linux-kernel - [PATCH v2 1/4] smp: Improve locality in smp_call_function

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Message-ID: <20250623000010.10124-2-yury.norov@gmail.com>
Date: Sun, 22 Jun 2025 20:00:06 -0400
From: Yury Norov <yury.norov@...il.com>
To: "Paul E. McKenney" <paulmck@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	"Yury Norov [NVIDIA]" <yury.norov@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Thorsten Blum <thorsten.blum@...ux.dev>,
	Zqiang <qiang.zhang1211@...il.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2 1/4] smp: Improve locality in smp_call_function_any()

From: "Yury Norov [NVIDIA]" <yury.norov@...il.com>

smp_call_function_any() tries to make a local call as it's the cheapest
option, or switches to a CPU in the same node. If it's not possible, the
algorithm gives up and searches for any CPU, in a numerical order.

Instead, we can search for the best CPU based on NUMA locality, including
2nd nearest hop (a set of equidistant nodes), and higher.

sched_numa_find_nth_cpu() does exactly that, and also helps to drop most
of housekeeping code.

Signed-off-by: Yury Norov [NVIDIA] <yury.norov@...il.com>
---
 kernel/smp.c | 19 +++----------------
 1 file changed, 3 insertions(+), 16 deletions(-)

diff --git a/kernel/smp.c b/kernel/smp.c
index 974f3a3962e8..7c8cfab0ce55 100644
--- a/kernel/smp.c
+++ b/kernel/smp.c
@@ -741,32 +741,19 @@ EXPORT_SYMBOL_GPL(smp_call_function_single_async);
  *
  * Selection preference:
  *	1) current cpu if in @mask
- *	2) any cpu of current node if in @mask
- *	3) any other online cpu in @mask
+ *	2) nearest cpu in @mask, based on NUMA topology
  */
 int smp_call_function_any(const struct cpumask *mask,
 			  smp_call_func_t func, void *info, int wait)
 {
 	unsigned int cpu;
-	const struct cpumask *nodemask;
 	int ret;
 
 	/* Try for same CPU (cheapest) */
 	cpu = get_cpu();
-	if (cpumask_test_cpu(cpu, mask))
-		goto call;
-
-	/* Try for same node. */
-	nodemask = cpumask_of_node(cpu_to_node(cpu));
-	for (cpu = cpumask_first_and(nodemask, mask); cpu < nr_cpu_ids;
-	     cpu = cpumask_next_and(cpu, nodemask, mask)) {
-		if (cpu_online(cpu))
-			goto call;
-	}
+	if (!cpumask_test_cpu(cpu, mask))
+		cpu = sched_numa_find_nth_cpu(mask, 0, cpu_to_node(cpu));
 
-	/* Any online will do: smp_call_function_single handles nr_cpu_ids. */
-	cpu = cpumask_any_and(mask, cpu_online_mask);
-call:
 	ret = smp_call_function_single(cpu, func, info, wait);
 	put_cpu();
 	return ret;
-- 
2.43.0