Messages in this thread | | | Date | Wed, 06 Feb 2013 11:07:23 +0800 | From | Tang Chen <> | Subject | Re: [PATCH v5 01/14] memory-hotplug: try to offline the memory twice to avoid dependence |
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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@cn.fujitsu.com> >>> >>> 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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