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SubjectRe: [PATCH] online CPU before memory failed in pcpu_alloc_pages()
On Fri, 21 May 2010 16:22:19 +0800
minskey guo <chaohong_guo@linux.intel.com> wrote:

> Yes. I can use cpu_to_mem(). only some little difference during
> CPU online: 1st cpu within memoryless node gets memory from current
> node or the node to which the cpu0 belongs,
>
>
> But I have a question about the patch:
>
> numa-slab-use-numa_mem_id-for-slab-local-memory-node.patch,
>
>
>
>
> @@ -2968,9 +2991,23 @@ static int __build_all_zonelists(void *d
> ...
>
> - for_each_possible_cpu(cpu)
> + for_each_possible_cpu(cpu) {
> setup_pageset(&per_cpu(boot_pageset, cpu), 0);
> ...
>
> +#ifdef CONFIG_HAVE_MEMORYLESS_NODES
> + if (cpu_online(cpu))
> + cpu_to_mem(cpu) = local_memory_node(cpu_to_node(cpu));
> +#endif
>
>
> Look at the last two lines, suppose that memory is onlined before CPUs,
> where will cpu_to_mem(cpu) be set to the right nodeid for the last
> onlined cpu ? Does that CPU always get memory from the node including
> cpu0 for slab allocator where cpu_to_mem() is used ?
>
build_all_zonelist() is called at boot, initialization.
And it calls local_memory_node(cpu_to_node(cpu)) for possible cpus.

So, "how cpu_to_node() for possible cpus is configured" is important.
At quick look, root/arch/x86/mm/numa_64.c has following code.


786 /*
787 * Setup early cpu_to_node.
788 *
789 * Populate cpu_to_node[] only if x86_cpu_to_apicid[],
790 * and apicid_to_node[] tables have valid entries for a CPU.
791 * This means we skip cpu_to_node[] initialisation for NUMA
792 * emulation and faking node case (when running a kernel compiled
793 * for NUMA on a non NUMA box), which is OK as cpu_to_node[]
794 * is already initialized in a round robin manner at numa_init_array,
795 * prior to this call, and this initialization is good enough
796 * for the fake NUMA cases.
797 *
798 * Called before the per_cpu areas are setup.
799 */
800 void __init init_cpu_to_node(void)
801 {
802 int cpu;
803 u16 *cpu_to_apicid = early_per_cpu_ptr(x86_cpu_to_apicid);
804
805 BUG_ON(cpu_to_apicid == NULL);
806
807 for_each_possible_cpu(cpu) {
808 int node;
809 u16 apicid = cpu_to_apicid[cpu];
810
811 if (apicid == BAD_APICID)
812 continue;
813 node = apicid_to_node[apicid];
814 if (node == NUMA_NO_NODE)
815 continue;
816 if (!node_online(node))
817 node = find_near_online_node(node);
818 numa_set_node(cpu, node);
819 }
820 }

So, cpu_to_node(cpu) for possible cpus will have NUMA_NO_NODE(-1)
or the number of the nearest node.

IIUC, if SRAT is not broken, all pxm has its own node_id. So,
cpu_to_node(cpu) will return the nearest node and cpu_to_mem() will
find the nearest node with memory.

Thanks,
-Kame





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