Re: [RFC Patch V1 28/30] mm: Update _mem_id_[] for every possible CPU when memory configuration changes

[Jiang Liu]

On 2014/7/22 1:47, Nishanth Aravamudan wrote:
> On 11.07.2014 [15:37:45 +0800], Jiang Liu wrote:
>> Current kernel only updates _mem_id_[cpu] for onlined CPUs when memory
>> configuration changes. So kernel may allocate memory from remote node
>> for a CPU if the CPU is still in absent or offline state even if the
>> node associated with the CPU has already been onlined.
> 
> This just sounds like the topology information is being updated at the
> wrong place/time? That is, the memory is online, the CPU is being
> brought online, but isn't associated with any node?
Hi Nishanth,
	Yes, that's the case.

> 
>> This patch tries to improve performance by updating _mem_id_[cpu] for
>> each possible CPU when memory configuration changes, thus kernel could
>> always allocate from local node once the node is onlined.
> 
> Ok, what is the impact? Do you actually see better performance?
No real data to support this yet, just with code analysis.
Regards!
Gerry
> 
>> We check node_online(cpu_to_node(cpu)) because:
>> 1) local_memory_node(nid) needs to access NODE_DATA(nid)
>> 2) try_offline_node(nid) just zeroes out NODE_DATA(nid) instead of free it
>>
>> Signed-off-by: Jiang Liu <jiang.liu@xxxxxxxxxxxxxxx>
>> ---
>>  mm/page_alloc.c |   10 +++++-----
>>  1 file changed, 5 insertions(+), 5 deletions(-)
>>
>> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
>> index 0ea758b898fd..de86e941ed57 100644
>> --- a/mm/page_alloc.c
>> +++ b/mm/page_alloc.c
>> @@ -3844,13 +3844,13 @@ static int __build_all_zonelists(void *data)
>>  		/*
>>  		 * We now know the "local memory node" for each node--
>>  		 * i.e., the node of the first zone in the generic zonelist.
>> -		 * Set up numa_mem percpu variable for on-line cpus.  During
>> -		 * boot, only the boot cpu should be on-line;  we'll init the
>> -		 * secondary cpus' numa_mem as they come on-line.  During
>> -		 * node/memory hotplug, we'll fixup all on-line cpus.
>> +		 * Set up numa_mem percpu variable for all possible cpus
>> +		 * if associated node has been onlined.
>>  		 */
>> -		if (cpu_online(cpu))
>> +		if (node_online(cpu_to_node(cpu)))
>>  			set_cpu_numa_mem(cpu, local_memory_node(cpu_to_node(cpu)));
>> +		else
>> +			set_cpu_numa_mem(cpu, NUMA_NO_NODE);
>>  #endif
> 
> 
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