Re: [PATCH 2/6] sched/topology: Introduce for_each_numa_node() iterator

Andrea Righi
2025-02-07T21:55:27Z

On Fri, Feb 07, 2025 at 11:46:51AM -1000, Tejun Heo wrote:
> On Fri, Feb 07, 2025 at 09:40:49PM +0100, Andrea Righi wrote:
> > +/**
> > + * for_each_numa_node - iterate over nodes at increasing distances from a
> > + * given starting node.
> > + * @node: the iteration variable and the starting node.
> > + * @unvisited: a nodemask to keep track of the unvisited nodes.
> > + * @state: state of NUMA nodes to iterate.
> > + *
> > + * This macro iterates over NUMA node IDs in increasing distance from the
> > + * starting @node and yields MAX_NUMNODES when all the nodes have been
> > + * visited.
> > + *
> > + * The difference between for_each_node() and for_each_numa_node() is that
> > + * the former allows to iterate over nodes in numerical order, whereas the
> > + * latter iterates over nodes in increasing order of distance.
> > + *
> > + * This complexity of this iterator is O(N^2), where N represents the
> > + * number of nodes, as each iteration involves scanning all nodes to
> > + * find the one with the shortest distance.
> > + *
> > + * Requires rcu_lock to be held.
> > + */
> > +#define for_each_numa_node(node, unvisited, state) \
> > + for (int start = (node), \
> > + node = numa_nearest_nodemask((start), (state), &(unvisited)); \
> > + node < MAX_NUMNODES; \
> > + node_clear(node, (unvisited)), \
> > + node = numa_nearest_nodemask((start), (state), &(unvisited)))
> > +
> > /**
> > * for_each_numa_hop_mask - iterate over cpumasks of increasing NUMA distance
> > * from a given node.
>
> Bikeshedding: Maybe this has already been argued back and forth but I find
> the distinction between for_each_node() and for_each_numa_node() way too
> subtle. I wouldn't suspect that they are doing different things when
> glancing through their usages in isolation. Can we add *something* to the
> name that indicates that this is iteration by distance? The next one uses
> "hop" which is fine, "_by_dist" can be fine too, or even "_from_nearest". I
> don't really care which but let's make the name clearly signal what it's
> doing.
>
> Thanks.

How about for_each_node_state_by_dist()? It's essentialy a variant of
for_each_node_state(), as it also accepts a state, with the only difference
that node IDs are returned in increasing distance order.

-Andrea