The cpuidle_haltpoll 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/guest-halt-polling.txt | 96 +++++++++++++++++
arch/x86/kernel/process.c | 2
drivers/cpuidle/Kconfig | 9 +
drivers/cpuidle/Makefile | 1
drivers/cpuidle/cpuidle-haltpoll.c | 145 +++++++++++++++++++++++++++
5 files changed, 252 insertions(+), 1 deletion(-)
Index: linux-2.6.git/Documentation/virtual/guest-halt-polling.txt
===================================================================
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ linux-2.6.git/Documentation/virtual/guest-halt-polling.txt 2019-06-11 16:38:55.072877644 -0300
@@ -0,0 +1,96 @@
+Guest halt polling
+==================
+
+The cpuidle_haltpoll 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 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.
+
+The basic logic as follows: A global value, guest_halt_poll_ns,
+is configured by the user, indicating the maximum amount of
+time polling is allowed. This value is fixed.
+
+Each vcpu has an adjustable guest_halt_poll_ns
+("per-cpu guest_halt_poll_ns"), which is adjusted by the algorithm
+in response to events (explained below).
+
+Module Parameters
+=================
+
+The cpuidle_haltpoll module has 5 tunable module parameters:
+
+1) guest_halt_poll_ns:
+
+Maximum amount of time, in nanoseconds, that polling is
+performed before halting.
+
+Default: 0
+
+2) guest_halt_poll_shrink:
+
+Division factor used to shrink per-cpu guest_halt_poll_ns when
+wakeup event occurs after the global guest_halt_poll_ns.
+
+Default: 2
+
+3) guest_halt_poll_grow:
+
+Multiplication factor used to grow per-cpu guest_halt_poll_ns
+when event occurs after per-cpu guest_halt_poll_ns
+but before global guest_halt_poll_ns.
+
+Default: 2
+
+4) guest_halt_poll_grow_start:
+
+The per-cpu guest_halt_poll_ns eventually reaches zero
+in case of an idle system. This value sets the initial
+per-cpu guest_halt_poll_ns when growing. This can
+be increased from 10000, to avoid misses during the initial
+growth stage:
+
+10000, 20000, 40000, ... (example assumes guest_halt_poll_grow=2).
+
+Default: 10000
+
+5) guest_halt_poll_allow_shrink:
+
+Bool parameter which allows shrinking. Set to N
+to avoid it (per-cpu guest_halt_poll_ns will remain
+high once achieves global guest_halt_poll_ns value).
+
+Default: Y
+
+The module parameters can be set from the debugfs files in:
+
+ /sys/module/cpuidle_haltpoll/parameters/
+
+Host and guest polling
+======================
+
+KVM also performs host side polling (that is, it can poll for a certain
+amount of time before halting) on behalf of guest vcpus.
+
+Modern hosts support poll control MSRs, which are used by
+cpuidle_haltpoll to disable host side polling on a per-VM basis.
+
+If the KVM host does not support this interface, then both guest side
+and host side polling can be performed, which can incur extra CPU and
+energy consumption. One might consider disabling host side polling
+manually if upgrading to a new host is not an option.
+
+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.
+
Index: linux-2.6.git/arch/x86/kernel/process.c
===================================================================
--- linux-2.6.git.orig/arch/x86/kernel/process.c 2019-06-11 12:14:37.731286353 -0300
+++ linux-2.6.git/arch/x86/kernel/process.c 2019-06-11 12:14:44.699424799 -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_HALTPOLL_CPUIDLE_MODULE)
EXPORT_SYMBOL(default_idle);
#endif
Index: linux-2.6.git/drivers/cpuidle/Kconfig
===================================================================
--- linux-2.6.git.orig/drivers/cpuidle/Kconfig 2019-06-11 12:14:37.731286353 -0300
+++ linux-2.6.git/drivers/cpuidle/Kconfig 2019-06-11 16:38:13.984060707 -0300
@@ -51,6 +51,15 @@
source "drivers/cpuidle/Kconfig.powerpc"
endmenu
+config HALTPOLL_CPUIDLE
+ tristate "Halt poll cpuidle driver"
+ depends on X86
+ default y
+ help
+ This option enables halt poll 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).
+
endif
config ARCH_NEEDS_CPU_IDLE_COUPLED
Index: linux-2.6.git/drivers/cpuidle/Makefile
===================================================================
--- linux-2.6.git.orig/drivers/cpuidle/Makefile 2019-06-11 12:14:37.731286353 -0300
+++ linux-2.6.git/drivers/cpuidle/Makefile 2019-06-11 12:14:44.700424819 -0300
@@ -7,6 +7,7 @@
obj-$(CONFIG_ARCH_NEEDS_CPU_IDLE_COUPLED) += coupled.o
obj-$(CONFIG_DT_IDLE_STATES) += dt_idle_states.o
obj-$(CONFIG_ARCH_HAS_CPU_RELAX) += poll_state.o
+obj-$(CONFIG_HALTPOLL_CPUIDLE) += cpuidle-haltpoll.o
##################################################################################
# ARM SoC drivers
Index: linux-2.6.git/drivers/cpuidle/cpuidle-haltpoll.c
===================================================================
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ linux-2.6.git/drivers/cpuidle/cpuidle-haltpoll.c 2019-06-11 16:37:23.328053569 -0300
@@ -0,0 +1,145 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * cpuidle driver for halt polling.
+ *
+ * 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/sched/clock.h>
+#include <linux/sched/idle.h>
+
+static unsigned int guest_halt_poll_ns __read_mostly = 200000;
+module_param(guest_halt_poll_ns, uint, 0644);
+
+/* division factor to shrink halt_poll_ns */
+static unsigned int guest_halt_poll_shrink __read_mostly = 2;
+module_param(guest_halt_poll_shrink, uint, 0644);
+
+/* multiplication factor to grow per-cpu halt_poll_ns */
+static unsigned int guest_halt_poll_grow __read_mostly = 2;
+module_param(guest_halt_poll_grow, uint, 0644);
+
+/* value in ns to start growing per-cpu halt_poll_ns */
+static unsigned int guest_halt_poll_grow_start __read_mostly = 10000;
+module_param(guest_halt_poll_grow_start, uint, 0644);
+
+/* value in ns to start growing per-cpu halt_poll_ns */
+static bool guest_halt_poll_allow_shrink __read_mostly = true;
+module_param(guest_halt_poll_allow_shrink, bool, 0644);
+
+static DEFINE_PER_CPU(unsigned int, halt_poll_ns);
+
+static void adjust_haltpoll_ns(unsigned int block_ns,
+ unsigned int *cpu_halt_poll_ns)
+{
+ unsigned int val;
+
+ /* Grow cpu_halt_poll_ns if
+ * cpu_halt_poll_ns < block_ns < guest_halt_poll_ns
+ */
+ if (block_ns > *cpu_halt_poll_ns && block_ns <= guest_halt_poll_ns) {
+ val = *cpu_halt_poll_ns * guest_halt_poll_grow;
+
+ if (val < guest_halt_poll_grow_start)
+ val = guest_halt_poll_grow_start;
+ if (val > guest_halt_poll_ns)
+ val = guest_halt_poll_ns;
+
+ *cpu_halt_poll_ns = val;
+ } else if (block_ns > guest_halt_poll_ns &&
+ guest_halt_poll_allow_shrink) {
+ unsigned int shrink = guest_halt_poll_shrink;
+
+ val = *cpu_halt_poll_ns;
+ if (shrink == 0)
+ val = 0;
+ else
+ val /= shrink;
+ *cpu_halt_poll_ns = val;
+ }
+}
+
+static int haltpoll_enter_idle(struct cpuidle_device *dev,
+ struct cpuidle_driver *drv, int index)
+{
+ unsigned int *cpu_halt_poll_ns;
+ unsigned long long start, now, block_ns;
+ int cpu = smp_processor_id();
+
+ cpu_halt_poll_ns = per_cpu_ptr(&halt_poll_ns, cpu);
+
+ if (current_set_polling_and_test()) {
+ local_irq_enable();
+ goto out;
+ }
+
+ start = sched_clock();
+ local_irq_enable();
+ for (;;) {
+ if (need_resched()) {
+ current_clr_polling();
+ goto out;
+ }
+
+ now = sched_clock();
+ if (now - start > *cpu_halt_poll_ns)
+ break;
+
+ cpu_relax();
+ }
+
+ local_irq_disable();
+ if (current_clr_polling_and_test()) {
+ local_irq_enable();
+ goto out;
+ }
+
+ default_idle();
+ block_ns = sched_clock() - start;
+ adjust_haltpoll_ns(block_ns, cpu_halt_poll_ns);
+
+out:
+ return index;
+}
+
+static struct cpuidle_driver haltpoll_driver = {
+ .name = "haltpoll_idle",
+ .owner = THIS_MODULE,
+ .states = {
+ { /* entry 0 is for polling */ },
+ {
+ .enter = haltpoll_enter_idle,
+ .exit_latency = 0,
+ .target_residency = 0,
+ .power_usage = -1,
+ .name = "Halt poll",
+ .desc = "Halt poll idle",
+ },
+ },
+ .safe_state_index = 0,
+ .state_count = 2,
+};
+
+static int __init haltpoll_init(void)
+{
+ return cpuidle_register(&haltpoll_driver, NULL);
+}
+
+static void __exit haltpoll_exit(void)
+{
+ cpuidle_unregister(&haltpoll_driver);
+}
+
+module_init(haltpoll_init);
+module_exit(haltpoll_exit);
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Marcelo Tosatti <mtosatti@xxxxxxxxxx>");
+
