On Mon, May 20, 2019 at 01:51:57PM +0200, Paolo Bonzini wrote:
> 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.
Agreed. Will call it: cpuidle_halt_poll, move it to drivers/cpuidle/
> 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?
Sure.
> 3) The spin time could use the same adaptive algorithm that KVM uses in
> the host.
Agreed... This can be done later, i suppose (the current fixed
setting works sufficiently well for our needs).
> 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.
> > +
> >
