Date:   Tue, 09 Nov 2021 15:53:21 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc:     <akpm@...ux-foundation.org>, <dave.hansen@...ux.intel.com>,
        <ziy@...dia.com>, <osalvador@...e.de>, <shy828301@...il.com>,
        <zhongjiang-ali@...ux.alibaba.com>, <xlpang@...ux.alibaba.com>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v2] mm: migrate: Support multiple target nodes demotion

Baolin Wang <baolin.wang@...ux.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@...ux.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[]?

> +};
> +
> +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?

> +	}
> +
>  	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.

>  		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

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