netdev - [PATCH 3/8] sched/topology: add for_each_numa

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Date:   Sat, 25 Mar 2023 11:55:09 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org
Cc:     Yury Norov <yury.norov@...il.com>,
        Saeed Mahameed <saeedm@...dia.com>,
        Pawel Chmielewski <pawel.chmielewski@...el.com>,
        Leon Romanovsky <leon@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Tariq Toukan <tariqt@...dia.com>,
        Gal Pressman <gal@...dia.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Barry Song <baohua@...nel.org>
Subject: [PATCH 3/8] sched/topology: add for_each_numa_cpu() macro

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@...il.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