Re: [PATCH v3] mm: migrate: Support multiple target nodes demotion

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

On 2021/11/13 3:05, Yang Shi wrote:
> On Thu, Nov 11, 2021 at 6:28 PM Baolin Wang
> <baolin.wang@linux.alibaba.com> wrote:
>>
>> We have some machines with multiple memory types like below, which
>> have one fast (DRAM) memory node and two slow (persistent memory) memory
>> nodes. According to current node demotion policy, if node 0 fills up,
>> its memory should be migrated to node 1, when node 1 fills up, its
>> memory will be migrated to node 2: node 0 -> node 1 -> node 2 ->stop.
>>
>> But this is not efficient and suitbale memory migration route
>> for our machine with multiple slow memory nodes. Since the distance
>> between node 0 to node 1 and node 0 to node 2 is equal, and memory
>> migration between slow memory nodes will increase persistent memory
>> bandwidth greatly, which will hurt the whole system's performance.
>>
>> Thus for this case, we can treat the slow memory node 1 and node 2
>> as a whole slow memory region, and we should migrate memory from
>> node 0 to node 1 and node 2 if node 0 fills up.
>>
>> This patch changes the node_demotion data structure to support multiple
>> target nodes, and establishes the migration path to support multiple
>> target nodes with validating if the node distance is the best or not.
>>
>> available: 3 nodes (0-2)
>> node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
>> node 0 size: 62153 MB
>> node 0 free: 55135 MB
>> node 1 cpus:
>> node 1 size: 127007 MB
>> node 1 free: 126930 MB
>> node 2 cpus:
>> node 2 size: 126968 MB
>> node 2 free: 126878 MB
>> node distances:
>> node   0   1   2
>>    0:  10  20  20
>>    1:  20  10  20
>>    2:  20  20  10
>>
>> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
>> ---
>> Changes from v2:
>>   - Redefine the DEMOTION_TARGET_NODES macro according to the
>>     MAX_NUMNODES.
>>   - Change node_demotion to a pointer and allocate it dynamically.
>>
>> Changes from v1:
>>   - Add a new patch to allocate the node_demotion dynamically.
>>   - Update some comments.
>>   - Simplify some variables' name.
>>
>> Changes from RFC v2:
>>   - Change to 'short' type for target nodes array.
>>   - Remove nodemask instead selecting target node directly.
>>   - Add WARN_ONCE() if the target nodes exceed the maximum value.
>>
>> Changes from RFC v1:
>>   - Re-define the node_demotion structure.
>>   - Set up multiple target nodes by validating the node distance.
>>   - Add more comments.
>> ---
>>   mm/migrate.c | 167 ++++++++++++++++++++++++++++++++++++++++++++++-------------
>>   1 file changed, 132 insertions(+), 35 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index cf25b00..9b8a813 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -50,6 +50,7 @@
>>   #include <linux/ptrace.h>
>>   #include <linux/oom.h>
>>   #include <linux/memory.h>
>> +#include <linux/random.h>
>>
>>   #include <asm/tlbflush.h>
>>
>> @@ -1119,12 +1120,25 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
>>    *
>>    * This is represented in the node_demotion[] like this:
>>    *
>> - *     {  1, // Node 0 migrates to 1
>> - *        2, // Node 1 migrates to 2
>> - *       -1, // Node 2 does not migrate
>> - *        4, // Node 3 migrates to 4
>> - *        5, // Node 4 migrates to 5
>> - *       -1} // Node 5 does not migrate
>> + *     {  nr=1, nodes[0]=1 }, // Node 0 migrates to 1
>> + *     {  nr=1, nodes[0]=2 }, // Node 1 migrates to 2
>> + *     {  nr=0, nodes[0]=-1 }, // Node 2 does not migrate
>> + *     {  nr=1, nodes[0]=4 }, // Node 3 migrates to 4
>> + *     {  nr=1, nodes[0]=5 }, // Node 4 migrates to 5
>> + *     {  nr=0, nodes[0]=-1 }, // Node 5 does not migrate
>> + *
>> + * Moreover some systems may have multiple slow memory nodes.
>> + * Suppose a system has one socket with 3 memory nodes, node 0
>> + * is fast memory type, and node 1/2 both are slow memory
>> + * type, and the distance between fast memory node and slow
>> + * memory node is same. So the migration path should be:
>> + *
>> + *     0 -> 1/2 -> stop
>> + *
>> + * This is represented in the node_demotion[] like this:
>> + *     { nr=2, {nodes[0]=1, nodes[1]=2} }, // Node 0 migrates to node 1 and node 2
>> + *     { nr=0, nodes[0]=-1, }, // Node 1 dose not migrate
>> + *     { nr=0, nodes[0]=-1, }, // Node 2 does not migrate
>>    */
>>
>>   /*
>> @@ -1135,8 +1149,20 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
>>    * must be held over all reads to ensure that no cycles are
>>    * observed.
>>    */
>> -static int node_demotion[MAX_NUMNODES] __read_mostly =
>> -       {[0 ...  MAX_NUMNODES - 1] = NUMA_NO_NODE};
>> +#define DEFAULT_DEMOTION_TARGET_NODES 15
>> +
>> +#if MAX_NUMNODES < DEFAULT_DEMOTION_TARGET_NODES
>> +#define DEMOTION_TARGET_NODES  (MAX_NUMNODES - 1)
>> +#else
>> +#define DEMOTION_TARGET_NODES  DEFAULT_DEMOTION_TARGET_NODES
>> +#endif
>> +
>> +struct demotion_nodes {
>> +       unsigned short nr;
>> +       short nodes[DEMOTION_TARGET_NODES];
>> +};
>> +
>> +static struct demotion_nodes *node_demotion __read_mostly;
>>
>>   /**
>>    * next_demotion_node() - Get the next node in the demotion path
>> @@ -1149,8 +1175,15 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
>>    */
>>   int next_demotion_node(int node)
>>   {
>> +       struct demotion_nodes *nd;
>> +       unsigned short target_nr, index;
>>          int target;
>>
>> +       if (!node_demotion)
>> +               return NUMA_NO_NODE;
>> +
>> +       nd = &node_demotion[node];
>> +
>>          /*
>>           * node_demotion[] is updated without excluding this
>>           * function from running.  RCU doesn't provide any
>> @@ -1161,9 +1194,28 @@ int next_demotion_node(int node)
>>           * node_demotion[] reads need to be consistent.
>>           */
>>          rcu_read_lock();
>> -       target = READ_ONCE(node_demotion[node]);
>> -       rcu_read_unlock();
>> +       target_nr = READ_ONCE(nd->nr);
>> +
>> +       switch (target_nr) {
>> +       case 0:
>> +               target = NUMA_NO_NODE;
>> +               goto out;
>> +       case 1:
>> +               index = 0;
>> +               break;
>> +       default:
>> +               /*
>> +                * If there are multiple target nodes, just select one
>> +                * target node randomly.
>> +                */
>> +               index = get_random_int() % target_nr;
> 
> Sorry for chiming in late. I don't get why not select demotion targe > node interleave? TBH, it makes more sense to me. Random is ok, but at
> least I'd expect to see some explanation about why random is used.

My first version patch[1] already did round-robin to select target node. 
For interleave (or round-robin), we should introduce another member to 
record last selected target node, as Dave and Ying said, that will cause 
cache ping-pong to hurt performance, or introduce per-cpu data to avoid 
this, which seems more complicated now.

[1] 
https://lore.kernel.org/all/c02bcbc04faa7a2c852534e9cd58a91c44494657.1636016609.git.baolin.wang@linux.alibaba.com/