From:   "Huang, Ying" <ying.huang@intel.com>
To:     Baolin Wang <baolin.wang@linux.alibaba.com>
Cc:     <akpm@linux-foundation.org>, <dave.hansen@linux.intel.com>,
        <ziy@nvidia.com>, <osalvador@suse.de>, <shy828301@gmail.com>,
        <zhongjiang-ali@linux.alibaba.com>, <xlpang@linux.alibaba.com>,
        <linux-mm@kvack.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v3] mm: migrate: Support multiple target nodes demotion
References: <a31dc065a7901bcdca0d9642d0def0f57e865e20.1636683991.git.baolin.wang@linux.alibaba.com>
Date:   Fri, 12 Nov 2021 10:44:43 +0800
In-Reply-To: <a31dc065a7901bcdca0d9642d0def0f57e865e20.1636683991.git.baolin.wang@linux.alibaba.com>
        (Baolin Wang's message of "Fri, 12 Nov 2021 10:27:56 +0800")
Message-ID: <87y25uks84.fsf@yhuang6-desk2.ccr.corp.intel.com>
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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 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;
> +		break;
> +	}
> +
> +	target = READ_ONCE(nd->nodes[index]);
>  
> +out:
> +	rcu_read_unlock();
>  	return target;
>  }
>  
> @@ -2974,10 +3026,16 @@ void migrate_vma_finalize(struct migrate_vma *migrate)
>  /* Disable reclaim-based migration. */
>  static void __disable_all_migrate_targets(void)
>  {
> -	int node;
> +	int node, i;
> +
> +	if (!node_demotion)
> +		return;
>  
> -	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 +3062,40 @@ 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 *nd;
>  
> -	/*
> -	 * 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 (node_demotion[node] != NUMA_NO_NODE)
> +	if (!node_demotion)
>  		return NUMA_NO_NODE;
>  
> +	nd = &node_demotion[node];
> +
>  	migration_target = find_next_best_node(node, used);
>  	if (migration_target == NUMA_NO_NODE)
>  		return NUMA_NO_NODE;
>  
> -	node_demotion[node] = migration_target;
> +	/*
> +	 * 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 (best_distance != -1) {
> +		val = node_distance(node, migration_target);
> +		if (val > best_distance)
> +			return NUMA_NO_NODE;
> +	}
> +
> +	index = nd->nr;
> +	if (WARN_ONCE(index >= DEMOTION_TARGET_NODES,
> +		      "Exceeds maximum demotion target nodes\n"))
> +		return NUMA_NO_NODE;
> +
> +	nd->nodes[index] = migration_target;
> +	nd->nr++;
>  
>  	return migration_target;
>  }
> @@ -3039,7 +3111,9 @@ 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 +3125,7 @@ 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 +3154,33 @@ 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
> @@ -3192,6 +3281,14 @@ static int __init migrate_on_reclaim_init(void)
>  {
>  	int ret;
>  
> +	/*
> +	 * Ignore allocation failure, if this kmalloc fails
> +	 * at boot time, we are likely in bigger trouble.
> +	 */
> +	node_demotion = kmalloc_array(nr_node_ids,
> +				      sizeof(struct demotion_nodes),
> +				      GFP_KERNEL);
> +

I think we should WARN_ON() here.

Best Regards,
Huang, Ying

>  	ret = cpuhp_setup_state_nocalls(CPUHP_MM_DEMOTION_DEAD, "mm/demotion:offline",
>  					NULL, migration_offline_cpu);
>  	/*