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

[Yury Norov <yury.norov@gmail.com>]

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@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.

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