==================
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Guest halt polling
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==================
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The cpuidle_haltpoll driver, with the haltpoll governor, allows
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the guest vcpus to poll for a specified amount of time before
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halting.
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This provides the following benefits to host side polling:
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1) The POLL flag is set while polling is performed, which allows
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a remote vCPU to avoid sending an IPI (and the associated
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cost of handling the IPI) when performing a wakeup.
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2) The VM-exit cost can be avoided.
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The downside of guest side polling is that polling is performed
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even with other runnable tasks in the host.
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The basic logic as follows: A global value, guest_halt_poll_ns,
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is configured by the user, indicating the maximum amount of
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time polling is allowed. This value is fixed.
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Each vcpu has an adjustable guest_halt_poll_ns
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("per-cpu guest_halt_poll_ns"), which is adjusted by the algorithm
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in response to events (explained below).
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Module Parameters
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=================
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The haltpoll governor has 5 tunable module parameters:
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1) guest_halt_poll_ns:
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Maximum amount of time, in nanoseconds, that polling is
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performed before halting.
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Default: 200000
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2) guest_halt_poll_shrink:
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Division factor used to shrink per-cpu guest_halt_poll_ns when
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wakeup event occurs after the global guest_halt_poll_ns.
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Default: 2
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3) guest_halt_poll_grow:
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Multiplication factor used to grow per-cpu guest_halt_poll_ns
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when event occurs after per-cpu guest_halt_poll_ns
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but before global guest_halt_poll_ns.
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Default: 2
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4) guest_halt_poll_grow_start:
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The per-cpu guest_halt_poll_ns eventually reaches zero
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in case of an idle system. This value sets the initial
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per-cpu guest_halt_poll_ns when growing. This can
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be increased from 10000, to avoid misses during the initial
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growth stage:
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10k, 20k, 40k, ... (example assumes guest_halt_poll_grow=2).
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Default: 50000
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5) guest_halt_poll_allow_shrink:
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Bool parameter which allows shrinking. Set to N
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to avoid it (per-cpu guest_halt_poll_ns will remain
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high once achieves global guest_halt_poll_ns value).
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Default: Y
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The module parameters can be set from the debugfs files in::
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/sys/module/haltpoll/parameters/
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Further Notes
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=============
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- Care should be taken when setting the guest_halt_poll_ns parameter as a
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large value has the potential to drive the cpu usage to 100% on a machine
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which would be almost entirely idle otherwise.
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