128 lines
6 KiB
Plaintext
128 lines
6 KiB
Plaintext
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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 parameter halt_poll_ns_grow.
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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: 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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Module Parameter | Description | Default Value
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--------------------------------------------------------------------------------
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halt_poll_ns | The global max polling interval | KVM_HALT_POLL_NS_DEFAULT
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| which defines the ceiling value |
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| of the 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 halt | 2
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| polling interval is multiplied |
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| in the 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 halt | 0
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| polling interval is divided in |
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| the 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 debugfs files in:
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/sys/module/kvm/parameters/
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Note: that 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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Further Notes
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=============
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- Care should be taken when setting the halt_poll_ns module parameter as a
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large value has the potential to drive the cpu usage to 100% on a machine which
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would be almost entirely idle otherwise. This is because even if a guest has
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wakeups during which very little work is done and which are quite far apart, if
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the period is shorter than the global max polling interval (halt_poll_ns) then
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the host will always poll for the entire block time and thus cpu utilisation
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will go to 100%.
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- Halt polling essentially presents a trade off between power usage and latency
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and the module parameters should be used to tune the affinity for this. Idle
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cpu time is essentially converted to host kernel time with the aim of decreasing
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latency when entering the guest.
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- Halt polling will only be conducted by the host when no other tasks are
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runnable on that cpu, otherwise the polling will cease immediately and
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schedule will be invoked to allow that other task to run. Thus this doesn't
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allow a guest to denial of service the cpu.
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