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Re: [Qemu-devel] [PATCH 1/2] qmp: add query-cpus-fast

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH 1/2] qmp: add query-cpus-fast
Date: Wed, 7 Feb 2018 12:49:38 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2

On 02/07/2018 11:50 AM, Luiz Capitulino wrote:
The query-cpus command has an extremely serious side effect:
it always interrupt all running vCPUs so that they can run


ioctl calls. This can cause a huge performance degradation for
some workloads. And most of the information retrieved by the
ioctl calls are not even used by query-cpus.

This commit introduces a replacement for query-cpus called
query-cpus-fast, which has the following features:

  o Never interrupt vCPUs threads. query-cpus-fast only returns
    vCPU information maintained by QEMU itself, which should be
    sufficient for most management software needs

  o Make "halted" field optional: we only return it if the
    halted state is maintained by QEMU. But this also gives
    the option of dropping the field in the future (see below)

  o Drop irrelevant fields such as "current", "pc" and "arch"

  o Rename some fields for better clarification & proper naming

The "halted" field is somewhat controversial. On the one hand,
it offers a convenient way to know if a guest CPU is idle or
running. On the other hand, it's a field that can change many
times a second. In fact, the halted state can change even
before query-cpus-fast has returned. This makes one wonder if
this field should be dropped all together. Having the "halted"
field as optional gives a better option for dropping it in
the future, since we can just stop returning it.

Makes sense to me.

Signed-off-by: Luiz Capitulino <address@hidden>
  cpus.c               | 44 ++++++++++++++++++++++++++++++++
  hmp-commands-info.hx | 14 +++++++++++
  hmp.c                | 24 ++++++++++++++++++
  hmp.h                |  1 +
  qapi-schema.json     | 71 ++++++++++++++++++++++++++++++++++++++++++++++++++++
  5 files changed, 154 insertions(+)

+++ b/hmp-commands-info.hx
@@ -157,6 +157,20 @@ STEXI
  @item info cpus
  @findex info cpus
  Show infos for each CPU.

Pre-existing, but...

+    {
+        .name       = "cpus_fast",
+        .args_type  = "",
+        .params     = "",
+        .help       = "show infos for each CPU without performance penalty",

...copied here. s/infos/information/ in public-facing documentation, while we're touching it.

+++ b/qapi-schema.json
@@ -558,6 +558,77 @@
  { 'command': 'query-cpus', 'returns': ['CpuInfo'] }
+# @CpuInfo2:

Good thing that type names aren't part of our introspection ABI. Would CpuInfoLite or CpuInfoFast make the code any more legible?

+# Information about a virtual CPU
+# @cpu-index: index of the virtual CPU
+# @halted: true if the virtual CPU is in the halt state.  Halt usually refers
+#          to a processor specific low power mode. This field is optional,
+#          it is only present if the halted state can be retrieved without
+#          a performance penalty

Disclaimer is good.

+# @qom-path: path to the CPU object in the QOM tree
+# @thread-id: ID of the underlying host thread
+# @props: properties describing to which node/socket/core/thread
+#         virtual CPU belongs to, provided if supported by board
+# Since: 2.12
+# Notes: @halted is a transient state that changes frequently.  By the time the
+#        data is sent to the client, the guest may no longer be halted.
+{ 'struct': 'CpuInfo2',
+  'data': {'cpu-index': 'int', '*halted': 'bool', 'qom-path': 'str',
+           'thread-id': 'int', '*props': 'CpuInstanceProperties' } }

Looks reasonable.

+# @query-cpus-fast:
+# Returns information about all virtual CPUs. This command does not
+# incur a performance penalty and should be used in production
+# instead of query-cpus.
+# Returns: list of @CpuInfo2
+# Notes: The CPU architecture name is not returned by query-cpus-fast.
+#        Use query-target to retreive that information.


+{ 'command': 'query-cpus-fast', 'returns': [ 'CpuInfo2' ] }

Also, please add some docs to 'query-cpus' pointing to 'query-cpus-fast'.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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