Subject: Re: [PATCH] IPI performance benchmark
To: Yury Norov <ynorov@caviumnetworks.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Ashish Kalra <Ashish.Kalra@cavium.com>,
        Christoffer Dall <christoffer.dall@linaro.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Linu Cherian <Linu.Cherian@cavium.com>,
        Sunil Goutham <Sunil.Goutham@cavium.com>
References: <20171211141600.24401-1-ynorov@caviumnetworks.com>
From: Christian Borntraeger <borntraeger@de.ibm.com>
Date: Mon, 11 Dec 2017 15:35:02 +0100
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On 12/11/2017 03:16 PM, Yury Norov wrote:
> 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@linux-foundation.org>
> CC: Ashish Kalra <Ashish.Kalra@cavium.com>
> CC: Christoffer Dall <christoffer.dall@linaro.org>
> CC: Geert Uytterhoeven <geert@linux-m68k.org>
> CC: Linu Cherian <Linu.Cherian@cavium.com>
> CC: Sunil Goutham <Sunil.Goutham@cavium.com>
> Signed-off-by: Yury Norov <ynorov@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);

you do not use NTIMES here to calculate the average value. Is that intended?