[PATCH] IPI performance benchmark

[Yury Norov]

This benchmark sends many IPIs in different modes and measures
time for IPI delivery (first column), and total time, ie including
time to acknowledge the receive by sender (second column).

The scenarios are:
Dry-run:	do everything except actually sending IPI. Useful
		to estimate system overhead.
Self-IPI:	Send IPI to self CPU.
Normal IPI:	Send IPI to some other CPU.
Broadcast IPI:	Send broadcast IPI to all online CPUs.

For virtualized guests, sending and reveiving IPIs causes guest exit.
I used this test to measure performance impact on KVM subsystem of
Christoffer Dall's series "Optimize KVM/ARM for VHE systems".

https://www.spinics.net/lists/kvm/msg156755.html

Test machine is ThunderX2, 112 online CPUs. Below the results normalized
to host dry-run time. Smaller - better.

Host, v4.14:
Dry-run:	  0	    1
Self-IPI:         9	   18
Normal IPI:      81	  110
Broadcast IPI:    0	 2106

Guest, v4.14:
Dry-run:          0	    1
Self-IPI:        10	   18
Normal IPI:     305	  525
Broadcast IPI:    0    	 9729

Guest, v4.14 + VHE:
Dry-run:          0	    1
Self-IPI:         9	   18
Normal IPI:     176	  343
Broadcast IPI:    0	 9885

CC: Andrew Morton <akpm at linux-foundation.org>
CC: Ashish Kalra <Ashish.Kalra at cavium.com>
CC: Christoffer Dall <christoffer.dall at linaro.org>
CC: Geert Uytterhoeven <geert at linux-m68k.org>
CC: Linu Cherian <Linu.Cherian at cavium.com>
CC: Sunil Goutham <Sunil.Goutham at cavium.com>
Signed-off-by: Yury Norov <ynorov at caviumnetworks.com>
---
 arch/Kconfig           |  10 ++++
 kernel/Makefile        |   1 +
 kernel/ipi_benchmark.c | 134 +++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 145 insertions(+)
 create mode 100644 kernel/ipi_benchmark.c
diff --git a/arch/Kconfig b/arch/Kconfig
index 057370a0ac4e..80d6ef439199 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -82,6 +82,16 @@ config JUMP_LABEL
 	 ( On 32-bit x86, the necessary options added to the compiler
 	   flags may increase the size of the kernel slightly. )
 
+config IPI_BENCHMARK
+	tristate "Test IPI performance on SMP systems"
+	depends on SMP
+	help
+	  Test IPI performance on SMP systems. If system has only one online
+	  CPU, sending IPI to other CPU is obviously not possible, and ENOENT
+	  is returned for corresponding test.
+
+	  If unsure, say N.
+
 config STATIC_KEYS_SELFTEST
 	bool "Static key selftest"
 	depends on JUMP_LABEL
diff --git a/kernel/Makefile b/kernel/Makefile
index 172d151d429c..04e550e1990c 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -101,6 +101,7 @@ obj-$(CONFIG_TRACEPOINTS) += trace/
 obj-$(CONFIG_IRQ_WORK) += irq_work.o
 obj-$(CONFIG_CPU_PM) += cpu_pm.o
 obj-$(CONFIG_BPF) += bpf/
+obj-$(CONFIG_IPI_BENCHMARK) += ipi_benchmark.o
 
 obj-$(CONFIG_PERF_EVENTS) += events/
 
diff --git a/kernel/ipi_benchmark.c b/kernel/ipi_benchmark.c
new file mode 100644
index 000000000000..35f1f7598c36
--- /dev/null
+++ b/kernel/ipi_benchmark.c
@@ -0,0 +1,134 @@
+/*
+ * Performance test for IPI on SMP machines.
+ *
+ * Copyright (c) 2017 Cavium Networks.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of version 2 of the GNU General Public
+ * License as published by the Free Software Foundation.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * General Public License for more details.
+ */
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/init.h>
+#include <linux/ktime.h>
+
+#define NTIMES 100000
+
+#define POKE_ANY	0
+#define DRY_RUN		1
+#define POKE_SELF	2
+#define POKE_ALL	3
+
+static void __init handle_ipi(void *t)
+{
+	ktime_t *time = (ktime_t *) t;
+
+	if (time)
+		*time = ktime_get() - *time;
+}
+
+static ktime_t __init send_ipi(int flags)
+{
+	ktime_t time;
+	unsigned int cpu = get_cpu();
+
+	switch (flags) {
+	case POKE_ALL:
+		/* If broadcasting, don't force all CPUs to update time. */
+		smp_call_function_many(cpu_online_mask, handle_ipi, NULL, 1);
+		/* Fall thru */
+	case DRY_RUN:
+		/* Do everything except actually sending IPI. */
+		time = 0;
+		break;
+	case POKE_ANY:
+		cpu = cpumask_any_but(cpu_online_mask, cpu);
+		if (cpu >= nr_cpu_ids) {
+			time = -ENOENT;
+			break;
+		}
+		/* Fall thru */
+	case POKE_SELF:
+		time = ktime_get();
+		smp_call_function_single(cpu, handle_ipi, &time, 1);
+		break;
+	default:
+		time = -EINVAL;
+	}
+
+	put_cpu();
+	return time;
+}
+
+static int __init __bench_ipi(unsigned long i, ktime_t *time, int flags)
+{
+	ktime_t t;
+
+	*time = 0;
+	while (i--) {
+		t = send_ipi(flags);
+		if ((int) t < 0)
+			return (int) t;
+
+		*time += t;
+	}
+
+	return 0;
+}
+
+static int __init bench_ipi(unsigned long times, int flags,
+				ktime_t *ipi, ktime_t *total)
+{
+	int ret;
+
+	*total = ktime_get();
+	ret = __bench_ipi(times, ipi, flags);
+	if (unlikely(ret))
+		return ret;
+
+	*total = ktime_get() - *total;
+
+	return 0;
+}
+
+static int __init init_bench_ipi(void)
+{
+	ktime_t ipi, total;
+	int ret;
+
+	ret = bench_ipi(NTIMES, DRY_RUN, &ipi, &total);
+	if (ret)
+		pr_err("Dry-run FAILED: %d\n", ret);
+	else
+		pr_err("Dry-run:       %18llu, %18llu ns\n", ipi, total);
+
+	ret = bench_ipi(NTIMES, POKE_SELF, &ipi, &total);
+	if (ret)
+		pr_err("Self-IPI FAILED: %d\n", ret);
+	else
+		pr_err("Self-IPI:      %18llu, %18llu ns\n", ipi, total);
+
+	ret = bench_ipi(NTIMES, POKE_ANY, &ipi, &total);
+	if (ret)
+		pr_err("Normal IPI FAILED: %d\n", ret);
+	else
+		pr_err("Normal IPI:    %18llu, %18llu ns\n", ipi, total);
+
+	ret = bench_ipi(NTIMES, POKE_ALL, &ipi, &total);
+	if (ret)
+		pr_err("Broadcast IPI FAILED: %d\n", ret);
+	else
+		pr_err("Broadcast IPI: %18llu, %18llu ns\n", ipi, total);
+
+	/* Return error to avoid annoying rmmod. */
+	return -EINVAL;
+}
+module_init(init_bench_ipi);
+
+MODULE_LICENSE("GPL");
-- 
2.11.0