[PATCH 3/8] sched/topology: add for_each_numa_cpu() macro

Yury Norov
2023-03-25T18:55:45Z

for_each_cpu() is widely used in the kernel, and it's beneficial to
create a NUMA-aware version of the macro.

Recently added for_each_numa_hop_mask() works, but switching existing
codebase to it is not an easy process.

New for_each_numa_cpu() is designed to be similar to the for_each_cpu().
It allows to convert existing code to NUMA-aware as simple as adding a
hop iterator variable and passing it inside new macro. for_each_numa_cpu()
takes care of the rest.

At the moment, we have 2 users of NUMA-aware enumerators. One is
Melanox's in-tree driver, and another is Intel's in-review driver:

https://lore.kernel.org/lkml/20230216145455.661709-1-pawel.chmielewski@intel.com/

Both real-life examples follow the same pattern:

for_each_numa_hop_mask(cpus, prev, node) {
for_each_cpu_andnot(cpu, cpus, prev) {
if (cnt++ == max_num)
goto out;
do_something(cpu);
}
prev = cpus;
}

With the new macro, it would look like this:

for_each_numa_cpu(cpu, hop, node, cpu_possible_mask) {
if (cnt++ == max_num)
break;
do_something(cpu);
}

Straight conversion of existing for_each_cpu() codebase to NUMA-aware
version with for_each_numa_hop_mask() is difficult because it doesn't
take a user-provided cpu mask, and eventually ends up with open-coded
double loop. With for_each_numa_cpu() it shouldn't be a brainteaser.
Consider the NUMA-ignorant example:

cpumask_t cpus = get_mask();
int cnt = 0, cpu;

for_each_cpu(cpu, cpus) {
if (cnt++ == max_num)
break;
do_something(cpu);
}

Converting it to NUMA-aware version would be as simple as:

cpumask_t cpus = get_mask();
int node = get_node();
int cnt = 0, hop, cpu;

for_each_numa_cpu(cpu, hop, node, cpus) {
if (cnt++ == max_num)
break;
do_something(cpu);
}

The latter looks more verbose and avoids from open-coding that annoying
double loop. Another advantage is that it works with a 'hop' parameter with
the clear meaning of NUMA distance, and doesn't make people not familiar
to enumerator internals bothering with current and previous masks machinery.

Signed-off-by: Yury Norov <yury.norov@gmail.com>
---
include/linux/topology.h | 20 ++++++++++++++++++++
1 file changed, 20 insertions(+)

diff --git a/include/linux/topology.h b/include/linux/topology.h
index 4a63154fa036..62a9dd8edd77 100644
--- a/include/linux/topology.h
+++ b/include/linux/topology.h
@@ -286,4 +286,24 @@ sched_numa_hop_mask(unsigned int node, unsigned int hops)
!IS_ERR_OR_NULL(mask); \
__hops++)

+/**
+ * for_each_numa_cpu - iterate over cpus in increasing order taking into account
+ * NUMA distances from a given node.
+ * @cpu: the (optionally unsigned) integer iterator
+ * @hop: the iterator variable, must be initialized to a desired minimal hop.
+ * @node: the NUMA node to start the search from.
+ *
+ * Requires rcu_lock to be held.
+ *
+ * Because it's implemented as double-loop, using 'break' inside the body of
+ * iterator may lead to undefined behaviour. Use 'goto' instead.
+ *
+ * Yields intersection of @mask and cpu_online_mask if @node == NUMA_NO_NODE.
+ */
+#define for_each_numa_cpu(cpu, hop, node, mask) \
+ for ((cpu) = 0, (hop) = 0; \
+ (cpu) = sched_numa_find_next_cpu((mask), (cpu), (node), &(hop)),\
+ (cpu) < nr_cpu_ids; \
+ (cpu)++)
+
#endif /* _LINUX_TOPOLOGY_H */
--
2.34.1