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84 lines
2.3 KiB
84 lines
2.3 KiB
================== |
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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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