On Sat, Mar 25, 2023 at 11:55:09AM -0700, Yury Norov wrote: > 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@xxxxxxxxx/ > > 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@xxxxxxxxx> > --- > 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. The comments below are incorrect (copy-paste error). I'll remove them in v2. > + * > + * 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
