentionally due to
memory error message, the script can't understand it is intended, and hot-add
the error section. I think this is one of reason why this link is necessary.

When it comes time to replace that misbehaving memory I bet the
user will be delighted to know which node it is in. This sort
of benefit will of course _not_ limited to the small community
of node hot-add/remove capable systems.

Gary

--
Gary Hade
System x Enablement
IBM Linux Technology Center
503-578-4503 IBM T/L: 775-4503
garyhade@xxxxxxxxxx
http://www.ibm.com/linux/ltc

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