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

[Igor Mammedov <imammedo@redhat.com>]

On Fri, 11 Nov 2022 07:47:16 +0100
Markus Armbruster <armbru@redhat.com> wrote:

> Gavin Shan <gshan@redhat.com> writes:
> 
> > Hi Zhenyu,
> >
> > 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.

> 
> > 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)
>