Re: [RFC Patch V1 30/30] x86, NUMA: Online node earlier when doing CPU hot-addition

[Nishanth Aravamudan <nacc@linux.vnet.ibm.com>]

On 11.07.2014 [15:37:47 +0800], Jiang Liu wrote:
> With typical CPU hot-addition flow on x86, PCI host bridges embedded
> in physical processor are always associated with NOMA_NO_NODE, which
> may cause sub-optimal performance.
> 1) Handle CPU hot-addition notification
> 	acpi_processor_add()
> 		acpi_processor_get_info()
> 			acpi_processor_hotadd_init()
> 				acpi_map_lsapic()
> 1.a)					acpi_map_cpu2node()
> 
> 2) Handle PCI host bridge hot-addition notification
> 	acpi_pci_root_add()
> 		pci_acpi_scan_root()
> 2.a)			if (node != NUMA_NO_NODE && !node_online(node)) node = NUMA_NO_NODE;
> 
> 3) Handle memory hot-addition notification
> 	acpi_memory_device_add()
> 		acpi_memory_enable_device()
> 			add_memory()
> 3.a)				node_set_online();
> 
> 4) Online CPUs through sysfs interfaces
> 	cpu_subsys_online()
> 		cpu_up()
> 			try_online_node()
> 4.a)				node_set_online();
> 
> So associated node is always in offline state because it is onlined
> until step 3.a or 4.a.
> 
> We could improve performance by online node at step 1.a. This change
> also makes the code symmetric. Nodes are always created when handling
> CPU/memory hot-addition events instead of handling user requests from
> sysfs interfaces, and are destroyed when handling CPU/memory hot-removal
> events.

It seems like this patch has little to nothing to do with the rest of
the series and can be sent on its own?

> It also close a race window caused by kmalloc_node(cpu_to_node(cpu)),

To be clear, the race is that on some x86 platforms, there is a period
of time where a node ID returned by cpu_to_node() is offline.

<snip>

> Signed-off-by: Jiang Liu <jiang.liu@linux.intel.com>
> ---
>  arch/x86/kernel/acpi/boot.c |    1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c
> index 3b5641703a49..00c2ed507460 100644
> --- a/arch/x86/kernel/acpi/boot.c
> +++ b/arch/x86/kernel/acpi/boot.c
> @@ -611,6 +611,7 @@ static void acpi_map_cpu2node(acpi_handle handle, int cpu, int physid)
>  	nid = acpi_get_node(handle);
>  	if (nid != -1) {
>  		set_apicid_to_node(physid, nid);
> +		try_online_node(nid);

try_online_node() seems like it can fail? I assume it's a pretty rare
case, but should the return code be checked?

If it does fail, it seems like there are pretty serious problems and we
shouldn't be onlining this CPU, etc.?

>  		numa_set_node(cpu, nid);
>  		if (node_online(nid))
>  			set_cpu_numa_mem(cpu, local_memory_node(nid));

Which means you can remove this check presuming try_online_node()
returned 0.

Thanks,
Nish

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