On Wed, Feb 12, 2025 at 05:48:13PM +0100, Andrea Righi wrote:
> @@ -90,6 +131,78 @@ s32 scx_pick_idle_cpu(const struct cpumask *cpus_allowed, u64 flags)
> goto retry;
> }
>
> +static s32 pick_idle_cpu_from_other_nodes(const struct cpumask *cpus_allowed, int node, u64 flags)
'From other node' sounds a bit vague
> +{
> + static DEFINE_PER_CPU(nodemask_t, per_cpu_unvisited);
> + nodemask_t *unvisited = this_cpu_ptr(&per_cpu_unvisited);
> + s32 cpu = -EBUSY;
> +
> + preempt_disable();
> + unvisited = this_cpu_ptr(&per_cpu_unvisited);
> +
> + /*
> + * Restrict the search to the online nodes, excluding the current
> + * one.
> + */
> + nodes_clear(*unvisited);
> + nodes_or(*unvisited, *unvisited, node_states[N_ONLINE]);
nodes_clear() + nodes_or() == nodes_copy()
Yeah, we miss it. The attached patch adds nodes_copy(). Can you
consider taking it for your series?
> + node_clear(node, *unvisited);
> +
> + /*
> + * Traverse all nodes in order of increasing distance, starting
> + * from @node.
> + *
> + * This loop is O(N^2), with N being the amount of NUMA nodes,
> + * which might be quite expensive in large NUMA systems. However,
> + * this complexity comes into play only when a scheduler enables
> + * SCX_OPS_BUILTIN_IDLE_PER_NODE and it's requesting an idle CPU
> + * without specifying a target NUMA node, so it shouldn't be a
> + * bottleneck is most cases.
> + *
> + * As a future optimization we may want to cache the list of nodes
> + * in a per-node array, instead of actually traversing them every
> + * time.
> + */
> + for_each_node_numadist(node, *unvisited) {
> + cpu = pick_idle_cpu_in_node(cpus_allowed, node, flags);
> + if (cpu >= 0)
> + break;
> + }
> + preempt_enable();
> +
> + return cpu;
> +}
> +
> +/*
> + * Find an idle CPU in the system, starting from @node.
> + */
> +s32 scx_pick_idle_cpu(const struct cpumask *cpus_allowed, int node, u64 flags)
> +{
> + s32 cpu;
> +
> + /*
> + * Always search in the starting node first (this is an
> + * optimization that can save some cycles even when the search is
> + * not limited to a single node).
> + */
> + cpu = pick_idle_cpu_in_node(cpus_allowed, node, flags);
> + if (cpu >= 0)
> + return cpu;
> +
> + /*
> + * Stop the search if we are using only a single global cpumask
> + * (NUMA_NO_NODE) or if the search is restricted to the first node
> + * only.
> + */
> + if (node == NUMA_NO_NODE || flags & SCX_PICK_IDLE_IN_NODE)
> + return -EBUSY;
> +
> + /*
> + * Extend the search to the other nodes.
> + */
> + return pick_idle_cpu_from_other_nodes(cpus_allowed, node, flags);
> +}
