Re: [PATCH v5 01/14] memory-hotplug: try to offline the memory twiceto avoid dependence

[Tang Chen]

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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