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

[Igor Mammedov <imammedo@redhat.com>]

On Fri, 11 Nov 2022 17:34:04 +0800
Gavin Shan <gshan@redhat.com> wrote:

> 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

default was not supposed to be consistent with legacy mem-prealloc
and sugar property takes care of mem-prealloc=on case.

so commit message in its current form looks fine to me.

> >>  
> >>> 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
> 
>