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%)
---
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.
+
