Re: [PATCH v3 3/3] cpus-common: implement dirty limit on vCPU

[Markus Armbruster <armbru@redhat.com>]

Hyman Huang <huangy81@chinatelecom.cn> writes:

> 在 2021/11/22 17:10, Markus Armbruster 写道:
>> Hyman Huang <huangy81@chinatelecom.cn> writes:
>> 
>>> =E5=9C=A8 2021/11/22 15:35, Markus Armbruster =E5=86=99=E9=81=93:
>>>> huangy81@chinatelecom.cn writes:
>>>>
>>>>> From: Hyman Huang(=E9=BB=84=E5=8B=87) <huangy81@chinatelecom.cn>
>>>>>
>>>>> implement dirtyrate calculation periodically basing on
>>>>> dirty-ring and throttle vCPU until it reachs the quota
>>>>> dirtyrate given by user.
>>>>>
>>>>> introduce qmp commands set-dirty-limit/cancel-dirty-limit to
>>>>> set/cancel dirty limit on vCPU.
>>>>
>>>> Please start sentences with a capital letter.
>>>>
>>> Ok，i'll check the syntax problem next version.
>>>>>
>>>>> Signed-off-by: Hyman Huang(黄勇) <huangy81@chinatelecom.cn>
>>>>
>>>>
>>>> [...]
>>>>
>>>>> diff --git a/qapi/misc.json b/qapi/misc.json
>>>>> index 358548a..98e6001 100644
>>>>> --- a/qapi/misc.json
>>>>> +++ b/qapi/misc.json
>>>>> @@ -527,3 +527,42 @@
>>>>>     'data': { '*option': 'str' },
>>>>>     'returns': ['CommandLineOptionInfo'],
>>>>>     'allow-preconfig': true }
>>>>> +
>>>>> +##
>>>>> +# @set-dirty-limit:
>>>>> +#
>>>>> +# This command could be used to cap the vCPU memory load, which is also
>>>>> +# refered as dirtyrate. One should use "calc-dirty-rate" with "dirty-ring"
>>>>> +# and to calculate vCPU dirtyrate and query it with "query-dirty-rate".
>>>>> +# Once getting the vCPU current dirtyrate, "set-dirty-limit" can be used
>>>>> +# to set the upper limit of dirtyrate for the interested vCPU.
>>>>
>>>> "dirtyrate" is not a word.  Let's spell it "dirty page rate", for
>>>> consistency with the documentation in migration.json.
>>> Ok, sounds good.
>>>>
>>>> Regarding "One should use ...": sounds like you have to run
>>>> calc-dirty-rate with argument @mode set to @dirty-ring before this
>>>> command.  Correct?  What happens when you don't?  set-dirty-limit fails?
>> You didn't answer this question.
> set-dirty-limit doesn't do any pre-check about if calc-dirty-rate has
> executed, so it doesn't fail.

Peeking at qmp_set_dirty_limit()... it fails when
!kvm_dirty_ring_enabled().  kvm_dirty_ring_enabled() returns true when
kvm_state->kvm_dirty_ring_size is non-zero.  How can it become non-zero?

> Since only executing calc-dirty-rate with dirty-ring mode can we get
> the vCPU dirty page rate currently(while the dirty-bitmap only get the
> vm dirty page rate), "One should use ..." maybe misleading, what i
> actually want to say is "One should use the dirty-ring mode to
> calculate the vCPU dirty page rate".

I'm still confused on what exactly users must do for the page dirty rate
limiting to work as intended, and at least as importantly, what happens
when they get it wrong.

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