On 17/05/19 19:48, Marcelo Tosatti wrote:
>
> The cpuidle_kvm driver allows the guest vcpus to poll for a specified
> amount of time before halting. This provides the following benefits
> to host side polling:
>
> 1) The POLL flag is set while polling is performed, which allows
> a remote vCPU to avoid sending an IPI (and the associated
> cost of handling the IPI) when performing a wakeup.
>
> 2) The HLT VM-exit cost can be avoided.
>
> The downside of guest side polling is that polling is performed
> even with other runnable tasks in the host.
>
> Results comparing halt_poll_ns and server/client application
> where a small packet is ping-ponged:
>
> host --> 31.33
> halt_poll_ns=300000 / no guest busy spin --> 33.40 (93.8%)
> halt_poll_ns=0 / guest_halt_poll_ns=300000 --> 32.73 (95.7%)
>
> For the SAP HANA benchmarks (where idle_spin is a parameter
> of the previous version of the patch, results should be the
> same):
>
> hpns == halt_poll_ns
>
> idle_spin=0/ idle_spin=800/ idle_spin=0/
> hpns=200000 hpns=0 hpns=800000
> DeleteC06T03 (100 thread) 1.76 1.71 (-3%) 1.78 (+1%)
> InsertC16T02 (100 thread) 2.14 2.07 (-3%) 2.18 (+1.8%)
> DeleteC00T01 (1 thread) 1.34 1.28 (-4.5%) 1.29 (-3.7%)
> UpdateC00T03 (1 thread) 4.72 4.18 (-12%) 4.53 (-5%)
Hi Marcelo,
some quick observations:
1) This is actually not KVM-specific, so the name and placement of the
docs should be adjusted.
2) Regarding KVM-specific code, however, we could add an MSR so that KVM
disables halt_poll_ns for this VM when this is active in the guest?
3) The spin time could use the same adaptive algorithm that KVM uses in
the host.
Thanks,
Paolo
> ---
> Documentation/virtual/kvm/guest-halt-polling.txt | 39 ++++++++
> arch/x86/Kconfig | 9 +
> arch/x86/kernel/Makefile | 1
> arch/x86/kernel/cpuidle_kvm.c | 105 +++++++++++++++++++++++
> arch/x86/kernel/process.c | 2
> 5 files changed, 155 insertions(+), 1 deletion(-)
>
> Index: linux-2.6.git/arch/x86/Kconfig
> ===================================================================
> --- linux-2.6.git.orig/arch/x86/Kconfig 2019-04-22 13:49:42.858303265 -0300
> +++ linux-2.6.git/arch/x86/Kconfig 2019-05-16 14:18:41.254852745 -0300
> @@ -805,6 +805,15 @@
> underlying device model, the host provides the guest with
> timing infrastructure such as time of day, and system time
>
> +config KVM_CPUIDLE
> + tristate "KVM cpuidle driver"
> + depends on KVM_GUEST
> + default y
> + help
> + This option enables KVM cpuidle driver, which allows to poll
> + before halting in the guest (more efficient than polling in the
> + host via halt_poll_ns for some scenarios).
> +
> config PVH
> bool "Support for running PVH guests"
> ---help---
> Index: linux-2.6.git/arch/x86/kernel/Makefile
> ===================================================================
> --- linux-2.6.git.orig/arch/x86/kernel/Makefile 2019-04-22 13:49:42.869303331 -0300
> +++ linux-2.6.git/arch/x86/kernel/Makefile 2019-05-17 12:59:51.673274881 -0300
> @@ -112,6 +112,7 @@
> obj-$(CONFIG_DEBUG_NMI_SELFTEST) += nmi_selftest.o
>
> obj-$(CONFIG_KVM_GUEST) += kvm.o kvmclock.o
> +obj-$(CONFIG_KVM_CPUIDLE) += cpuidle_kvm.o
> obj-$(CONFIG_PARAVIRT) += paravirt.o paravirt_patch_$(BITS).o
> obj-$(CONFIG_PARAVIRT_SPINLOCKS)+= paravirt-spinlocks.o
> obj-$(CONFIG_PARAVIRT_CLOCK) += pvclock.o
> Index: linux-2.6.git/arch/x86/kernel/process.c
> ===================================================================
> --- linux-2.6.git.orig/arch/x86/kernel/process.c 2019-04-22 13:49:42.876303374 -0300
> +++ linux-2.6.git/arch/x86/kernel/process.c 2019-05-17 13:19:18.055435117 -0300
> @@ -580,7 +580,7 @@
> safe_halt();
> trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, smp_processor_id());
> }
> -#ifdef CONFIG_APM_MODULE
> +#if defined(CONFIG_APM_MODULE) || defined(CONFIG_KVM_CPUIDLE_MODULE)
> EXPORT_SYMBOL(default_idle);
> #endif
>
> Index: linux-2.6.git/arch/x86/kernel/cpuidle_kvm.c
> ===================================================================
> --- /dev/null 1970-01-01 00:00:00.000000000 +0000
> +++ linux-2.6.git/arch/x86/kernel/cpuidle_kvm.c 2019-05-17 13:38:02.553941356 -0300
> @@ -0,0 +1,105 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * cpuidle driver for KVM guests.
> + *
> + * Copyright 2019 Red Hat, Inc. and/or its affiliates.
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2. See
> + * the COPYING file in the top-level directory.
> + *
> + * Authors: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> + */
> +
> +#include <linux/init.h>
> +#include <linux/cpuidle.h>
> +#include <linux/module.h>
> +#include <linux/timekeeping.h>
> +#include <linux/sched/idle.h>
> +
> +unsigned int guest_halt_poll_ns;
> +module_param(guest_halt_poll_ns, uint, 0644);
> +
> +static int kvm_enter_idle(struct cpuidle_device *dev,
> + struct cpuidle_driver *drv, int index)
> +{
> + int do_halt = 0;
> +
> + /* No polling */
> + if (guest_halt_poll_ns == 0) {
> + if (current_clr_polling_and_test()) {
> + local_irq_enable();
> + return index;
> + }
> + default_idle();
> + return index;
> + }
> +
> + local_irq_enable();
> + if (!current_set_polling_and_test()) {
> + ktime_t now, end_spin;
> +
> + now = ktime_get();
> + end_spin = ktime_add_ns(now, guest_halt_poll_ns);
> +
> + while (!need_resched()) {
> + cpu_relax();
> + now = ktime_get();
> +
> + if (!ktime_before(now, end_spin)) {
> + do_halt = 1;
> + break;
> + }
> + }
> + }
> +
> + if (do_halt) {
> + /*
> + * No events while busy spin window passed,
> + * halt.
> + */
> + local_irq_disable();
> + if (current_clr_polling_and_test()) {
> + local_irq_enable();
> + return index;
> + }
> + default_idle();
> + } else {
> + current_clr_polling();
> + }
> +
> + return index;
> +}
> +
> +static struct cpuidle_driver kvm_idle_driver = {
> + .name = "kvm_idle",
> + .owner = THIS_MODULE,
> + .states = {
> + { /* entry 0 is for polling */ },
> + {
> + .enter = kvm_enter_idle,
> + .exit_latency = 0,
> + .target_residency = 0,
> + .power_usage = -1,
> + .name = "KVM",
> + .desc = "KVM idle",
> + },
> + },
> + .safe_state_index = 0,
> + .state_count = 2,
> +};
> +
> +static int __init kvm_cpuidle_init(void)
> +{
> + return cpuidle_register(&kvm_idle_driver, NULL);
> +}
> +
> +static void __exit kvm_cpuidle_exit(void)
> +{
> + cpuidle_unregister(&kvm_idle_driver);
> +}
> +
> +module_init(kvm_cpuidle_init);
> +module_exit(kvm_cpuidle_exit);
> +MODULE_LICENSE("GPL");
> +MODULE_AUTHOR("Marcelo Tosatti <mtosatti@xxxxxxxxxx>");
> +
> Index: linux-2.6.git/Documentation/virtual/kvm/guest-halt-polling.txt
> ===================================================================
> --- /dev/null 1970-01-01 00:00:00.000000000 +0000
> +++ linux-2.6.git/Documentation/virtual/kvm/guest-halt-polling.txt 2019-05-17 13:36:39.274703710 -0300
> @@ -0,0 +1,39 @@
> +KVM guest halt polling
> +======================
> +
> +The cpuidle_kvm driver allows the guest vcpus to poll for a specified
> +amount of time before halting. This provides the following benefits
> +to host side polling:
> +
> + 1) The POLL flag is set while polling is performed, which allows
> + a remote vCPU to avoid sending an IPI (and the associated
> + cost of handling the IPI) when performing a wakeup.
> +
> + 2) The HLT VM-exit cost can be avoided.
> +
> +The downside of guest side polling is that polling is performed
> +even with other runnable tasks in the host.
> +
> +Module Parameters
> +=================
> +
> +The cpuidle_kvm module has 1 tuneable module parameter: guest_halt_poll_ns,
> +the amount of time, in nanoseconds, that polling is performed before
> +halting.
> +
> +This module parameter can be set from the debugfs files in:
> +
> + /sys/module/cpuidle_kvm/parameters/
> +
> +Further Notes
> +=============
> +
> +- Care should be taken when setting the guest_halt_poll_ns parameter as a
> +large value has the potential to drive the cpu usage to 100% on a machine which
> +would be almost entirely idle otherwise.
> +
> +- The effective amount of time that polling is performed is the host poll
> +value (see halt-polling.txt) plus guest_halt_poll_ns. If all guests
> +on a host system support and have properly configured guest_halt_poll_ns,
> +then setting halt_poll_ns to 0 in the host is probably the best choice.
> +
>
