| Message ID | 20190613225023.011025297@redhat.com (mailing list archive) |
|---|---|
| State | Changes Requested, archived |
| Headers | show |
| Series | cpuidle haltpoll driver and governor (v4) | expand |
Hi, I tried your patch set, but only to check that they didn't cause any regression for situations where idle state 0 (Poll) is used a lot (teo governor). They didn't (my testing was not thorough). I do not know if the below matters or not. On 2019.06.13 15:46 Marcelo Tosatti wrote: ... [snip] ... > 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. ^ |_ While applying the patches, git complains about this space character before the TAB. It also complains about a few patches with a blank line before EOF. ... Doug
On Fri, Jun 14, 2019 at 12:55 AM Marcelo Tosatti <mtosatti@redhat.com> wrote: > > 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); What about PM QoS constraints? Especially if the resume latency limit is set to 0? > + > + 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. > + > >
On Thu, Jun 20, 2019 at 05:34:46PM -0700, Doug Smythies wrote: > Hi, > > I tried your patch set, but only to check > that they didn't cause any regression for situations > where idle state 0 (Poll) is used a lot (teo governor). > > They didn't (my testing was not thorough). > > I do not know if the below matters or not. > > On 2019.06.13 15:46 Marcelo Tosatti wrote: > > ... [snip] ... > > > 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. > ^ > |_ While applying the patches, git complains about this space character before the TAB. > > It also complains about a few patches with a blank line before EOF. > > ... Doug Hi Doug, Will fix those, thanks.
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. +
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(+)