Re: [PATCH v9 3/3] cpus-common: implement dirty page limit on vCPU

[Markus Armbruster]

huangy81@chinatelecom.cn writes:

> From: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
>
> Implement dirtyrate calculation periodically basing on
> dirty-ring and throttle vCPU until it reachs the quota
> dirty page rate given by user.
>
> Introduce qmp commands "vcpu-dirty-limit", "query-vcpu-dirty-limit"
> to enable, disable, query dirty page limit for virtual CPU.
>
> Meanwhile, introduce corresponding hmp commands "vcpu_dirty_limit",
> "info vcpu_dirty_limit" so developers can play with them easier.
>
> Signed-off-by: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>

[...]

I see you replaced the interface.  Back to square one...

> diff --git a/qapi/migration.json b/qapi/migration.json
> index 3da8fdf..dc15b3f 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -1872,6 +1872,54 @@
>              'current-rate': 'int64' } }
>  
>  ##
> +# @vcpu-dirty-limit:
> +#
> +# Set or cancel the upper limit of dirty page rate for a virtual CPU.
> +#
> +# Requires KVM with accelerator property "dirty-ring-size" set.
> +# A virtual CPU's dirty page rate is a measure of its memory load.
> +# To observe dirty page rates, use @calc-dirty-rate.
> +#
> +# @cpu-index: index of virtual CPU.
> +#
> +# @enable: true to enable, false to disable.
> +#
> +# @dirty-rate: upper limit of dirty page rate for virtual CPU.
> +#
> +# Since: 7.0
> +#
> +# Example:
> +#   {"execute": "vcpu-dirty-limit"}
> +#    "arguments": { "cpu-index": 0,
> +#                   "enable": true,
> +#                   "dirty-rate": 200 } }
> +#
> +##
> +{ 'command': 'vcpu-dirty-limit',
> +  'data': { 'cpu-index': 'int',
> +            'enable': 'bool',
> +            'dirty-rate': 'uint64'} }

When @enable is false, @dirty-rate makes no sense and is not used (I
checked the code), but users have to specify it anyway.  That's bad
design.

Better: drop @enable, make @dirty-rate optional, present means enable,
absent means disable.

> +
> +##
> +# @query-vcpu-dirty-limit:
> +#
> +# Returns information about the virtual CPU dirty limit status.
> +#
> +# @cpu-index: index of the virtual CPU to query, if not specified, all
> +#             virtual CPUs will be queried.
> +#
> +# Since: 7.0
> +#
> +# Example:
> +#   {"execute": "query-vcpu-dirty-limit"}
> +#    "arguments": { "cpu-index": 0 } }
> +#
> +##
> +{ 'command': 'query-vcpu-dirty-limit',
> +  'data': { '*cpu-index': 'int' },
> +  'returns': [ 'DirtyLimitInfo' ] }

Why would anyone ever want to specify @cpu-index?  Output isn't that
large even if you have a few hundred CPUs.

Let's keep things simple and drop the parameter.

> +
> +##
>  # @snapshot-save:
>  #
>  # Save a VM snapshot
> diff --git a/softmmu/vl.c b/softmmu/vl.c
> index 620a1f1..0f83ce3 100644
> --- a/softmmu/vl.c
> +++ b/softmmu/vl.c
> @@ -3777,5 +3777,6 @@ void qemu_init(int argc, char **argv, char **envp)
>      qemu_init_displays();
>      accel_setup_post(current_machine);
>      os_setup_post();
> +    dirtylimit_setup(current_machine->smp.max_cpus);
>      resume_mux_open();
>  }