Re: [PATCH v3] qapi/qom: Memory backend property prealloc-threads doc fix

[Gavin Shan <gshan@redhat.com>]

On 11/11/22 5:13 PM, Igor Mammedov wrote:
> On Fri, 11 Nov 2022 07:47:16 +0100
> Markus Armbruster <armbru@redhat.com> wrote: 
>> Gavin Shan <gshan@redhat.com> writes:
>>> On 11/11/22 11:05 AM, Zhenyu Zhang wrote:
>>>> Commit ffac16fab3 "hostmem: introduce "prealloc-threads" property"
>>>> (v5.0.0) changed the default number of threads from number of CPUs
>>>> to 1.  This was deemed a regression, and fixed in commit f8d426a685
>>>> "hostmem: default the amount of prealloc-threads to smp-cpus".
>>>> Except the documentation remained unchanged.  Update it now.
>>>> Signed-off-by: Zhenyu Zhang <zhenyzha@redhat.com>
>>>> ---
>>>> v3: Covers historical descriptions                  (Markus)
>>>> v2: The property is changed to smp-cpus since 5.0   (Phild)
>>>> ---
>>>>    qapi/qom.json | 2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>    
>>>
>>> With the following comments addressed:
>>>
>>> Reviewed-by: Gavin Shan <gshan@redhat.com>
>>>
>>> ---
>>>
>>> Please consider amending the commit log to something like below.
>>>
>>> The default "prealloc-threads" value is set to 1 when the property is
>>> added by commit ffac16fab33b ("hostmem: introduce "prealloc-threads"
>>> property") in v5.0.0. The default value is conflicting with the sugar
>>> property as the value provided by the sugar property is number of CPUs.
>>
>> What is the sugar property?  Can you explain the conflict in a bit more
>> detail?
> 
> my guess is that Gavin means mem_prealloc compat glue in qemu_process_sugar_options()
> 
> property value should be set according to following order
>       default -> compat -> explicit value
> so I don't see any conflict here.
> 
> PS:
> if it we up to me, default would have stayed 1,
> and prealloc-threads fixup to vCPUs number would happen in vl.c
> similar to what is done in qemu_process_sugar_options(),
> keeping backend clean of external dependencies.
> 

Yes, it's the sugar property I was talking about. I'm not sure if
we have a more popular name for this property: compat property or
sugar property.

When 'mem-prealloc=on' and 'prealloc-threads=xxx' aren't provided,
the value is 1 before commit f8d426a6852c is applied. It's not
inconsistent with 'mem-prealloc=on'. It's the conflict I was talking
about and it's fixed by commit f8d426a6852c

>>
>>> The conflict has been fixed by commit f8d426a6852c ("hostmem: default
>>> the amount of prealloc-threads to smp-cpus"). However, 'qapi/qom.json'
>>> was missed to be updated accordingly in the commit.
>>>
>>> Update 'qapi/qom.json' to reflect the change in commit f8d426a6852c.
>>>
>>> Signed-off-by: Zhenyu Zhang <zhenyzha@redhat.com>
>>>
>>> When a specific commit is mentioned in the commit log, we usually have
>>> fixed format like below.
>>>
>>> commit ffac16fab33b ("hostmem: introduce "prealloc-threads" property")
>>> commit f8d426a6852c ("hostmem: default the amount of prealloc-threads to smp-cpus")
>>
>> This is certainly a common format, but the other one is also in use.
>>
>>>> diff --git a/qapi/qom.json b/qapi/qom.json
>>>> index 30e76653ad..dfd89bc6d4 100644
>>>> --- a/qapi/qom.json
>>>> +++ b/qapi/qom.json
>>>> @@ -576,7 +576,7 @@
>>>>    #
>>>>    # @prealloc: if true, preallocate memory (default: false)
>>>>    #
>>>> -# @prealloc-threads: number of CPU threads to use for prealloc (default: 1)
>>>> +# @prealloc-threads: number of CPU threads to use for prealloc (default: number of CPUs) (since 5.0)
>>>>    #
>>>>    # @prealloc-context: thread context to use for creation of preallocation threads
>>>>    #                    (default: none) (since 7.2)
>>>>    
>>>
>>> The line seems exceeding 80 characters. It'd better to limit each line in 75 characters.
>>> So you probably need:
>>>
>>>      # @prealloc-threads: number of CPU threads to use for prealloc (default: number of CPUs)
>>>      #                    (since 5.0)
>>
>> Still exceeds :)
>>
>> I suggested
>>
>>        # @prealloc-threads: number of CPU threads to use for prealloc
>>        #                    (default: number of CPUs) (since 5.0)
>>
> 

Markus's suggestion works :)

Thanks,
Gavin