154 lines
7.1 KiB
ReStructuredText
154 lines
7.1 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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===========================
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The KVM halt polling system
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===========================
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The KVM halt polling system provides a feature within KVM whereby the latency
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of a guest can, under some circumstances, be reduced by polling in the host
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for some time period after the guest has elected to no longer run by cedeing.
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That is, when a guest vcpu has ceded, or in the case of powerpc when all of the
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vcpus of a single vcore have ceded, the host kernel polls for wakeup conditions
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before giving up the cpu to the scheduler in order to let something else run.
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Polling provides a latency advantage in cases where the guest can be run again
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very quickly by at least saving us a trip through the scheduler, normally on
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the order of a few micro-seconds, although performance benefits are workload
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dependant. In the event that no wakeup source arrives during the polling
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interval or some other task on the runqueue is runnable the scheduler is
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invoked. Thus halt polling is especially useful on workloads with very short
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wakeup periods where the time spent halt polling is minimised and the time
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savings of not invoking the scheduler are distinguishable.
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The generic halt polling code is implemented in:
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virt/kvm/kvm_main.c: kvm_vcpu_block()
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The powerpc kvm-hv specific case is implemented in:
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arch/powerpc/kvm/book3s_hv.c: kvmppc_vcore_blocked()
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Halt Polling Interval
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=====================
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The maximum time for which to poll before invoking the scheduler, referred to
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as the halt polling interval, is increased and decreased based on the perceived
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effectiveness of the polling in an attempt to limit pointless polling.
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This value is stored in either the vcpu struct:
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kvm_vcpu->halt_poll_ns
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or in the case of powerpc kvm-hv, in the vcore struct:
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kvmppc_vcore->halt_poll_ns
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Thus this is a per vcpu (or vcore) value.
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During polling if a wakeup source is received within the halt polling interval,
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the interval is left unchanged. In the event that a wakeup source isn't
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received during the polling interval (and thus schedule is invoked) there are
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two options, either the polling interval and total block time[0] were less than
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the global max polling interval (see module params below), or the total block
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time was greater than the global max polling interval.
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In the event that both the polling interval and total block time were less than
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the global max polling interval then the polling interval can be increased in
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the hope that next time during the longer polling interval the wake up source
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will be received while the host is polling and the latency benefits will be
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received. The polling interval is grown in the function grow_halt_poll_ns() and
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is multiplied by the module parameters halt_poll_ns_grow and
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halt_poll_ns_grow_start.
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In the event that the total block time was greater than the global max polling
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interval then the host will never poll for long enough (limited by the global
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max) to wakeup during the polling interval so it may as well be shrunk in order
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to avoid pointless polling. The polling interval is shrunk in the function
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shrink_halt_poll_ns() and is divided by the module parameter
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halt_poll_ns_shrink, or set to 0 iff halt_poll_ns_shrink == 0.
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It is worth noting that this adjustment process attempts to hone in on some
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steady state polling interval but will only really do a good job for wakeups
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which come at an approximately constant rate, otherwise there will be constant
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adjustment of the polling interval.
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[0] total block time:
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the time between when the halt polling function is
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invoked and a wakeup source received (irrespective of
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whether the scheduler is invoked within that function).
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Module Parameters
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=================
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The kvm module has 3 tuneable module parameters to adjust the global max
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polling interval as well as the rate at which the polling interval is grown and
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shrunk. These variables are defined in include/linux/kvm_host.h and as module
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parameters in virt/kvm/kvm_main.c, or arch/powerpc/kvm/book3s_hv.c in the
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powerpc kvm-hv case.
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+-----------------------+---------------------------+-------------------------+
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|Module Parameter | Description | Default Value |
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+-----------------------+---------------------------+-------------------------+
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|halt_poll_ns | The global max polling | KVM_HALT_POLL_NS_DEFAULT|
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| | interval which defines | |
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| | the ceiling value of the | |
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| | polling interval for | (per arch value) |
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| | each vcpu. | |
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+-----------------------+---------------------------+-------------------------+
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|halt_poll_ns_grow | The value by which the | 2 |
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| | halt polling interval is | |
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| | multiplied in the | |
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| | grow_halt_poll_ns() | |
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| | function. | |
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+-----------------------+---------------------------+-------------------------+
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|halt_poll_ns_grow_start| The initial value to grow | 10000 |
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| | to from zero in the | |
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| | grow_halt_poll_ns() | |
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| | function. | |
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+-----------------------+---------------------------+-------------------------+
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|halt_poll_ns_shrink | The value by which the | 0 |
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| | halt polling interval is | |
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| | divided in the | |
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| | shrink_halt_poll_ns() | |
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| | function. | |
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+-----------------------+---------------------------+-------------------------+
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These module parameters can be set from the sysfs files in:
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/sys/module/kvm/parameters/
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Note: these module parameters are system-wide values and are not able to
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be tuned on a per vm basis.
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Any changes to these parameters will be picked up by new and existing vCPUs the
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next time they halt, with the notable exception of VMs using KVM_CAP_HALT_POLL
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(see next section).
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KVM_CAP_HALT_POLL
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=================
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KVM_CAP_HALT_POLL is a VM capability that allows userspace to override halt_poll_ns
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on a per-VM basis. VMs using KVM_CAP_HALT_POLL ignore halt_poll_ns completely (but
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still obey halt_poll_ns_grow, halt_poll_ns_grow_start, and halt_poll_ns_shrink).
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See Documentation/virt/kvm/api.rst for more information on this capability.
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Further Notes
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=============
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- Care should be taken when setting the halt_poll_ns module parameter as a large value
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has the potential to drive the cpu usage to 100% on a machine which would be almost
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entirely idle otherwise. This is because even if a guest has wakeups during which very
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little work is done and which are quite far apart, if the period is shorter than the
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global max polling interval (halt_poll_ns) then the host will always poll for the
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entire block time and thus cpu utilisation will go to 100%.
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- Halt polling essentially presents a trade-off between power usage and latency and
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the module parameters should be used to tune the affinity for this. Idle cpu time is
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essentially converted to host kernel time with the aim of decreasing latency when
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entering the guest.
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- Halt polling will only be conducted by the host when no other tasks are runnable on
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that cpu, otherwise the polling will cease immediately and schedule will be invoked to
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allow that other task to run. Thus this doesn't allow a guest to cause denial of service
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of the cpu.
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