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docs: virt: convert halt-polling.txt to ReST format

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- Fix document title to match ReST format
- Convert the table to be properly recognized
- Some indentation fixes to match ReST syntax.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Mauro Carvalho Chehab 2020-02-10 07:02:43 +01:00 committed by Paolo Bonzini
parent c09708ccb4
commit 2756df60d0
2 changed files with 46 additions and 41 deletions
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86
Documentation/virt/kvm/halt-polling.txt → Documentation/virt/kvm/halt-polling.rst
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@ -1,3 +1,6 @@
.. SPDX-License-Identifier: GPL-2.0
===========================
The KVM halt polling system
===========================
@ -68,7 +71,8 @@ steady state polling interval but will only really do a good job for wakeups
which come at an approximately constant rate, otherwise there will be constant
adjustment of the polling interval.
[0] total block time: the time between when the halt polling function is
[0] total block time:
the time between when the halt polling function is
invoked and a wakeup source received (irrespective of
whether the scheduler is invoked within that function).
@ -81,31 +85,32 @@ shrunk. These variables are defined in include/linux/kvm_host.h and as module
parameters in virt/kvm/kvm_main.c, or arch/powerpc/kvm/book3s_hv.c in the
powerpc kvm-hv case.
Module Parameter | Description | Default Value
--------------------------------------------------------------------------------
halt_poll_ns | The global max polling | KVM_HALT_POLL_NS_DEFAULT
| interval which defines |
| the ceiling value of the |
| polling interval for | (per arch value)
| each vcpu. |
--------------------------------------------------------------------------------
halt_poll_ns_grow | The value by which the | 2
| halt polling interval is |
| multiplied in the |
| grow_halt_poll_ns() |
| function. |
--------------------------------------------------------------------------------
halt_poll_ns_grow_start | The initial value to grow | 10000
| to from zero in the |
| grow_halt_poll_ns() |
| function. |
--------------------------------------------------------------------------------
halt_poll_ns_shrink | The value by which the | 0
| halt polling interval is |
| divided in the |
| shrink_halt_poll_ns() |
| function. |
--------------------------------------------------------------------------------
+-----------------------+---------------------------+-------------------------+
|Module Parameter | Description | Default Value |
+-----------------------+---------------------------+-------------------------+
|halt_poll_ns | The global max polling | KVM_HALT_POLL_NS_DEFAULT|
| | interval which defines | |
| | the ceiling value of the | |
| | polling interval for | (per arch value) |
| | each vcpu. | |
+-----------------------+---------------------------+-------------------------+
|halt_poll_ns_grow | The value by which the | 2 |
| | halt polling interval is | |
| | multiplied in the | |
| | grow_halt_poll_ns() | |
| | function. | |
+-----------------------+---------------------------+-------------------------+
|halt_poll_ns_grow_start| The initial value to grow | 10000 |
| | to from zero in the | |
| | grow_halt_poll_ns() | |
| | function. | |
+-----------------------+---------------------------+-------------------------+
|halt_poll_ns_shrink | The value by which the | 0 |
| | halt polling interval is | |
| | divided in the | |
| | shrink_halt_poll_ns() | |
| | function. | |
+-----------------------+---------------------------+-------------------------+
These module parameters can be set from the debugfs files in:
@ -117,20 +122,19 @@ Note: that these module parameters are system wide values and are not able to
Further Notes
=============
- Care should be taken when setting the halt_poll_ns module 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. This is because even if a guest has
wakeups during which very little work is done and which are quite far apart, if
the period is shorter than the global max polling interval (halt_poll_ns) then
the host will always poll for the entire block time and thus cpu utilisation
will go to 100%.
- Care should be taken when setting the halt_poll_ns module 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. This is because even if a guest has wakeups during which very
little work is done and which are quite far apart, if the period is shorter than the
global max polling interval (halt_poll_ns) then the host will always poll for the
entire block time and thus cpu utilisation will go to 100%.
- Halt polling essentially presents a trade off between power usage and latency
and the module parameters should be used to tune the affinity for this. Idle
cpu time is essentially converted to host kernel time with the aim of decreasing
latency when entering the guest.
- Halt polling essentially presents a trade off between power usage and latency and
the module parameters should be used to tune the affinity for this. Idle cpu time is
essentially converted to host kernel time with the aim of decreasing latency when
entering the guest.
- Halt polling will only be conducted by the host when no other tasks are
runnable on that cpu, otherwise the polling will cease immediately and
schedule will be invoked to allow that other task to run. Thus this doesn't
allow a guest to denial of service the cpu.
- Halt polling will only be conducted by the host when no other tasks are runnable on
that cpu, otherwise the polling will cease immediately and schedule will be invoked to
allow that other task to run. Thus this doesn't allow a guest to denial of service the
cpu.

1
Documentation/virt/kvm/index.rst
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@ -9,6 +9,7 @@ KVM
amd-memory-encryption
cpuid
halt-polling
vcpu-requests
arm/index