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cpuidle: add haltpoll governor

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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>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
This commit is contained in:
Marcelo Tosatti 2019-07-03 20:51:28 -03:00 committed by Rafael J. Wysocki
parent 7d4daeedd5
commit 2cffe9f6b9
4 changed files with 240 additions and 0 deletions
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78
Documentation/virtual/guest-halt-polling.txt Normal file
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@ -0,0 +1,78 @@
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_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 haltpoll governor has 5 tunable module parameters:
1) guest_halt_poll_ns:
Maximum amount of time, in nanoseconds, that polling is
performed before halting.
Default: 200000
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:
10k, 20k, 40k, ... (example assumes guest_halt_poll_grow=2).
Default: 50000
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/haltpoll/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.

11
drivers/cpuidle/Kconfig
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@ -33,6 +33,17 @@ config CPU_IDLE_GOV_TEO
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

1
drivers/cpuidle/governors/Makefile
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@ -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

150
drivers/cpuidle/governors/haltpoll.c Normal file
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@ -0,0 +1,150 @@
// 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_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 poll_limit_ns */
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_ns */
static unsigned int guest_halt_poll_grow_start __read_mostly = 50000;
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);
/**
* 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)
{
int latency_req = cpuidle_governor_latency_req(dev->cpu);
if (!drv->state_count || latency_req == 0) {
*stop_tick = false;
return 0;
}
if (dev->poll_limit_ns == 0)
return 1;
/* Last state was poll? */
if (dev->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_poll_limit(struct cpuidle_device *dev, unsigned int block_us)
{
unsigned int val;
u64 block_ns = block_us*NSEC_PER_USEC;
/* Grow cpu_halt_poll_us if
* cpu_halt_poll_us < block_ns < guest_halt_poll_us
*/
if (block_ns > dev->poll_limit_ns && block_ns <= guest_halt_poll_ns) {
val = dev->poll_limit_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;
dev->poll_limit_ns = val;
} else if (block_ns > guest_halt_poll_ns &&
guest_halt_poll_allow_shrink) {
unsigned int shrink = guest_halt_poll_shrink;
val = dev->poll_limit_ns;
if (shrink == 0)
val = 0;
else
val /= shrink;
dev->poll_limit_ns = 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)
{
dev->last_state_idx = index;
if (index != 0)
adjust_poll_limit(dev, dev->last_residency);
}
/**
* 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)
{
dev->poll_limit_ns = 0;
return 0;
}
static struct cpuidle_governor haltpoll_governor = {
.name = "haltpoll",
.rating = 21,
.enable = haltpoll_enable_device,
.select = haltpoll_select,
.reflect = haltpoll_reflect,
};
static int __init init_haltpoll(void)
{
if (kvm_para_available())
return cpuidle_register_governor(&haltpoll_governor);
return 0;
}
postcore_initcall(init_haltpoll);