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Date: Tue, 9 Nov 2021 17:54:58 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: "Huang, Ying" <ying.huang@...el.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
On 2021/11/9 15:53, Huang, Ying writes:
> 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[]?
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
>
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