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Message-ID: <b7d1ab3b-e92c-d3aa-72cb-b80cc1a61e85@linux.alibaba.com>
Date: Tue, 29 Mar 2022 20:26:05 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Jagdish Gediya <jvgediya@...ux.ibm.com>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org
Cc: akpm@...ux-foundation.org, aneesh.kumar@...ux.ibm.com,
dave.hansen@...ux.intel.com, ying.huang@...el.com
Subject: Re: [PATCH] mm: migrate: set demotion targets differently
Hi Jagdish,
On 3/29/2022 7:52 PM, Jagdish Gediya wrote:
> The current implementation to identify the demotion
> targets limits some of the opportunities to share
> the demotion targets between multiple source nodes.
>
> Implement a logic to identify the loop in the demotion
> targets such that all the possibilities of demotion can
> be utilized. Don't share the used targets between all
> the nodes, instead create the used targets from scratch
> for each individual node based on for what all node this
> node is a demotion target. This helps to share the demotion
> targets without missing any possible way of demotion.
>
> e.g. with below NUMA topology, where node 0 & 1 are
> cpu + dram nodes, node 2 & 3 are equally slower memory
> only nodes, and node 4 is slowest memory only node,
>
> available: 5 nodes (0-4)
> node 0 cpus: 0 1
> node 0 size: n MB
> node 0 free: n MB
> node 1 cpus: 2 3
> node 1 size: n MB
> node 1 free: n MB
> node 2 cpus:
> node 2 size: n MB
> node 2 free: n MB
> node 3 cpus:
> node 3 size: n MB
> node 3 free: n MB
> node 4 cpus:
> node 4 size: n MB
> node 4 free: n MB
> node distances:
> node 0 1 2 3 4
> 0: 10 20 40 40 80
> 1: 20 10 40 40 80
> 2: 40 40 10 40 80
> 3: 40 40 40 10 80
> 4: 80 80 80 80 10
>
> The existing implementation gives below demotion targets,
>
> node demotion_target
> 0 3, 2
> 1 4
> 2 X
> 3 X
> 4 X
>
> With this patch applied, below are the demotion targets,
>
> node demotion_target
> 0 3, 2
> 1 3, 2
> 2 3
> 3 4
> 4 X
Node 2 and node 3 both are slow memory and have same distance, why node
2 should demote cold memory to node 3? They should have the same target
demotion node 4, which is the slowest memory node, right?
>
> e.g. with below NUMA topology, where node 0, 1 & 2 are
> cpu + dram nodes and node 3 is slow memory node,
>
> available: 4 nodes (0-3)
> node 0 cpus: 0 1
> node 0 size: n MB
> node 0 free: n MB
> node 1 cpus: 2 3
> node 1 size: n MB
> node 1 free: n MB
> node 2 cpus: 4 5
> node 2 size: n MB
> node 2 free: n MB
> node 3 cpus:
> node 3 size: n MB
> node 3 free: n MB
> node distances:
> node 0 1 2 3
> 0: 10 20 20 40
> 1: 20 10 20 40
> 2: 20 20 10 40
> 3: 40 40 40 10
>
> The existing implementation gives below demotion targets,
>
> node demotion_target
> 0 3
> 1 X
> 2 X
> 3 X
>
> With this patch applied, below are the demotion targets,
>
> node demotion_target
> 0 3
> 1 3
> 2 3
> 3 X
Sounds reasonable.
>
> with below NUMA topology, where node 0 & 2 are cpu + dram
> nodes and node 1 & 3 are slow memory nodes,
>
> available: 4 nodes (0-3)
> node 0 cpus: 0 1
> node 0 size: n MB
> node 0 free: n MB
> node 1 cpus:
> node 1 size: n MB
> node 1 free: n MB
> node 2 cpus: 2 3
> node 2 size: n MB
> node 2 free: n MB
> node 3 cpus:
> node 3 size: n MB
> node 3 free: n MB
> node distances:
> node 0 1 2 3
> 0: 10 40 20 80
> 1: 40 10 80 80
> 2: 20 80 10 40
> 3: 80 80 40 10
>
> The existing implementation gives below demotion targets,
>
> node demotion_target
> 0 3
> 1 X
> 2 3
> 3 X
If I understand correctly, this is not true. The demotion route should
be as below with existing implementation:
node 0 ---> node 1
node 1 ---> X
node 2 ---> node 3
node 3 ---> X
>
> With this patch applied, below are the demotion targets,
>
> node demotion_target
> 0 1
> 1 3
> 2 3
> 3 X
>
> As it can be seen above, node 3 can be demotion target for node
> 1 but existing implementation doesn't configure it that way. It
> is better to move pages from node 1 to node 3 instead of moving
> it from node 1 to swap.
Which means node 3 is the slowest memory node?
>
> Signed-off-by: Jagdish Gediya <jvgediya@...ux.ibm.com>
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com>
> ---
> mm/migrate.c | 75 ++++++++++++++++++++++++++++------------------------
> 1 file changed, 41 insertions(+), 34 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 3d60823afd2d..7ec8d934e706 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -2381,10 +2381,13 @@ static int establish_migrate_target(int node, nodemask_t *used,
> */
> 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, best_distance;
> + nodemask_t *src_nodes;
> +
> + src_nodes = kcalloc(nr_node_ids, sizeof(nodemask_t), GFP_KERNEL);
> + if (!src_nodes)
> + return;
>
> /*
> * Avoid any oddities like cycles that could occur
> @@ -2393,29 +2396,39 @@ static void __set_migration_target_nodes(void)
> */
> disable_all_migrate_targets();
>
> - /*
> - * Allocations go close to CPUs, first. Assume that
> - * the migration path starts at the nodes with CPUs.
> - */
> - next_pass = node_states[N_CPU];
> -again:
> - this_pass = next_pass;
> - next_pass = NODE_MASK_NONE;
> - /*
> - * To avoid cycles in the migration "graph", ensure
> - * that migration sources are not future targets by
> - * setting them in 'used_targets'. Do this only
> - * once per pass so that multiple source nodes can
> - * share a target node.
> - *
> - * 'used_targets' will become unavailable in future
> - * passes. This limits some opportunities for
> - * multiple source nodes to share a destination.
> - */
> - nodes_or(used_targets, used_targets, this_pass);
> + for_each_online_node(node) {
> + int tmp_node;
>
> - for_each_node_mask(node, this_pass) {
> best_distance = -1;
> + used_targets = NODE_MASK_NONE;
> +
> + /*
> + * Avoid adding same node as the demotion target.
> + */
> + node_set(node, used_targets);
> +
> + /*
> + * Add CPU NUMA nodes to the used target list so that it
> + * won't be considered a demotion target.
> + */
> + nodes_or(used_targets, used_targets, node_states[N_CPU]);
> +
> + /*
> + * Add all nodes that has appeared as source node of demotion
> + * for this target node.
> + *
> + * To avoid cycles in the migration "graph", ensure
> + * that migration sources are not future targets by
> + * setting them in 'used_targets'.
> + */
> + for_each_node_mask(tmp_node, src_nodes[node])
> + nodes_or(used_targets, used_targets, src_nodes[tmp_node]);
> +
> + /*
> + * Now update the demotion src nodes with other nodes in graph
> + * which got computed above.
> + */
> + nodes_or(src_nodes[node], src_nodes[node], used_targets);
>
> /*
> * Try to set up the migration path for the node, and the target
> @@ -2434,20 +2447,14 @@ static void __set_migration_target_nodes(void)
> 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'.
> + * Add this node in the src_nodes list so that we can
> + * detect the looping.
> */
> - node_set(target_node, next_pass);
> + node_set(node, src_nodes[target_node]);
> } while (1);
> }
> - /*
> - * 'next_pass' contains nodes which became migration
> - * targets in this pass. Make additional passes until
> - * no more migrations targets are available.
> - */
> - if (!nodes_empty(next_pass))
> - goto again;
> +
> + kfree(src_nodes);
> }
>
> /*
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