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

[Hyman Huang]

在 2021/12/3 20:34, Markus Armbruster 写道:
> 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.
Uh, if we drop @enable, enabling dirty limit should be like:
vcpu-dirty-limit cpu-index=0 dirty-rate=1000

And disabling dirty limit like:
vcpu-dirty-limit cpu-index=0

For disabling case, there is no hint of disabling in command 
"vcpu-dirty-limit".

How about make @dirty-rate optional, when enable dirty limit, it should
present, ignored otherwise?

> > +
> > +##
> > +# @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.
Ok, this make things simple.
> > +
> > +##
> >   # @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();
> >   }

--
Best regard

Hyman Huang(黄勇)