Hi Glauber, all,
An old thing I want to discuss with you. :)
On 01/09/2013 11:09 PM, Glauber Costa wrote:
memory can't be offlined when CONFIG_MEMCG is selected.
For example: there is a memory device on node 1. The address range
is [1G, 1.5G). You will find 4 new directories memory8, memory9, memory10,
and memory11 under the directory /sys/devices/system/memory/.
If CONFIG_MEMCG is selected, we will allocate memory to store page cgroup
when we online pages. When we online memory8, the memory stored page cgroup
is not provided by this memory device. But when we online memory9, the memory
stored page cgroup may be provided by memory8. So we can't offline memory8
now. We should offline the memory in the reversed order.
When the memory device is hotremoved, we will auto offline memory provided
by this memory device. But we don't know which memory is onlined first, so
offlining memory may fail. In such case, iterate twice to offline the memory.
1st iterate: offline every non primary memory block.
2nd iterate: offline primary (i.e. first added) memory block.
This idea is suggested by KOSAKI Motohiro.
Signed-off-by: Wen Congyang<wency@xxxxxxxxxxxxxx>
Maybe there is something here that I am missing - I admit that I came
late to this one, but this really sounds like a very ugly hack, that
really has no place in here.
Retrying, of course, may make sense, if we have reasonable belief that
we may now succeed. If this is the case, you need to document - in the
code - while is that.
The memcg argument, however, doesn't really cut it. Why can't we make
all page_cgroup allocations local to the node they are describing? If
memcg is the culprit here, we should fix it, and not retry. If there is
still any benefit in retrying, then we retry being very specific about why.
We try to make all page_cgroup allocations local to the node they are describing
now. If the memory is the first memory onlined in this node, we will allocate
it from the other node.
For example, node1 has 4 memory blocks: 8-11, and we online it from 8 to 11
1. memory block 8, page_cgroup allocations are in the other nodes
2. memory block 9, page_cgroup allocations are in memory block 8
So we should offline memory block 9 first. But we don't know in which order
the user online the memory block.
I think we can modify memcg like this:
allocate the memory from the memory block they are describing
I am not sure it is OK to do so.
I don't see a reason why not.
You would have to tweak a bit the lookup function for page_cgroup, but
assuming you will always have the pfns and limits, it should be easy to do.
I think the only tricky part is that today we have a single
node_page_cgroup, and we would of course have to have one per memory
block. My assumption is that the number of memory blocks is limited and
likely not very big. So even a static array would do.
About the idea "allocate the memory from the memory block they are
describing",
online_pages()
|-->memory_notify(MEM_GOING_ONLINE, &arg) ----------- memory of this
section is not in buddy yet.
|-->page_cgroup_callback()
|-->online_page_cgroup()
|-->init_section_page_cgroup()
|-->alloc_page_cgroup() --------- allocate
page_cgroup from buddy system.
When onlining pages, we allocate page_cgroup from buddy. And the being
onlined pages are not in
buddy yet. I think we can reserve some memory in the section for
page_cgroup, and return all the
rest to the buddy.
But when the system is booting,
start_kernel()
|-->setup_arch()
|-->mm_init()
| |-->mem_init()
| |-->numa_free_all_bootmem() -------------- all the pages are
in buddy system.
|-->page_cgroup_init()
|-->init_section_page_cgroup()
|-->alloc_page_cgroup() ------------------ I don't know how
to reserve memory in each section.
So any idea about how to deal with it when the system is booting please?
And one more question, a memory section is 128MB in Linux. If we reserve
part of the them for page_cgroup,
then anyone who wants to allocate a contiguous memory larger than 128MB,
it will fail, right ?
Is it OK ?
Thanks. :)
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