1 ==================
2 Guest halt polling
3 ==================
4
5 The cpuidle_haltpoll driver, with the haltpoll
6 the guest vcpus to poll for a specified amount
7 halting.
8
9 This provides the following benefits to host s
10
11 1) The POLL flag is set while polling
12 a remote vCPU to avoid sending an I
13 cost of handling the IPI) when perf
14
15 2) The VM-exit cost can be avoided.
16
17 The downside of guest side polling is that pol
18 even with other runnable tasks in the host.
19
20 The basic logic as follows: A global value, gu
21 is configured by the user, indicating the maxi
22 time polling is allowed. This value is fixed.
23
24 Each vcpu has an adjustable guest_halt_poll_ns
25 ("per-cpu guest_halt_poll_ns"), which is adjus
26 in response to events (explained below).
27
28 Module Parameters
29 =================
30
31 The haltpoll governor has 5 tunable module par
32
33 1) guest_halt_poll_ns:
34
35 Maximum amount of time, in nanoseconds, that p
36 performed before halting.
37
38 Default: 200000
39
40 2) guest_halt_poll_shrink:
41
42 Division factor used to shrink per-cpu guest_h
43 wakeup event occurs after the global guest_hal
44
45 Default: 2
46
47 3) guest_halt_poll_grow:
48
49 Multiplication factor used to grow per-cpu gue
50 when event occurs after per-cpu guest_halt_pol
51 but before global guest_halt_poll_ns.
52
53 Default: 2
54
55 4) guest_halt_poll_grow_start:
56
57 The per-cpu guest_halt_poll_ns eventually reac
58 in case of an idle system. This value sets the
59 per-cpu guest_halt_poll_ns when growing. This
60 be increased from 10000, to avoid misses durin
61 growth stage:
62
63 10k, 20k, 40k, ... (example assumes guest_halt
64
65 Default: 50000
66
67 5) guest_halt_poll_allow_shrink:
68
69 Bool parameter which allows shrinking. Set to
70 to avoid it (per-cpu guest_halt_poll_ns will r
71 high once achieves global guest_halt_poll_ns v
72
73 Default: Y
74
75 The module parameters can be set from the sysf
76
77 /sys/module/haltpoll/parameters/
78
79 Further Notes
80 =============
81
82 - Care should be taken when setting the guest_
83 large value has the potential to drive the c
84 which would be almost entirely idle otherwis
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