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

[Jiang Liu]

On 2014/7/25 7:30, Nishanth Aravamudan wrote:
> 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@xxxxxxxxxxxxxxx>
>> ---
>>  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.
Good suggestion, will try to enhance the error handling path.

> 
> Thanks,
> Nish
> 
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