[patch 3/5] cpuidle: add haltpoll governor

Marcelo Tosatti
2019-06-13T22:55:24Z

The cpuidle_haltpoll governor, in conjunction with the haltpoll cpuidle
driver, allows 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.

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%)

Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

---
Documentation/virtual/guest-halt-polling.txt | 79 ++++++++++++
drivers/cpuidle/Kconfig | 11 +
drivers/cpuidle/governors/Makefile | 1
drivers/cpuidle/governors/haltpoll.c | 175 +++++++++++++++++++++++++++
4 files changed, 266 insertions(+)

Index: linux-2.6.git/drivers/cpuidle/Kconfig
===================================================================
--- linux-2.6.git.orig/drivers/cpuidle/Kconfig 2019-06-13 18:05:46.456294042 -0400
+++ linux-2.6.git/drivers/cpuidle/Kconfig 2019-06-13 18:14:58.981570277 -0400
@@ -33,6 +33,17 @@
Some workloads benefit from using it and it generally should be safe
to use. Say Y here if you are not happy with the alternatives.

+config CPU_IDLE_GOV_HALTPOLL
+ bool "Haltpoll governor (for virtualized systems)"
+ depends on KVM_GUEST
+ help
+ This governor implements haltpoll idle state selection, to be
+ used in conjunction with the haltpoll cpuidle driver, allowing
+ for polling for a certain amount of time before entering idle
+ state.
+
+ Some virtualized workloads benefit from using it.
+
config DT_IDLE_STATES
bool

Index: linux-2.6.git/drivers/cpuidle/governors/Makefile
===================================================================
--- linux-2.6.git.orig/drivers/cpuidle/governors/Makefile 2019-06-13 18:05:46.456294042 -0400
+++ linux-2.6.git/drivers/cpuidle/governors/Makefile 2019-06-13 18:10:53.861444033 -0400
@@ -6,3 +6,4 @@
obj-$(CONFIG_CPU_IDLE_GOV_LADDER) += ladder.o
obj-$(CONFIG_CPU_IDLE_GOV_MENU) += menu.o
obj-$(CONFIG_CPU_IDLE_GOV_TEO) += teo.o
+obj-$(CONFIG_CPU_IDLE_GOV_HALTPOLL) += haltpoll.o
Index: linux-2.6.git/drivers/cpuidle/governors/haltpoll.c
===================================================================
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ linux-2.6.git/drivers/cpuidle/governors/haltpoll.c 2019-06-13 18:12:46.581615748 -0400
@@ -0,0 +1,175 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * haltpoll.c - haltpoll idle governor
+ *
+ * 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@redhat.com>
+ */
+
+#include <linux/kernel.h>
+#include <linux/cpuidle.h>
+#include <linux/time.h>
+#include <linux/ktime.h>
+#include <linux/hrtimer.h>
+#include <linux/tick.h>
+#include <linux/sched.h>
+#include <linux/module.h>
+#include <linux/kvm_para.h>
+
+static unsigned int guest_halt_poll_us __read_mostly = 200;
+module_param(guest_halt_poll_us, uint, 0644);
+
+/* division factor to shrink halt_poll_us */
+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_us */
+static unsigned int guest_halt_poll_grow __read_mostly = 2;
+module_param(guest_halt_poll_grow, uint, 0644);
+
+/* value in us to start growing per-cpu halt_poll_us */
+static unsigned int guest_halt_poll_grow_start __read_mostly = 50;
+module_param(guest_halt_poll_grow_start, uint, 0644);
+
+/* allow shrinking guest halt poll */
+static bool guest_halt_poll_allow_shrink __read_mostly = true;
+module_param(guest_halt_poll_allow_shrink, bool, 0644);
+
+struct haltpoll_device {
+ int last_state_idx;
+ unsigned int halt_poll_us;
+};
+
+static DEFINE_PER_CPU_ALIGNED(struct haltpoll_device, hpoll_devices);
+
+/**
+ * haltpoll_select - selects the next idle state to enter
+ * @drv: cpuidle driver containing state data
+ * @dev: the CPU
+ * @stop_tick: indication on whether or not to stop the tick
+ */
+static int haltpoll_select(struct cpuidle_driver *drv,
+ struct cpuidle_device *dev,
+ bool *stop_tick)
+{
+ struct haltpoll_device *hdev = this_cpu_ptr(&hpoll_devices);
+
+ if (!drv->state_count) {
+ *stop_tick = false;
+ return 0;
+ }
+
+ if (hdev->halt_poll_us == 0)
+ return 1;
+
+ /* Last state was poll? */
+ if (hdev->last_state_idx == 0) {
+ /* Halt if no event occurred on poll window */
+ if (dev->poll_time_limit == true)
+ return 1;
+
+ *stop_tick = false;
+ /* Otherwise, poll again */
+ return 0;
+ }
+
+ *stop_tick = false;
+ /* Last state was halt: poll */
+ return 0;
+}
+
+static void adjust_haltpoll_us(unsigned int block_us,
+ struct haltpoll_device *dev)
+{
+ unsigned int val;
+
+ /* Grow cpu_halt_poll_us if
+ * cpu_halt_poll_us < block_ns < guest_halt_poll_us
+ */
+ if (block_us > dev->halt_poll_us && block_us <= guest_halt_poll_us) {
+ val = dev->halt_poll_us * guest_halt_poll_grow;
+
+ if (val < guest_halt_poll_grow_start)
+ val = guest_halt_poll_grow_start;
+ if (val > guest_halt_poll_us)
+ val = guest_halt_poll_us;
+
+ dev->halt_poll_us = val;
+ } else if (block_us > guest_halt_poll_us &&
+ guest_halt_poll_allow_shrink) {
+ unsigned int shrink = guest_halt_poll_shrink;
+
+ val = dev->halt_poll_us;
+ if (shrink == 0)
+ val = 0;
+ else
+ val /= shrink;
+ dev->halt_poll_us = val;
+ }
+}
+
+/**
+ * haltpoll_reflect - update variables and update poll time
+ * @dev: the CPU
+ * @index: the index of actual entered state
+ */
+static void haltpoll_reflect(struct cpuidle_device *dev, int index)
+{
+ struct haltpoll_device *hdev = this_cpu_ptr(&hpoll_devices);
+
+ hdev->last_state_idx = index;
+
+ if (index != 0)
+ adjust_haltpoll_us(dev->last_residency, hdev);
+}
+
+/**
+ * haltpoll_enable_device - scans a CPU's states and does setup
+ * @drv: cpuidle driver
+ * @dev: the CPU
+ */
+static int haltpoll_enable_device(struct cpuidle_driver *drv,
+ struct cpuidle_device *dev)
+{
+ struct haltpoll_device *hdev = &per_cpu(hpoll_devices, dev->cpu);
+
+ memset(hdev, 0, sizeof(struct haltpoll_device));
+
+ return 0;
+}
+
+/**
+ * haltpoll_get_poll_time - return amount of poll time
+ * @drv: cpuidle driver
+ * @dev: the CPU
+ */
+static u64 haltpoll_get_poll_time(struct cpuidle_driver *drv,
+ struct cpuidle_device *dev)
+{
+ struct haltpoll_device *hdev = &per_cpu(hpoll_devices, dev->cpu);
+
+ return hdev->halt_poll_us * NSEC_PER_USEC;
+}
+
+static struct cpuidle_governor haltpoll_governor = {
+ .name = "haltpoll",
+ .rating = 21,
+ .enable = haltpoll_enable_device,
+ .select = haltpoll_select,
+ .reflect = haltpoll_reflect,
+ .get_poll_time = haltpoll_get_poll_time,
+};
+
+static int __init init_haltpoll(void)
+{
+ if (kvm_para_available())
+ return cpuidle_register_governor(&haltpoll_governor);
+
+ return 0;
+}
+
+postcore_initcall(init_haltpoll);
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 2019-06-13 18:16:22.414262777 -0400
@@ -0,0 +1,79 @@
+Guest halt polling
+==================
+
+The cpuidle_haltpoll driver, with the haltpoll governor, 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_us,
+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_us
+("per-cpu guest_halt_poll_us"), which is adjusted by the algorithm
+in response to events (explained below).
+
+Module Parameters
+=================
+
+The haltpoll governor has 5 tunable module parameters:
+
+1) guest_halt_poll_us:
+Maximum amount of time, in microseconds, that polling is
+performed before halting.
+
+Default: 200
+
+2) guest_halt_poll_shrink:
+Division factor used to shrink per-cpu guest_halt_poll_us when
+wakeup event occurs after the global guest_halt_poll_us.
+
+Default: 2
+
+3) guest_halt_poll_grow:
+Multiplication factor used to grow per-cpu guest_halt_poll_us
+when event occurs after per-cpu guest_halt_poll_us
+but before global guest_halt_poll_us.
+
+Default: 2
+
+4) guest_halt_poll_grow_start:
+The per-cpu guest_halt_poll_us eventually reaches zero
+in case of an idle system. This value sets the initial
+per-cpu guest_halt_poll_us when growing. This can
+be increased from 10, to avoid misses during the initial
+growth stage:
+
+10, 20, 40, ... (example assumes guest_halt_poll_grow=2).
+
+Default: 50
+
+5) guest_halt_poll_allow_shrink:
+
+Bool parameter which allows shrinking. Set to N
+to avoid it (per-cpu guest_halt_poll_us will remain
+high once achieves global guest_halt_poll_us value).
+
+Default: Y
+
+The module parameters can be set from the debugfs files in:
+
+ /sys/module/haltpoll/parameters/
+
+Further Notes
+=============
+
+- Care should be taken when setting the guest_halt_poll_us 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.
+