Re: [RFC PATCH v2] mm: migrate: Support multiple target nodes demotion

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

On 2021/11/9 15:53, Huang, Ying writes:
> Baolin Wang <baolin.wang@linux.alibaba.com> writes:
> 
>> 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, 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 RFC v1:
>>   - Re-define the node_demotion structure.
>>   - Set up multiple target nodes by validating the node distance.
>>   - Add more comments.
>> ---
>>   mm/migrate.c | 132 +++++++++++++++++++++++++++++++++++++++++++----------------
>>   1 file changed, 96 insertions(+), 36 deletions(-)
>>
>> diff --git a/mm/migrate.c b/mm/migrate.c
>> index cf25b00..95f170d 100644
>> --- a/mm/migrate.c
>> +++ b/mm/migrate.c
>> @@ -1119,12 +1119,25 @@ 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 same class memory
>> + * types. 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 +1148,13 @@ 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 DEMOTION_TARGET_NODES 15
>> +struct demotion_nodes {
>> +	unsigned short nr;
>> +	int nodes[DEMOTION_TARGET_NODES];
> 
> Why we cannot use "unsigned short" for nodes[]?

I think the default value of target node should be NUMA_NO_NODE(-1), so 
a signed type is more suitable. I can change to 'short' type.

> 
>> +};
>> +
>> +static struct demotion_nodes node_demotion[MAX_NUMNODES] __read_mostly;
>>   
>>   /**
>>    * next_demotion_node() - Get the next node in the demotion path
>> @@ -1149,7 +1167,9 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
>>    */
>>   int next_demotion_node(int node)
>>   {
>> -	int target;
>> +	struct demotion_nodes *current_node_demotion = &node_demotion[node];
>> +	int target, i;
>> +	nodemask_t target_nodes = NODE_MASK_NONE;
>>   
>>   	/*
>>   	 * node_demotion[] is updated without excluding this
>> @@ -1161,9 +1181,21 @@ int next_demotion_node(int node)
>>   	 * node_demotion[] reads need to be consistent.
>>   	 */
>>   	rcu_read_lock();
>> -	target = READ_ONCE(node_demotion[node]);
>> +	for (i = 0; i < DEMOTION_TARGET_NODES; i++) {
>> +		target = READ_ONCE(current_node_demotion->nodes[i]);
>> +		if (target == NUMA_NO_NODE)
>> +			break;
>> +
>> +		node_set(target, target_nodes);
> 
> Why do we need a nodemask?  Why not just find a target node from
> current_node_demotion->nodes[] randomly and directly?

I think nodemask is scalable in future if we want to add more 
requirements to select the target node if necessary. Anyway now I have 
no strong preference with the nodemask, and can change to select the 
target node randomly and directly, which are something like below.

+       target_nr = READ_ONCE(current_node_demotion->nr);
+
+       if (target_nr == 0) {
+               target = NUMA_NO_NODE;
+               goto out;
+       } else if (target_nr == 1) {
+               index = 0;
+       } else {
+               /*
+                * If there are multiple target nodes, just select one
+                * target node randomly.
+                */
+               index = get_random_int() % target_nr;
+       }
+
+       target = READ_ONCE(current_node_demotion->nodes[index]);

> 
>> +	}
>> +
>>   	rcu_read_unlock();
>>   
>> +	if (nodes_empty(target_nodes))
>> +		return NUMA_NO_NODE;
>> +
>> +	/* TODO: Select a target node randomly */
>> +	target = node_random(&target_nodes);
>>   	return target;
>>   }
>>   
>> @@ -2974,10 +3006,13 @@ void migrate_vma_finalize(struct migrate_vma *migrate)
>>   /* Disable reclaim-based migration. */
>>   static void __disable_all_migrate_targets(void)
>>   {
>> -	int node;
>> +	int node, i;
>>   
>> -	for_each_online_node(node)
>> -		node_demotion[node] = NUMA_NO_NODE;
>> +	for_each_online_node(node) {
>> +		node_demotion[node].nr = 0;
>> +		for (i = 0; i < DEMOTION_TARGET_NODES; i++)
>> +			node_demotion[node].nodes[i] = NUMA_NO_NODE;
>> +	}
>>   }
>>   
>>   static void disable_all_migrate_targets(void)
>> @@ -3004,26 +3039,34 @@ static void disable_all_migrate_targets(void)
>>    * Failing here is OK.  It might just indicate
>>    * being at the end of a chain.
>>    */
>> -static int establish_migrate_target(int node, nodemask_t *used)
>> +static int establish_migrate_target(int node, nodemask_t *used,
>> +				    int best_distance)
>>   {
>> -	int migration_target;
>> +	int migration_target, index, val;
>> +	struct demotion_nodes *current_node_demotion = &node_demotion[node];
>> +
>> +	migration_target = find_next_best_node(node, used);
>> +	if (migration_target == NUMA_NO_NODE)
>> +		return NUMA_NO_NODE;
>>   
>>   	/*
>> -	 * Can not set a migration target on a
>> -	 * node with it already set.
>> -	 *
>> -	 * No need for READ_ONCE() here since this
>> -	 * in the write path for node_demotion[].
>> -	 * This should be the only thread writing.
>> +	 * If the node has been set a migration target node before,
>> +	 * which means it's the best distance between them. Still
>> +	 * check if this node can be demoted to other target nodes
>> +	 * if they have a same best distance.
>>   	 */
>> -	if (node_demotion[node] != NUMA_NO_NODE)
>> -		return NUMA_NO_NODE;
>> +	if (best_distance != -1) {
>> +		val = node_distance(node, migration_target);
>> +		if (val > best_distance)
>> +			return NUMA_NO_NODE;
>> +	}
>>   
>> -	migration_target = find_next_best_node(node, used);
>> -	if (migration_target == NUMA_NO_NODE)
>> +	index = current_node_demotion->nr;
>> +	if (index >= DEMOTION_TARGET_NODES)
> 
> I think we need WARN_ONCE() here, so we can increase
> DEMOTION_TARGET_NODES if necessary.

Sure, will do. Thanks for your comments.

> 
>>   		return NUMA_NO_NODE;
>>   
>> -	node_demotion[node] = migration_target;
>> +	current_node_demotion->nodes[index] = migration_target;
>> +	current_node_demotion->nr++;
>>   
>>   	return migration_target;
>>   }
>> @@ -3039,7 +3082,9 @@ static int establish_migrate_target(int node, nodemask_t *used)
>>    *
>>    * The difference here is that cycles must be avoided.  If
>>    * node0 migrates to node1, then neither node1, nor anything
>> - * node1 migrates to can migrate to node0.
>> + * node1 migrates to can migrate to node0. Also one node can
>> + * be migrated to multiple nodes if the target nodes all have
>> + * a same best-distance against the source node.
>>    *
>>    * This function can run simultaneously with readers of
>>    * node_demotion[].  However, it can not run simultaneously
>> @@ -3051,7 +3096,7 @@ static void __set_migration_target_nodes(void)
>>   	nodemask_t next_pass	= NODE_MASK_NONE;
>>   	nodemask_t this_pass	= NODE_MASK_NONE;
>>   	nodemask_t used_targets = NODE_MASK_NONE;
>> -	int node;
>> +	int node, best_distance;
>>   
>>   	/*
>>   	 * Avoid any oddities like cycles that could occur
>> @@ -3080,18 +3125,33 @@ static void __set_migration_target_nodes(void)
>>   	 * multiple source nodes to share a destination.
>>   	 */
>>   	nodes_or(used_targets, used_targets, this_pass);
>> -	for_each_node_mask(node, this_pass) {
>> -		int target_node = establish_migrate_target(node, &used_targets);
>>   
>> -		if (target_node == NUMA_NO_NODE)
>> -			continue;
>> +	for_each_node_mask(node, this_pass) {
>> +		best_distance = -1;
>>   
>>   		/*
>> -		 * Visit targets from this pass in the next pass.
>> -		 * Eventually, every node will have been part of
>> -		 * a pass, and will become set in 'used_targets'.
>> +		 * Try to set up the migration path for the node, and the target
>> +		 * migration nodes can be multiple, so doing a loop to find all
>> +		 * the target nodes if they all have a best node distance.
>>   		 */
>> -		node_set(target_node, next_pass);
>> +		do {
>> +			int target_node =
>> +				establish_migrate_target(node, &used_targets,
>> +							 best_distance);
>> +
>> +			if (target_node == NUMA_NO_NODE)
>> +				break;
>> +
>> +			if (best_distance == -1)
>> +				best_distance = node_distance(node, target_node);
>> +
>> +			/*
>> +			 * Visit targets from this pass in the next pass.
>> +			 * Eventually, every node will have been part of
>> +			 * a pass, and will become set in 'used_targets'.
>> +			 */
>> +			node_set(target_node, next_pass);
>> +		} while (1);
>>   	}
>>   	/*
>>   	 * 'next_pass' contains nodes which became migration
> 
> Best Regards,
> Huang, Ying
>