Re: [RFC PATCH] iothread: add set_iothread_poll_* commands

[Zhenyu Ye <yezhenyu2@huawei.com>]

On 2019/10/24 22:38, Dr. David Alan Gilbert wrote:
> * Zhenyu Ye (yezhenyu2@huawei.com) wrote:
>>
>>
>> On 2019/10/24 21:56, Dr. David Alan Gilbert wrote:
>>> * Zhenyu Ye (yezhenyu2@huawei.com) wrote:
>>>>
>>>>
>>>> On 2019/10/23 23:19, Stefan Hajnoczi wrote:
>>>>> On Tue, Oct 22, 2019 at 04:12:03PM +0800, yezhenyu (A) wrote:
>>>>>> Since qemu2.9, QEMU added three AioContext poll parameters to struct
>>>>>> IOThread: poll_max_ns, poll_grow and poll_shrink. These properties are
>>>>>> used to control iothread polling time.
>>>>>>
>>>>>> However, there isn't properly hmp commands to adjust them when the VM is
>>>>>> alive. It's useful to adjust them online when observing the impact of
>>>>>> different property value on performance.
>>>>>>
>>>>>> This patch add three hmp commands to adjust iothread poll-* properties
>>>>>> for special iothread:
>>>>>>
>>>>>> set_iothread_poll_max_ns: set the maximum polling time in ns;
>>>>>> set_iothread_poll_grow: set how many ns will be added to polling time;
>>>>>> set_iothread_poll_shrink: set how many ns will be removed from polling
>>>>>> time.
>>>>>>
>>>>>> Signed-off-by: Zhenyu Ye <yezhenyu2@huawei.com>
>>>>>> ---
>>>>>> hmp-commands.hx | 42 ++++++++++++++++++++
>>>>>> hmp.c | 30 +++++++++++++++
>>>>>> hmp.h | 3 ++
>>>>>> include/sysemu/iothread.h | 6 +++
>>>>>> iothread.c | 80 +++++++++++++++++++++++++++++++++++++++
>>>>>> qapi/misc.json | 23 +++++++++++
>>>>>> 6 files changed, 184 insertions(+)
>>>>>
>>>>> poll-max-ns, poll-grow, poll-shrink are properties of IOThread objects.
>>>>> They can already be modified at runtime using:
>>>>>
>>>>>   $ qemu -object iothread,id=iothread1
>>>>>   (qemu) qom-set /objects/iothread1 poll-max-ns 100000
>>>>>
>>>>> I think there is no need for a patch.
>>>>>
>>>>> Stefan
>>>>>
>>>>
>>>> Thanks for your review. I have considered using the `qom-set` command to modify
>>>> IOThread object's properties, however, this command is not friendly to primary
>>>> users. The help info for this command is only:
>>>>
>>>>     qom-set path property value -- set QOM property
>>>>
>>>> It's almost impossible to get the correct `path` parameter for primary user.
>>>
>>> Is this just a matter of documenting how to do it?
>>>
>>> It sounds like there's no need for a new QMP command though;  if you
>>> want an easier HMP command I'd probably still take it (because HMP is ok
>>> at having things for convenience) - but not if it turns out that just
>>> adding a paragraph of documentation is enough.
>>>
>>> Dave
>>>
>>
>> I will show the differences in QMP and HMP:
>> If I want to set iothread1.poll-max-ns=1000 and iothread1.poll-grow=2:
>>
>> Without this patch:
>> QMP command:
>>
>>     qom-set /objects/iothread1 poll-max-ns 1000
>>     qom-set /objects/iothread1 poll-grow 2
>>
>> HMP command:
>>
>>     { "execute": "qom-set", "arguments": { "path": "/objects/iothread1",
>>                                            "property": "poll-max-ns", "value": 1000 } }
>>     { "execute": "qom-set", "arguments": { "path": "/objects/iothread1",
>>                                            "property": "poll-grow", "value": 2} }
>>
>> with this patch:
>> QMP command:
>>
>>     iothread_set_parameter iothread1 max-ns 1000
>>     iothread_set_parameter iothread1 grow 2
>>
>> HMP command:
>>
>>     { "execute": "set-iothread-poll-params", "arguments': { "iothread-id": "iothread1",
>>                                                             "max-ns": 1000, "grow": 2 } }
>>
>>
>> I think the inconvenience of qom-set is how to get the correct `path` parameter.
>> Anyway, I will consider your advice.
> 
> So it depends how obvious the path is;  if it's just   /objects/
> followed by whatever you used with id=  when you created the iothread
> then I think it's easy - we just need to update the docs.
> Is there a case where it's harder to know?
> 
> Dave
> 

You are right, it's just /objects/ followed by the id. Maybe we just need
to update the docs for qom-set.

>>
>>>> This patch provides a more convenient and easy-use hmp&qmp interface to modify
>>>> these IOThread properties. I think this patch still has a little value.
>>>>
>>>> And I can implement this patch compactly by reusing your code.
>>>>
>>>> Waiting for your reply.
>>>>
>>> --
>>> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>>>
>>>
>>> .
>>>
>>
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> 
> 
> .
>