Re: [RFC PATCH v2 6/6] hw/arm/virt: Replace smp_parse with one that prefers cores

[wangyanan (Y)]

On 2021/4/27 22:58, Andrew Jones wrote:
> On Tue, Apr 13, 2021 at 04:07:45PM +0800, Yanan Wang wrote:
> > From: Andrew Jones <drjones@redhat.com>
> >
> > The virt machine type has never used the CPU topology parameters, other
> > than number of online CPUs and max CPUs. When choosing how to allocate
> > those CPUs the default has been to assume cores. In preparation for
> > using the other CPU topology parameters let's use an smp_parse that
> > prefers cores over sockets. We can also enforce the topology matches
> > max_cpus check because we have no legacy to preserve.
> >
> > Signed-off-by: Andrew Jones <drjones@redhat.com>
> > Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
> > ---
> >   hw/arm/virt.c | 76 +++++++++++++++++++++++++++++++++++++++++++++++++++
> >   1 file changed, 76 insertions(+)
> Thanks, this patch matches [1]. Of course, I've always considered this
> patch to be something of an RFC, though. Is there any harm in defaulting
> to sockets over cores? If not, I wonder if we shouldn't just leave the
> default as it is to avoid a mach-virt specific smp parser. The "no
> topology" compat variable will keep existing machine types from switching
> from cores to sockets, so we don't need to worry about that.
>
> [1] 
> https://github.com/rhdrjones/qemu/commit/c0670b1bccb4d08c7cf7c6957cc8878a2af131dd
For CPU topology population, ARM64 kernel will firstly try to parse ACPI 
PPTT table
and then DT in function init_cpu_topology(), if failed it will rely on 
the MPIDR value
in function store_cpu_topology(). But MPIDR can not be trusted and is 
ignored for
any topology deduction. And instead, topology of one single socket with 
multiple
cores is made, which can not represent the real underlying system topology.
I think this is the reason why VMs will in default prefer cores over 
sockets if no
topology description is provided.

With the feature introduced by this series, guest kernel can 
successfully get cpu
information from one of the two (ACPI or DT) for topology population.

According to above analysis, IMO, whether the parsing logic is "sockets 
over cores" or
"cores over sockets", it just provide different topology information and 
consequently
different scheduling performance. Apart from this, I think there will 
not any harm or
problems caused.

So maybe it's fine that we just use the arch-neutral parsing logic?
How do you think ?

Thanks,
Yanan
> Thanks,
> drew
>
>
> .