Re: [RFC PATCH] mm: migrate: Add new node demotion strategy

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2021/11/8 14:48, Huang, Ying writes:
> Baolin Wang <baolin.wang@linux.alibaba.com> writes:
> 
>> On 2021/11/7 23:20, Dave Hansen wrote:
>>> On 11/7/21 1:33 AM, Baolin Wang wrote:
>>>> Thanks for your suggestion. After some thinking, can we change the
>>>> node_demotion[] structure like below? Which means one source node can be
>>>> demoted to mutiple target node, and we can set up the target node mask
>>>> according to the node distance. How do you think? Thanks.
>>>>
>>>> static nodemask_t node_demotion[MAX_NUMNODES] __read_mostly =
>>>>           {[0 ...  MAX_NUMNODES - 1] = NODE_MASK_NONE};
>>> How large is that in the worst case?
>>
>> For the worst case (MAX_NUMNODES=1024), the size of the node_demotion
>> is 131072 bytes, while the size of original data structure is 4096
>> bytes. Maybe we can allocate the node_demotion dynamically?
> 
> Per my understanding, in most cases, the number of demotion target nodes
> should be quite small.  So why not restrict the number of demotion
> target nodes to make it some kind of simple array?

Yes, agree. Something like below is reasonable for you?

#define DEMOTION_TARGET_NODES 16
typedef struct { DECLARE_BITMAP(bits, DEMOTION_TARGET_NODES); } 
demotemask_t;

static demotemask_t node_demotion[MAX_NUMNODES];