On Thu, Apr 1, 2021 at 11:35 AM Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> wrote:
> +/*
> + * node_demotion[] example:
> + *
> + * Consider a system with two sockets. Each socket has
> + * three classes of memory attached: fast, medium and slow.
> + * Each memory class is placed in its own NUMA node. The
> + * CPUs are placed in the node with the "fast" memory. The
> + * 6 NUMA nodes (0-5) might be split among the sockets like
> + * this:
> + *
> + * Socket A: 0, 1, 2
> + * Socket B: 3, 4, 5
> + *
> + * When Node 0 fills up, its memory should be migrated to
> + * Node 1. When Node 1 fills up, it should be migrated to
> + * Node 2. The migration path start on the nodes with the
> + * processors (since allocations default to this node) and
> + * fast memory, progress through medium and end with the
> + * slow memory:
> + *
> + * 0 -> 1 -> 2 -> stop
> + * 3 -> 4 -> 5 -> stop
> + *
> + * 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
> + */
In this example, if we want to support multiple nodes as the demotion
target of a source node, we can group these nodes into three tiers
(classes):
fast class:
0 -> {1, 4} // 1 is the preferred
3 -> {4, 1} // 4 is the preferred
medium class:
1 -> {2, 5} // 2 is the preferred
4 -> {5, 2} // 5 is the preferred
slow class:
2 -> stop
5 -> stop
This can guarantee there are no cycles, either. Does it sound sensible?
> +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_node_mask(node, this_pass) {
> + int target_node = establish_migrate_target(node, &used_targets);
> +
> + if (target_node == NUMA_NO_NODE)
> + continue;
> +
> + /* Visit targets from this pass in the next pass: */
> + node_set(target_node, next_pass);
> + }
> + /* Is another pass necessary? */
> + if (!nodes_empty(next_pass))
> + goto again;
This goto seems like exactly a "do {} while" loop. Any particular reason not to
use "do {} while" here?
