From 5d1646d90e1f2cceb9f0828f4b28318cd0ec7744 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Sat, 27 Apr 2024 12:05:51 +0200 Subject: Adding upstream version 5.10.209. Signed-off-by: Daniel Baumann --- Documentation/virt/guest-halt-polling.rst | 84 +++++++++++++++++++++++++++++++ 1 file changed, 84 insertions(+) create mode 100644 Documentation/virt/guest-halt-polling.rst (limited to 'Documentation/virt/guest-halt-polling.rst') diff --git a/Documentation/virt/guest-halt-polling.rst b/Documentation/virt/guest-halt-polling.rst new file mode 100644 index 000000000..b4e747942 --- /dev/null +++ b/Documentation/virt/guest-halt-polling.rst @@ -0,0 +1,84 @@ +================== +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_ns, +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_ns +("per-cpu guest_halt_poll_ns"), 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_ns: + +Maximum amount of time, in nanoseconds, that polling is +performed before halting. + +Default: 200000 + +2) guest_halt_poll_shrink: + +Division factor used to shrink per-cpu guest_halt_poll_ns when +wakeup event occurs after the global guest_halt_poll_ns. + +Default: 2 + +3) guest_halt_poll_grow: + +Multiplication factor used to grow per-cpu guest_halt_poll_ns +when event occurs after per-cpu guest_halt_poll_ns +but before global guest_halt_poll_ns. + +Default: 2 + +4) guest_halt_poll_grow_start: + +The per-cpu guest_halt_poll_ns eventually reaches zero +in case of an idle system. This value sets the initial +per-cpu guest_halt_poll_ns when growing. This can +be increased from 10000, to avoid misses during the initial +growth stage: + +10k, 20k, 40k, ... (example assumes guest_halt_poll_grow=2). + +Default: 50000 + +5) guest_halt_poll_allow_shrink: + +Bool parameter which allows shrinking. Set to N +to avoid it (per-cpu guest_halt_poll_ns will remain +high once achieves global guest_halt_poll_ns 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_ns 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. -- cgit v1.2.3