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

[Tang Chen]

Hi all,

On 02/06/2013 11:07 AM, Tang Chen wrote:
> 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?

How about this way.

1) Add a new flag PAGE_CGROUP_INFO, like SECTION_INFO and MIX_SECTION_INFO.
2) In sparse_init(), reserve some beginning pages of each section as 
bootmem.
3) In register_page_bootmem_info_section(), set these pages as
     page->lru.next = PAGE_CGROUP_INFO;

Then these pages will not go to buddy system.

But I do worry about the fragment problem because part of each section will
be used in the very beginning.

Thanks. :)

> 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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