samples: add a benchmark for smp_processor_id()

This module can be used to benchmark different implementations of
smp_processor_id();

Signed-off-by: Puranjay Mohan <puranjay@kernel.org>

Author: puranjaymohan

samples/Kconfig

@@ -291,6 +291,12 @@ config SAMPLE_CGROUP
 	help
 	  Build samples that demonstrate the usage of the cgroup API.
 
+config SAMPLE_SMP_PROCESSOR_ID
+	tristate "Build a benchmark for smp_processor_id()"
+	help
+	  This builds a bechmark that calls smp_processor_id() a given
+	  number of times and calculates the runtime overhead.
+
 source "samples/rust/Kconfig"
 
 endif # SAMPLES

samples/Makefile

@@ -39,3 +39,4 @@ obj-$(CONFIG_SAMPLE_KMEMLEAK)		+= kmemleak/
 obj-$(CONFIG_SAMPLE_CORESIGHT_SYSCFG)	+= coresight/
 obj-$(CONFIG_SAMPLE_FPROBE)		+= fprobe/
 obj-$(CONFIG_SAMPLES_RUST)		+= rust/
+obj-$(CONFIG_SAMPLE_SMP_PROCESSOR_ID) += smp_processor_id/

samples/smp_processor_id/Makefile

@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: GPL-2.0-only
+
+obj-$(CONFIG_SAMPLE_SMP_PROCESSOR_ID) += smp_processor_id.o

samples/smp_processor_id/smp_processor_id.c

@@ -0,0 +1,55 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#define pr_fmt(fmt)       KBUILD_MODNAME ": " fmt
+
+#include <linux/ktime.h>
+#include <linux/module.h>
+#include <linux/smp.h>
+
+/*
+ * Arbitrary large value chosen to be sufficiently large to minimize noise but
+ * sufficiently small to complete quickly.
+ */
+static unsigned int nr_function_calls = 1000000;
+module_param(nr_function_calls, uint, 0);
+MODULE_PARM_DESC(nr_function_calls, "How many times to call the smp_processor_id()");
+
+static noinline int get_cpu_id(void)
+{
+	return smp_processor_id();
+}
+
+static int __init smp_processor_id_sample_init(void)
+{
+	volatile int cpu;
+	ktime_t start, end;
+	u64 period;
+
+	start = ktime_get();
+	for (unsigned int i = 0; i < nr_function_calls; i++)
+		cpu = get_cpu_id();
+	end = ktime_get();
+
+	period = ktime_to_ns(ktime_sub(end, start));
+
+	pr_info("Attempted %u calls to %ps in %lluns (%lluns / call)\n",
+		nr_function_calls, get_cpu_id,
+		period, div_u64(period, nr_function_calls));
+
+	/*
+	 * The benchmark completed successfully, but there's no reason to keep
+	 * the module around. Return an error do the user doesn't have to
+	 * manually unload the module.
+	 */
+	return -EINVAL;
+}
+module_init(smp_processor_id_sample_init);
+
+static void __exit smp_processor_id_sample_exit(void)
+{
+}
+module_exit(smp_processor_id_sample_exit);
+
+MODULE_AUTHOR("Puranjay Mohan");
+MODULE_DESCRIPTION("Benchmark for smp_processor_id function");
+MODULE_LICENSE("GPL");